PATENT DOCUMENT

Publication Number: US-9014544-B2
Application Number: US-201213720988-A
Country: US
Kind Code: B2

Title: User interface for retiming in a media authoring tool

Abstract:
A novel method for creating multimedia presentations in a media editing application is provided. The method selects a portion of a media clip and retimes the selected portion of the media clip from a first playback speed to a second playback speed. The method defines a transition interval for a discontinuity between a first section of the media clip with the first playback speed and a second section of the media clip with the second playback speed. The media clip is played back according to a smoothed curve that is constrained by the transition interval, the first playback speed, and the second playback speed. The smoothed curve transitions gradually from the first playback speed at the start of the transition interval to the second playback speed at the end of the transition interval.

Claims:
What is claimed is: 
     
       1. A method for creating a multimedia presentation, the method comprising:
 receiving a media clip for the multimedia presentation, the media clip comprising a first section having a first playback speed and a second section having a second playback speed, the first section transitioning to the second section at a user-defined point of discontinuity; 
 defining a transition interval encompassing the user-defined point of discontinuity, wherein the start of the transition interval is after the start of the first section and the end of the transition interval is before the end of the second section; 
 presenting the transition interval in a graphical user interface (GUI), wherein boundaries of the transition interval are presented as a plurality of user-selectable items for adjusting a duration of the transition interval; and 
 playing back the media clip according to a curve that is constrained by the transition interval, the first playback speed, and the second playback speed. 
 
     
     
       2. The method of  claim 1 , wherein the curve transitions gradually from the first playback speed at the start of the transition interval to the second playback speed at the end of the transition interval. 
     
     
       3. The method of  claim 1 , wherein the curve is a playback curve for specifying a playback timing of the media clip. 
     
     
       4. The method of  claim 1  further comprising defining a discontinuity keyframe at the user-defined point of discontinuity. 
     
     
       5. The method of  claim 4 , wherein the curve is not constrained to go through the discontinuity keyframe. 
     
     
       6. The method of  claim 1  further comprising:
 selecting a portion of the media clip; and 
 retiming the selected portion of the media clip from the first playback speed to the second playback speed, wherein the selected portion is the second section and an unselected portion is the first section. 
 
     
     
       7. A method of defining a media editing application for creating multimedia presentations, the method comprising:
 defining a range selector for user-selecting a portion of a media clip in a multimedia presentation by a user; 
 defining a retiming engine for applying a retiming operation that changes a playback timing of the media clip from a first playback speed to a second playback speed at a first transition point that is at an intersection of the selected portion and an unselected portion; and 
 defining a transition engine for determining a playback speed of the media clip around the first transition point by defining second and third transition points, the second transition point earlier than the first transition point by a fraction of the duration of the selected portion and the third transition point later than the first transition point by a fraction of the duration of the unselected portion. 
 
     
     
       8. The method of  claim 7 , wherein the playback speed of the media clip gradually changes from the first playback speed at the second transition point to the second playback speed at the third transition point without overshooting the second playback speed before the third transition point. 
     
     
       9. The method of  claim 7 , wherein the playback speed of the media clip at the second transition point is the first playback speed. 
     
     
       10. The method of  claim 9 , wherein the playback speed of the media clip at third transition point is the second playback speed. 
     
     
       11. The method of  claim 10 , wherein the playback speed of the media clip at the first transition point is between the first playback speed and the second playback speed. 
     
     
       12. The method of  claim 7 , wherein the transition engine determines the playback speed of the media clip by defining a playback curve that specifies a relationship between content of the media clip and the playback timing of the media clip. 
     
     
       13. The method of  claim 7 , wherein the retiming engine applies a retiming operation by defining a keyframe associated with the first transition point, wherein the keyframe defines a particular point in the content of the media clip that is played back at a particular playback time. 
     
     
       14. A system comprising: a set of processing units for executing sets of instructions; and a non-transitory machine readable medium storing a media editing application for execution by at least one of the processing units, the media editing application comprising sets of instructions for: editing a playback curve of a media clip of a multimedia presentation for specifying playback timing of the media clip, wherein editing the playback curve comprises changing a playback speed of a section of the playback curve; introducing a transition interval at a user-adjustable point of discontinuity between a first section with a first playback speed and a second section with a second playback speed, wherein the playback speed of the media clip gradually changes without any discontinuity from the first playback speed at a start of the transition interval to the second playback speed at an end of the transition interval without crossing the second playback speed before the end of the transition interval; and playing back the media clip according to the edited playback curve and the transition interval. 
     
     
       15. The system of  claim 14 , wherein introducing the transition interval comprises determining a first fraction of a duration of the first section and a second fraction of a duration of the second section. 
     
     
       16. The system of  claim 14  further comprising a storage for storing the playback curve, wherein a data structure of the playback curve stores the start point and the end point for the transition interval. 
     
     
       17. The system of  claim 16 , wherein the data structure stores a plurality of keyframes, each keyframe defining a particular point in the content of the media clip that is played back at a particular playback time. 
     
     
       18. The system of  claim 14 , wherein playing back the media clip according to the edited playback curve comprises creating a smoothed playback curve, the smoothed playback curve comprising a Bezier curve that is constrained at the first playback speed at the start of the transition interval and at the second playback speed at the end of the transition interval. 
     
     
       19. The system of  claim 18 , wherein the user-adjustable point of discontinuity is a keyframe that anchors the end of the first section and the start of the second section, wherein the playback curve is not constrained to go through the keyframe. 
     
     
       20. A non-transitory computer readable medium storing a computer program for execution by one or more processing units, the computer program comprising sets of instructions for:
 receiving a media clip that changes playback timing from a first playback speed to a second playback speed at a first transition point between a first media clip section and a second media clip section; 
 introducing a transition interval that is defined by a second transition point and a third transition point, the second transition point earlier than the first transition point and the third transition point later than the first transition point in the media clip, wherein the second and third transition points are user-selectable items that are adjustable within a graphical user interface (GUI); and 
 determining the playback speed of the media clip around the first transition point based on a playback time of the second and third transition points. 
 
     
     
       21. The non-transitory computer readable medium of  claim 20 , wherein the program further comprises a set of instructions for displaying a speed indicator for indicating the playback speed of the media clip. 
     
     
       22. The non-transitory computer readable medium of  claim 21 , wherein the speed indicator is partitioned into a plurality of sections based on transitions of playback speeds in the media clip. 
     
     
       23. The non-transitory computer readable medium of  claim 22 , wherein one of the plurality of sections is a transition section that starts at the second transition point and ends at the third transition point. 
     
     
       24. The non-transitory computer readable medium of  claim 23 , where the transition section is associated with said user-selectable items. 
     
     
       25. The non-transitory computer readable medium of  claim 20 , wherein the second transition point is initially defined to be earlier than the first transition point by a fraction of the duration of the first media clip section and the third transition point is initially defined to be later than the first transition point in the media clip by a fraction of the duration of the second media clip section. 
     
     
       26. The non-transitory computer readable medium of  claim 20 , wherein the playback speed of the media clip gradually changes from the first playback speed at the second transition point to the second playback speed at the third transition point. 
     
     
       27. The non-transitory computer readable medium of  claim 20 , wherein the set of instructions for determining the playback speed of the media clip comprises a set of instructions for defining a playback curve that specifies a relationship between the content of the media clip and the playback timing of the media clip.

Description:
BACKGROUND 
     Digital graphic design, image editing, audio editing, and video editing applications (hereafter collectively referred to as media content editing applications or media editing applications) provide graphical designs, media artists, and other users with the necessary tools to create a variety of media content. Examples of such applications include Final Cut Pro® and iMovie®, both sold by Apple Inc. These applications give users the ability to edit, combine, transition, overlay and piece together different media content in a variety of manners to create a resulting media project. The resulting media project specifies a particular sequenced composition of any number of text, audio clips, images, or video content that is used to create a media presentation. 
     Various media editing applications facilitate such compositions through electronic means. Specifically, a computer or other electronic device with a processor and a computer readable storage medium executes the media content editing applications. In so doing, the computer generates a graphical interface whereby designers digitally manipulate graphical representation of the media content to produce a desired result. 
     One difficulty in media editing is that a user cannot easily and intuitively alter the timing of media clips in the graphical interface. For example, the user may wish to graphically specify that media content within a particular range to be played back at a particular playback speed (e.g., slow motion or accelerated motion). The user may also wish to apply other speed or timing effects (e.g., instant replay or rewind) to the particular range of media content. 
     Some existing media editing applications facilitate the application of speed or timing effect by providing a playback curve. A playback curve is an abstract representation of a media content that specifies the relationship between the media content and the playback time. A user can graphically manipulate the playback curve in order to adjust the playback timing of the media content. Unfortunately, such a timing adjustment is based on manipulations of an abstract representation of the media content that does not intuitively relate to the user what has happened to the media content. Worse yet, allowing direct user manipulation of the playback curve in some instances can cause unintended visual effects (such as playback speed overshoot). 
     BRIEF SUMMARY 
     Some embodiments of the invention provide a media editing application for creating multimedia presentations. The media editing application selects a portion of a media clip and retimes the selected portion of the media clip from a first playback speed to a second playback speed. The media editing application defines a transition interval for a discontinuity between a first section of the media clip with the first playback speed and a second section of the media clip with the second playback speed. The media clip is played back according to a smoothed curve that is constrained by the transition interval, the first playback speed, and the second playback speed. The smoothed curve transitions gradually from the first playback speed at the start of the transition interval to the second playback speed at the end of the transition interval. The smoothed curve does not exceed or overshoot the second playback speed before the end of the transition interval. 
     In some embodiments, the media editing application sets the start and end of the transition interval based predefined fractions of the section before the discontinuity and of the section after the discontinuity. In some other embodiments, the media editing application sets the start of the transition interval based on a fixed time interval. 
     A retiming operation changes the timing characteristics of a portion of a piece of media content such as a composite presentation or a media clip. In some embodiments, the media editing application includes a user interface for defining a range in order to select the portion of the media content. The media editing application then performs a retiming operation by applying a preset speed effect to the portion of the media content selected by the defined range. 
     In some embodiments, the timing of the playback of a piece of media content is determined by a playback curve associated with the piece of media content. The playback curve is for mapping each video frame that needs to be played back at a particular instant in time to one or more video frame in the source media clips. In some embodiments, the same playback curve is also used to map audio playback such that the slope of the playback curve at a particular instant in time that corresponds to the audio playback speed at that particular instant in time. Each retiming operation is implemented based on adjustments of the playback curve. 
     Some embodiments define a playback curve by keyframes. Each keyframe specifies a particular point in the media content (i.e., a specific media time) that is to be played back at a particular playback time. In some embodiments, keyframes are used to anchor different sections of the playback curve by specifying the start and the end of the section. These different sections of the playback curve are created by retiming operations, which applies speed effects to portions of the media content to cause sections of the playback back curve to have different speeds. In some of these embodiments, the adjustment of the playback curve (and hence the playback timing of the media content) is accomplished by moving one or more keyframes to a different point in playback time and/or media time. 
     The initially created speed effect sections can be used as a template for further adjustments and refinements by the user for desired result. In some embodiments, the adjustment of the speed effect is accomplished by adjusting individual speed effect sections (including transition intervals) that have been created by the earlier preset speed effects. In some of these embodiments, effects bars associated with each speed effect section have graphical handles that can be manipulated by the user to adjust the speed effect. After each speed effect adjustment, some embodiments examine each transition interval. Based on such examination of the transition intervals, some embodiments eliminate one or more transition intervals, adjust one or more of the transition intervals, and/or perform new curve smoothing operation based on the remaining transition intervals. 
     The preceding Summary is intended to serve as a brief introduction to some embodiments of the invention. It is not meant to be an introduction or overview of all inventive subject matter disclosed in this document. The Detailed Description that follows and the Drawings that are referred to in the Detailed Description will further describe the embodiments described in the Summary as well as other embodiments. Accordingly, to understand all the embodiments described by this document, a full review of the Summary, Detailed Description and the Drawings is needed. Moreover, the claimed subject matters are not to be limited by the illustrative details in the Summary, Detailed Description and the Drawing, but rather are to be defined by the appended claims, because the claimed subject matters can be embodied in other specific forms without departing from the spirit of the subject matters. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features of the invention are set forth in the appended claims. However, for purpose of explanation, several embodiments of the invention are set forth in the following figures. 
         FIG. 1  illustrates a retiming operation of a media clip that is followed by a curve smoothing operation. 
         FIG. 2  illustrates a retiming operation and a curve smoothing operation from the perspective of a playback curve. 
         FIG. 3  conceptually illustrates a process for performing a retiming operation that is followed by a curve smoothing operation. 
         FIG. 4  illustrates an example of a media editing application that performs retiming operation on portions of a media clip. 
         FIGS. 5   a - b  illustrates a playback curve at different stages of the retiming operations described in  FIG. 4 . 
         FIG. 6  illustrates an example ramp retiming operation in a timeline. 
         FIG. 7  illustrates a playback curve that is maintained by the media editing application while performing the ramp retiming operation of  FIG. 6 . 
         FIG. 8  illustrates an example “conform speed” retiming operation. 
         FIG. 9  illustrates a playback curve that corresponds to the “conform speed” retiming operation of  FIG. 8 . 
         FIG. 10  illustrates a curve smoothing operation that uses transition intervals between different playback speeds in some embodiments. 
         FIG. 11  illustrates a playback speed plot that is based on the smoothed playback curve that is constrained by specified start and end times of the transition intervals. 
         FIG. 12  illustrates the creation of a smoothed curve that is based on a Bezier curve. 
         FIG. 13  conceptually illustrates a process for specifying a smoothed playback curve following a retiming operation by using transition intervals. 
         FIG. 14  illustrates an example media clip for which the start and the end of the transition intervals are initially defined according to a predefined fraction. 
         FIG. 15  illustrates an example media clip for which the start and the end of the transition intervals are initially defined based on fixed time intervals. 
         FIG. 16  conceptually illustrates a process that uses the stored information of the transitional intervals to construct the actual smoothed playback curve for media clip playback. 
         FIGS. 17   a - b  illustrate an example speed effect adjustment operation of a media clip that has been partitioned by an earlier preset retiming operation. 
         FIGS. 18   a - b  illustrates an example of a user adjustment of speed effect that shrinks a section of the effects bar. 
         FIGS. 19   a - b  illustrates an example range adjustment operation of a media clip that has been partitioned by an earlier preset retiming operation. 
         FIG. 20  conceptually illustrates a process for performing a user adjustment of the media clip. 
         FIG. 21  illustrates the mapping of a playback curve from playback time to media time and then to actual video frames in the source of the media content. 
         FIG. 22  conceptually illustrates a process for mapping playback times to actual or interpolated video frames. 
         FIG. 23  conceptually illustrates the software architecture of a media editing application of some embodiments. 
         FIG. 24  conceptually illustrates a data structure for a playback curve. 
         FIG. 25  illustrates an example graphical user interface (“GUI”) of a media-editing application. 
         FIG. 26  conceptually illustrates a computer system with which some embodiments of the invention are implemented. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous details are set forth for the purpose of explanation. However, one of ordinary skill in the art will realize that the invention may be practiced without the use of these specific details. In other instances, well-known structures and devices are shown in block diagram form in order not to obscure the description of the invention with unnecessary detail. 
     Some embodiments of the invention provide a media editing application for creating multimedia presentations. The media editing application selects a portion of a media clip and retimes the selected portion of the media clip from a first playback speed to a second playback speed. The media editing application defines a transition interval for a discontinuity between a first section of the media clip with the first playback speed and a second section of the media clip with the second playback speed. The media clip is played back according to a smoothed curve that is constrained by the transition interval, the first playback speed, and the second playback speed. The smoothed curve transitions gradually from the first playback speed at the start of the transition interval to the second playback speed at the end of the transition interval. The smoothed curve does not exceed or overshoot the second playback speed before the end of the transition interval. 
     A retiming operation changes the timing characteristics of a portion of a piece of media content such as a composite presentation or a media clip. In some embodiments, the media editing application includes a user interface for defining a range in order to select the portion of the media content. The media editing application then performs retiming by applying a speed effect to the portion of the media content selected by the defined range. 
     In some embodiments, the timing of the playback of a piece of media content is determined by a playback curve associated with the piece of media content. The playback curve is for mapping each video frame that needs to be played back at a particular instant in time to one or more video frame in the source media clips. In some embodiments, the same playback curve is also used to map audio playback such that the slope of the playback curve at a particular instant in time that corresponds to the audio playback speed at that particular instant in time. Each retiming operation is implemented based on adjustments of the playback curve. In some embodiments, a playback curve has playback time as its x-axis and media time as its y-axis. Media time is the time native to a piece of media content. An instant in the media time associated with a video frame specifies the time that the video frame is captured or intended to be displayed. Playback time is the time that the piece of media content is actually being played back as part of the composite presentation. As such, each point in the playback curve maps a particular instant in playback time with a particular instant in media time. The slope of the playback curve at any particular instant in playback time indicates the playback speed at that particular instant. A slope of 100% in a playback curve indicates that the playback time is elapsing at the same rate as the media time, or that the playback speed is the same as the normal speed of the media content. 
     Some embodiments define a playback curve by keyframes. Each keyframe specifies a particular point in the media clip (i.e., a specific media time) that is to be played back at a particular playback time. In some embodiments, keyframes are used to anchor different sections of the playback curve by specifying the start and the end of each section. These different sections of the playback curve are created by retiming operations, which applies speed effects to portions of the media content to cause sections of the playback back curve to have different speeds. These keyframes therefore, in addition to anchoring the start and end of each section, also marks the discontinuity in playback speed between the different sections. In some of these embodiments, the adjustment of the playback curve (and hence the playback timing of the media content) is accomplished by moving a discontinuity keyframe to a different point in playback time and/or media time while the discontinuity keyframe is still anchoring the end of one section and the start of another section. 
     In some embodiments, the media application performs a curve smoothing operation after a retiming operation. Application of speed effect changes playback speed on a portion of a media clip, which results in one or more discontinuities in playback speed of the media clip. Such speed discontinuity can be visually distracting to a viewer. Some embodiments thus adjust the playback curve in order to ameliorate the playback speed discontinuity. A curve smoothing operation adjusts the playback curve near the discontinuities such that the slope or tangent (i.e., playback speed) of the smoothed playback curve is continuous throughout the curve. Some embodiments introduce a transition interval to allow the playback curve to smoothly transition from a first section with one playback speed to a second section with another playback speed. In some embodiments, the smoothed curve is not constrained to go through the discontinuity keyframe between the two speed sections. 
     For some embodiments,  FIG. 1  illustrates a retiming operation of a media clip that is followed by a curve smoothing operation. The curve smoothing operation is based on transition intervals between different playback speeds.  FIG. 1  illustrates a timeline  100  in a GUI of a media editing application. Within the timeline  100  is a representation of a media clip  150 . The representation of the media clip displays a series of thumbnail images sampled at regular intervals of the playback time from the media clip  150 . Each thumbnail image is labeled with an index that reflects the position of the thumbnail image in media time for purpose of illustration. The media clip representation  150  also includes an effects bar  160  that indicates the playback speed of the media content in the media clip. 
       FIG. 1  will be described by reference to  FIG. 2 .  FIG. 2  illustrates the retiming operation and the curve smoothing operation from the perspective of a playback curve  200  that controls the playback timing of the media clip  150 . The curve smoothing operation is based on transition intervals with specified start and end times.  FIG. 2  illustrates the playback curve  200  during the retiming operation and the subsequent curve smoothing operation of  FIG. 1 . As the retiming operation and curve smooth operations alter the playback curve  200 , the playback speed of the media clip  150  are also accordingly altered.  FIG. 2  illustrates the changes to the playback curve  200  in six stages  201 - 206 . 
       FIG. 1  illustrates the creation and adjustment of transition intervals following the retiming operation in six stages  101 - 106  of the timeline  100 . The first stage  101  correspond to the playback curve stage  201  of  FIG. 2 . The stage  101  shows the selection of a range  170  for retiming operation that corresponds to the selection of range  210 . At the first stage  101 , the effects bar  160  of the content media clip  150  indicates that the entire media clip is at 100% of normal speed. The thumbnails are indexed at increments of 4 (0, 4, 8, 12 . . . ) at the current zoom level, which correspond to playing back the media content at the standard frame rate. The first stage  101  also illustrates the selection of a range  170  that starts at playback time t 0  and ends at playback time t 1 . The user is illustrated as selecting the “slower” “at 50% playback rate” retiming operation preset from an effects menu  140 . The “slower” preset retiming operation will be applied to the media content in the selected range  170  between the playback times t 0  and t 1 . Some embodiments provide preset speed effects so a user can quickly achieve an initial retiming effect on a selected range in the composite presentation. The initial retiming effect can then be used as a template for further adjustments and refinements by the user for desired result. The second stage  102  corresponds to the playback curve stage  202  of  FIG. 2 . The stage  102  shows the results of the slower speed retiming operation in the GUI. A new section  162  has appeared in the effects bar  160  that correspond to the selected range  170 . The new section  162  ends at t′ 1  instead of t 1  because its duration is twice as long as the selected range  170 . Since the source frame rate of the media content is twice the playback frame rate, the playback speed of the section  162  is effectively reduced to 50% of normal. The thumbnail images under the section  162  reflect the reduced playback speed, as they are indexed at increments of 2 (8, 10, 12, 14, 16, etc.). The retiming operation also creates sections  161  and  163  in the effects bar  160 . The new effects bar section  162  has a different appearance (e.g., different color) than the sections  161  and  163  to indicate that the playback speed of the section  162  (50%) is different from the playback speed of the sections  161  and  163  (both at 100%). 
     In some embodiments that introduce transition intervals for smoothing playback curves, the second stage  102  is not shown to the user. For these embodiments, the second stage  102  is for illustrative purpose only, and the GUI of the media editing application proceeds directly from the first stage  101  to the third stage  103  without displaying the second stage  102 . 
     The third stage  103  corresponds to the playback curve stages  203  and  204  of  FIG. 2 . The stage  103  illustrates the introduction of transition intervals  164  and  165  in the timeline  100 . In some embodiments, the transition interval is initially defined by the media editing application. The transition interval  164  spans over a portion of the 100% speed section  161  and a portion of the 50% speed section  162 . The transition interval  164  starts at the time t 1a  and ends at the time t 1b , where t 1a  is before t 1  and t 1b  is after t 1  (t 1  is the end of the 100% playback speed section  161  and also the start of the 50% playback speed section  162 ). The transition interval  165  spans over a portion of the 50% speed section  162  and a portion of the 100% speed section  163 . The transition interval  165  starts at the time t 2a  and ends at the time t 2b , where t 2a  is before t 2  and t 2b  is after t′ 2  (t′ 2  is the end of the 50% playback speed section  162  and also the start of the 100% playback speed section  163 ). 
     In some embodiments, the sections of the effects bar  160  that correspond to the transition intervals  164  and  165  also have visual features that indicate transition between speeds. For example, for embodiments that uses color to indicate speed in the effects bar, the transition interval shows a color gradient that gradually transition from the color of one speed to the color of another speed. In the example of  FIG. 1 , the color rendering of the transition interval  164  gradually transition from the color of the section  161  to the color of the section  162 , and the color rendering of the transition interval  164  gradually transition from the color of the section  162  to the color of the section  163 . In some embodiments, the color gradient in the transition intervals is based on the playback speed (as determined by the smoothed playback curve) within the transition intervals. In some embodiments, other indicia of transition appear in sections of effects bars that correspond to transition intervals. For example, some embodiments depict a curve in the effects bar section that corresponds to the playback speed during the transition interval (as illustrated in effects bar section  164  and  165 ). Some embodiments depict transition intervals as slightly opaque rectangles that transparently superimposed on top of the effect bar. This type of representation of transition intervals allows the features of the effects bar to be still visible and accessible to the user. One of ordinary skill would understand that other visual depictions of speed transitions are also possible, such as hash pattern with density gradients that shows a transition from one speed to another. 
     As mentioned, the stage  103  corresponds to the playback curve stages  203  and  204 . In other words, at the end of the stage  103 , the media editing application has not only defined the transition intervals  164  and  165  as illustrated in playback curve stage  203 , but it has also completed a curve smoothing operation that corresponds to the playback curve stage  204 . 
     The fourth stage  104  illustrates the start of an operation to adjust the duration of the transition intervals. The stage  104  illustrates a cursor (or another form of user selection) being placed over a handle GUI item  180  in order to adjust the transition interval  165 . The handle  180  is positioned at the right edge of transition interval  165  over the speed section  162 . In some embodiments, the proximity of the cursor to the edge of the transition interval causes the appearance of the handle  180 . The cursor drags the handle  180  to the right from the time t 2a  to the time t 2a ′. 
     The fifth stage  105  correspond to the playback curve stage  205  of  FIG. 2  as it illustrates the result of the adjustment to the transition interval  165 . The transition interval  165  has been shortened by the adjustment operation as it now spans from t 2a ′ to t 2b . The smoothing of the playback curve  200  follows this shortened transition interval is shown in the playback curve stage  205 . 
     The stage  105  also illustrates the start of another transition interval adjustment operation. As illustrated, a cursor (or another form of user selection) is being placed over a handle GUI item  182  in order to adjust the transition interval  164 . The handle  182  is positioned at the left edge of the transition interval  164  over the 100% speed section. In some embodiments, the proximity of the cursor to the edge causes the appearance of the handle  182 . The cursor drags the handle  180  to the left, from the time t 1a  to the time t 1a ′. 
     The sixth stage  106  corresponds to the playback curve stage  206  of  FIG. 2 . The stage  106  illustrates the result of the adjustment to the transition interval  164 . The transition interval  164  has been lengthened by the adjustment operation as it now spans from t 1a ′ to t 1b . The smoothing of the playback curve follows this shortened transition interval as shown in the playback curve stage  206 . 
     The six stages  201 - 206  of  FIG. 2  will now be described. At the first stage  201 , the media clip has not experienced any retiming operations. Correspondingly, the playback curve  200  is a straight line with constant slope of 1:1 to indicate that the playback speed of the media clip has not been altered and will play at 100% of the original speed from the beginning to the end. The first stage  201  also shows a range of the media clip being selected by the range selector for a retiming operation (conceptually illustrated by a box  210 ). The selected range  210  of the media clip starts at playback time t 1  and ends at playback time t 2 . 
     The second stage  202  shows the result of the retiming operation. The example retiming operation alters the playback curve  250  by slowing the playback speed of the selected range  210  to 50% of the original playback speed. The discontinuity in speed thereby divides the playback curve  200  into three sections  221  (at 100% playback speed),  222  (at 50% playback speed), and  223  (at 100% playback speed). The retiming operation also creates two keyframes  211  and  212 . The keyframe  211  is at playback time t 1  and the keyframe  212  is at playback time t 2 ′. The keyframe  211  is at the end of the 100% playback speed section  221  and at the start of the 50% playback speed section  222 . The keyframe  212  is at the end the 50% playback speed section  222  and at the start of the second 100% playback speed section  223 . 
     The third stage  203  shows the introduction of the transition intervals between different playback speeds. Specifically,  FIG. 2  at stage  203  illustrates the two transition intervals  164  and  165  being defined by the media editing application. The transition interval  164  is for the transition from 100% speed to 50% playback speed around the playback time t 1 , while the transition interval  165  is for the transition from 50% playback speed to 100% playback speed around the playback time t 2 ′. The transition interval  164  is anchored by two keyframes  241  and  242 , while the transition interval  165  is anchored by two keyframes  243  and  244 . The keyframe  241  is at playback time t 1a  and the keyframe  242  is at playback time t 1b , t 1a  being before t 1  and t 1b  being after t 1 . The keyframe  243  is at playback time t 2a  and the keyframe  244  is at playback time t 2b , t 2a  being before t 2 ′ and t 2b  being after t 2 ′. In other words, the transition interval  164  starts before the speed discontinuity at t 1  (50% to 100%, or the keyframe  211 ) and ends after t 1 . Likewise, the transition interval  165  starts before the speed discontinuity at t 2  (100% to 50%, or the keyframe  212 ) and ends after t 2 . 
     The fourth stage  204  illustrates curve smoothing by using transition intervals. The stage  204  does not illustrate the key frames  211  and  212 , because the curve smoothing operation is not based on those keyframes, which were the original keyframes that separates the different playback speeds. The curve smoothing operation is instead based on the keyframes  241 - 244 , which defines that start and ends of the transition intervals  164  and  165 . The curve smoothing operation produces smooth Bezier curves  231  and  232  that go through the keyframes  241 - 244  while maintaining continuity in playback speed. The smoothed curves  231  and  232  that fall within transition intervals  164  (between t 1a  and t 1b ) and  165  (between t 2a  and t 2b ) are illustrated as darkened. In some embodiments, only the portions of the media clip that fall within transition intervals are Bezier curves, while media clips that are outside of transition intervals remain straight (i.e., constant playback speed). A Bezier curve of a transition interval starts with a slope (i.e., playback speed) that is identical to the slope of the section before the transition interval and ends with a slope that is identical to the slope of the section after the transition interval. 
     The keyframes  241 - 244  are therefore transition interval keyframes that define the start and the end of the transition intervals. In some embodiments, the transition interval keyframes are automatically set by the media editing application. Some of these embodiments let the user of the media editing application adjusts the start and end time of the transition intervals (e.g., by moving the transition interval keyframes.) The automatic setting of the transition interval keyframes is discussed above by reference to  FIG. 1  and below by reference to 13-14. 
     The fifth stage  205  illustrates the adjustment of the transition interval  165 . As illustrated, the user has moved the start of the transition interval  165  later, from playback time t 2a  to playback time t 2a ′, which shortens the transition interval  165  to between t 2a ′ and t 2b . In some embodiments, this adjustment is accomplished by moving the keyframe  243  from the playback time t 2a  to playback time t 2a ′ along the 50% playback speed slope of the section  222 , since the move does not alter the playback speed in the section  222 . This adjustment of the transition interval  165  also causes some embodiments to regenerate the smoothed curve  232  in order to maintain the continuity in speed. 
     The sixth stage  206  illustrates the adjustment of the transition interval  164 . As illustrated, the user has moved the start of the transition interval  164  earlier, from playback time t 1a  to playback time t 1a ′, which lengthen the transition interval  164  to between t 1a ′ and t 1b  while shortening the first 100% playback speed section to between t 0  and t 1a ′. In some embodiments, this adjustment is accomplished by moving the keyframe  241  from the playback time t 1a  to playback time t 1a ′ along the 100% playback speed slope of the section  221 , since the move does not alter the playback speed in the section  221 . This adjustment of the transition interval  164  also causes some embodiments to regenerate the smoothed curve  231  in order to maintain the continuity in speed 
     For some embodiments,  FIG. 3  conceptually illustrates a process  300  for performing a preset retiming operation that is followed by a curve smoothing operation. The process  300  will be described by reference to  FIGS. 1 and 2 . Preset retiming operations will be further described below by reference to  FIG. 4  and Section I. The process  300  receives (at  310 ) a range selection (such as the range  170  in  FIG. 1 ) for the preset retiming operation on a set of media clips. 
     Next, the process  300  inserts (at  320 ) one or more keyframes in the playback curve associated with the clip or a set of clips according to the selected range. In some embodiments, such keyframes are inserted at locations that correspond to the start and end points of the selected range (such as keyframes  211  and  212  in the stage  202  of  FIG. 2 ). 
     The process  300  next receives (at  330 ) a retiming command that specifies a set of retiming parameters. In the example of  FIG. 1 , such retiming command is received after the user uses the effects menu  140  to select one of the preset retiming operations (such as “slower” and “faster”). 
     Next, the process updates (at  340 ) the playback curve of the clip or set of clips in the media clip according to the retiming command and associated parameters. In the example of  FIG. 2 , the playback curve  200  is updated according to the retiming preset commands by moving the inserted keyframes along the playback curve in order to create the speed effect specified by the retiming commands. 
     The process next performs (at  350 ) a curve-smoothing operation on the updated playback curve. The curve-smoothing operations make changes to the curve to minimize or eliminate playback speed discontinuities as described above in stage  204  of  FIG. 2 . In some embodiments, the curve smoothing operation is based on transition intervals that are introduced to encompass discontinuities between sections of the playback curve with different playback speeds (such as the transition interval  231  between the section  221  and the section  222 ). Curve smoothing operations will be described further in Section II below. After performing the curve smoothing operation, the process  300  ends. 
     Some embodiments provide user with an option to turn off/on the transition interval between segments. In some of these embodiments, the curve smoothing operation will not be performed and the GUI will not display the graphical representations of the transition intervals when the user elects to not introduce transition intervals between different speed sections. 
     Several more detailed embodiments of the invention are described below. Section I describes various preset retiming operations. Section II describes in further detail curve smoothing operations for smoothing discontinuities between sections of different playback speeds. Section III describes user adjustment of speed effects. Section IV describes the mapping of playback time using the playback curve. Section V describes the software architecture of a media editing application that performs retiming and curve smoothing. Section VI describes in detail an example media editing application. Finally, Section VII describes an electronic system with which some embodiments of the invention are implemented. 
     I. Preset Retiming Operations 
     The media editing application in some embodiments provides tools for performing retiming operations on a piece of media content, media content such as a composite presentation or a media clip. A retiming operation changes the timing characteristics (e.g., playback speed) of the portion of a piece of media content. In some embodiments, the media editing application includes a user interface for defining a range in order to select the portion of the media content. The media editing application then performs retiming by applying a preset speed effect to the portion of the media content selected by the defined range. 
     For some embodiments,  FIG. 4  illustrates an example of a media editing application that performs retiming operation on portions of a media clip.  FIG. 4  illustrates the retiming operation in six different stages  401 - 406  of a graphical user interface (GUI)  400  of the media editing application. As shown in this figure, the GUI  400  includes a media library area  410 , a preview display area  420 , a timeline  430 , an effects menu  440 , a playback activation item  422  and a retiming tool activation item  424 . In some embodiments, the GUI  400  also includes a user interaction indicator such as a cursor  490 . 
     The media library  410  is an area in the GUI  400  through which the application&#39;s user can select media clips (video or audio) to add to a media presentation that the user is compositing with the application. In the example of  FIG. 4 , the clips in the media library  410  are presented as thumbnails that can be selected and added to the timeline  430  (e.g., through a drag-and-drop operation or a menu selection operation). The clips in the media library may also be presented as a list, a set of icons, or some other visual representations that allows a user to view and select the various clips in the library. In some embodiments, the media library  410  may include audio clips, video clips, text overlays, pictures, and/or other media. The preview display area  420  provides a preview of the actual playback of the composite presentation. 
     The timeline  430  provides a visual representation of a composite presentation being created by the user. In some embodiments, a composite presentation in the timeline  430  includes one or more containers of media clips. Media clips such as video and/or audio clips can be brought into one of the containers in the timeline  430  from the media library  420  for creating the composite presentation. 
     In some embodiments, the media editing application provides preset speed effects so a user can quickly achieve an initial retiming effect on a selected range in the composite presentation. The initial retiming effect can then be used as a template for further adjustments and refinements by the user for desired result.  FIG. 4  above illustrates two such preset speed effect, namely “slower” and “faster” playback speed for a selected range. In addition to playing a portion of a media clip faster or slower, the media editing application in some embodiments also supports other speed effect or retiming operations.  FIGS. 6-8  below illustrate the operation of several such speed effects. 
     The timeline  430  includes a central compositing lane  450  that includes clips  452  (clip A) and  454  (clip B). The inclusion of clips A and B are graphically indicated by graphical representation of the clips in the central compositing lane  450 . A clip can contain a single piece of media content from a single source. A clip can also be a compound clip that includes several pieces of media content from multiple sources. A clip in the timeline is therefore referred to as a media container in some embodiments. A central compositing lane in some embodiments is the main track of the composite presentation, upon which other video clips and audio clips can be overlaid. In some embodiments, the timeline  430  has only one track and the central compositing lane  450  is that only track of the timeline. In some other embodiments, the timeline has multiple tracks and the central compositing lane  450  is one of the tracks. 
     Clip  452  includes an effect bar  460  that indicates the status of an effect being applied to the clip A. Clip  454  includes an effect bar  470  that indicates the status of an effect being applied to clip B. In the example of  FIG. 4 , the effect bars  460  and  470  indicate the playback speeds of the clips  452  and  454 . In some embodiments, clips in the central compositing lane can include one or more additional effect bars for indicating the status of other effects being applied. An effect bar can also be broken into multiple sections or portions to indicate the status of effects being applied to different sections or portions of a clip. 
     The effects menu  440  provides a menu of effects that can be selected and applied to the composite presentation. One of ordinary skill in the art would recognize that the effects menu can be implemented in the GUI  400  as a fixed panel or as a pop menu that appears only when specifically invoked by the user (e.g., by a mouse click or a selection of a particular GUI item). The effects menu  440  includes speed effects such as “slower”, “faster”, “ramp” and “hold”, which are described in more detail further below. In some embodiments, the effects menu  440  also includes other speed effects such as “hold”, “instant replay”, “rewind”, and “conform speed”, which are described in more detail further below. In addition to speed effects that affect the playback time of the composite presentation, the effects menu in some embodiments also includes one or more other visual or audio effects that do not affect the playback time of the composite presentation. In some embodiments, the effects in the effects menu  440  are “preset” effects. A preset effect is an effect that, once selected by the user, is automatically applied to a range of media contents. A user can then use the GUI to further manipulate the resultant composite presentation and adjust the applied speed effect. 
     In some embodiments, operations of the media editing application that changes the timing relationship between playback and content (such as applying a preset speed effects or adjusting a previously applied speed effects) are performed by a retiming engine of the media editing application. In some embodiments, the media editing application translates selection of preset speed effect and/or user adjustment of speed effect of a media clip into one or more retiming commands for the retiming engine, which generates, maintains, and adjusts a playback curve for the media clips according to the retiming command. Retiming engine or retiming module will be further described by reference to  FIG. 23  below. 
     The playback activation item  422  is a conceptual illustration of one or more UI items that allow the media editing application to activate its video and audio playback. The retiming tool activation item  424  is a conceptual illustration of one or more UI items that allow the media editing application to activate its retiming tool. Different embodiments of the invention implement these UI items differently. Some embodiments implement them as a selectable UI button, others as a command that can be selected in a pull-down or drop-down menu, and still others as a command that can be selected through one or more keystroke operations. Accordingly, the selection of the playback activation item  422  and retiming tool activation item  424  may be received from a cursor controller (e.g., a mouse, touchpad, trackball, etc.), from a touchscreen (e.g., a user touching a UI item on a touchscreen), or from a keyboard input (e.g., a hotkey or a key sequence), etc. Yet other embodiments allow the user to access the retiming tool feature through two or more of such UI implementations or other UI implementations. 
     The retiming operation of  FIG. 4  will be described by reference to  FIGS. 5   a - b .  FIGS. 5   a - b  illustrate a playback curve  500  at five different stages  501 - 505  of the retiming operations described in  FIG. 4 . The playback curve  500  has playback time as its x-axis and media time as its y-axis. Media time is the time native to a piece of media content. In some embodiments, an instant in the media time associated with a video frame specifies the time that the video frame is captured or intended to be displayed. The dashed line  510  marks the end of the media content and hence the end of media time. Playback time is the time that the piece of media content is actually being played back as part of the composite presentation. As such, each point in the playback curve  500  maps a particular instant in playback time with a particular instant in media time. The slope of the playback curve at any particular instant in playback time indicates the playback speed at that particular instant. A slope of 100% in a playback curve indicates that the playback time is elapsing at the same rate as the media time, or that the playback speed is the same as the normal speed of the media content.  FIGS. 5   a - b  also illustrate the keyframes  510 - 514  along the playback curve  500 . The keyframes  510 - 514  are points along the playback curve  500  that define the speed changes in the playback of the central compositing lane  450 . Each keyframe specifies a particular point in the media clip (i.e., a specific media time) that is to be played back at a particular playback time. 
     In some embodiments, the media editing application performs a curve smoothing operation after each retiming operation, because each application of speed effect changes the playback speed on a portion of the media clip and results in discontinuity in playback speed. A discontinuity in playback speed can be visually distracting to a viewer. Some embodiments thus adjust the playback curve in order to ameliorate the playback speed discontinuity. Some embodiments introduce a transition interval to allow the playback curve to smoothly transition from one playback speed to another playback speed. Hence, in addition to retiming operation stages  501 - 505 ,  FIG. 5  also illustrates two curve smoothing operation stages  503   a  and  505   a . The stage  503   a  is the curve smoothing stage for the retiming stage  503  and the stage  505   a  is the curve smoothing stage for the retiming stage  505 . The smoothing of the playback curve will also be further described below in Section II. 
     The stages  401 - 406  of  FIG. 4  will now be described by reference to stages of  FIG. 5 . The first stage  401  of  FIG. 4  shows the GUI  400  before the retiming operation. The cursor  490  is placed over the retiming activation item  424  in order to activate the retiming tool. At the stage  401 , the effect bar  460  has only one section, indicating that the playback speed of the clip A is at 100% of the normal speed. The stage  501  of  FIG. 5   a  corresponds to the stage  401  and illustrates the playback curve  500  before the retiming operation. The playback curve  500  is a straight line at a slope that corresponds to a playback speed at 100% of the normal speed. The corresponding period for the two clips in the central compositing lane  450  starts at playback time t0 and ends at playback time t1, which also maps to the start and the end of the media time according to the playback curve  500 . 
     The second stage  402  of  FIG. 4  shows the selection of a range for selecting a portion of the central compositing lane from t 2  to t 3 . In some embodiments, the user is able to graphically define the range (e.g., by clicking and dragging on the range indicator  480 ) after the selection of the retiming activation item  424 . Some embodiments also allow the user to determine the range textually by directly inputting numerical values for defining the start and end of the selection range. The selection of the range  480  corresponds to stage  502  of  FIG. 5   a . At stage  502 , keyframes  512  and  513  are inserted at t 2  and t 3  to mark the start and end time of the selection range  480  on the playback curve  500 . In some embodiments, range selection is performed by a range selector module that is part of a UI interaction module that handles user inputs for the media editing application. In this example, the range selector module receives user definition of the range (i.e., from the retiming activation item  424  and from the click and drag operation) and translates this range definition into commands for the retiming engine. 
     The third stage  403  of  FIG. 4  shows the selection of a preset effect to be applied to the portion of the central compositing lane  450  within the selected range  480 . Specifically, the cursor  490  is used to select from the effects menu  440  a “slower” speed effect that slows the playback speed down to 50% of the normal playback speed. The sub-menu of the effects menu  440  displays playback speed options 75%, 66.7%, 50% and 33.3%. One of ordinary skill in the art would recognize that the “slower” speed effect preset can have other playback speed options. For example, the “slower” speed effect preset includes playback speed options 50%, 25% and 10% in some embodiments. 
     The fourth stage  404  of  FIG. 4  shows the result of the application of the “slower” speed effect. The effect bar  460  of clip A is now partitioned into four sections  461 ,  462 ,  463  and  467 , each section of the effect bar specifies a playback speed for a portion of the clip  452  that falls under the section. The first section  461  specifies that the portion of the clip  452  from t 0  to t 2  is to be played back at 100% of the normal speed. The second section  462  specifies that the portion of the clip  452  from t 2  to t′ 3  is to be played back at 50% of the normal speed. The third section  463  specifies that the portion of the central compositing lane from t′ 3  to f 1  is to be played back at 100% of the normal speed. The fourth section remains at 100% playback speed. 
     The second section  462  corresponds to the selected range  480  after the application of the “slower” preset speed effect. The duration of second section  462  (t′ 3 −t 2 ) is longer than the duration of the selected range  480  (t 3 −t 2 ) because the portion of the second section  462  is being played back at 50% of the normal speed (thus the duration of the section is twice as long as before the speed effect). The section  462  of the effects bar is also marked with a visual indication (i.e., diagonal hash pattern) to indicate that this section is to be played back at a speed slower than normal. In some embodiments, each section of the effects bar is associated with a visual indication of the effect being applied. For example, some embodiments color code each section of the effect bar according to the speed of the playback (e.g., green for normal speed, orange for speed slower than normal, blue for speed faster than normal, and red for stoppage or pause during playback.) Some of these embodiments use different color intensity levels to indicate different levels of speed. Some embodiments use different patterns and or different texts on the effect bar to provide visual indications of effects being applied. 
     The application of the “slower” speed effect preset is illustrated by stage  503  of  FIG. 5   a . At stage  503 , the “slower” speed effect has caused the keyframes  513  and  511  to both shifted to the right (from t 3  and t 1  to t′ 3  and t′ 1 ) because of the longer duration of the second section  462 . The longer duration of the second section  462  also results in a slower playback speed (50%) between t 2  and t′ 3 , as the same duration of media time is being stretched out for longer playback time. The speed effect also created one speed discontinuity t 2  that corresponds to the keyframe  512  and another speed discontinuity t′ 3  that corresponds to the keyframe  513 . A curve smoothing operation is then performed at these two discontinuities after the stage  503 . The stage  503   a  illustrates this curve smoothing operation: a smoothed curve based on a transition interval  522  is introduced around time t 2  (i.e., the discontinuity at keyframe  512 ) and another smoothed curve based on a transition interval  523  is introduced around time t′ 3  (i.e., the discontinuity at the keyframe  513 ). 
     The fourth stage  404  of  FIG. 4  also illustrates the selection of a second range  485  by the cursor  490 . The second range  485  starts at playback time t 4  and ends at playback time t 5 . The selection of the range  485  corresponds to the stage  504  of  FIG. 5   b , in which additional keyframes  514  and  515  are defined along the playback curve at the playback time t 4  and t 5 . 
     The fifth stage  405  of  FIG. 4  shows the selection of another effect preset from the effects menu  440 . Specifically, the “faster” speed effect is selected to accelerate the playback speed within the range  485  to 200% of the normal speed. The sub-menu of the effects menu  440  displays playback speed options 125%, 150%, 200% and 300%. One of ordinary skill in the art would recognize that the “faster” speed effect preset can have other playback speed options. For example, the “faster” speed effect preset includes playback speed options 200%, 400%, 800% and 2000% in some embodiments. 
     The last stage  406  of  FIG. 4  illustrates the result of the “faster” speed effect on the composite presentation and the central compositing lane  450 . The effects bar  460  is now partitioned into six sections  461 ,  463 ,  464 ,  465 ,  466  and  467 . The section  461  remains unchanged because it falls outside of the selected range  485 . The section  463  also falls outside the selected range, but has shifted to the left (from t′ 3  to t″ 3 ). What was once section  462  with playback speed 50% is partitioned into sections  464 - 466  by the retiming operation performed on the selected range  485 . The section  465  corresponds to the selected range  485 , and its duration has been shrunk because its playback speed has been accelerated to 200% of normal speed. The 50% speed section before the selected range  485  becomes the newly created section  464 , which starts at t 2  and ends at t 4 . The 50% speed section after the selected range  485  becomes the newly created section  466 , which starts at t′ 5  and ends at t″ 3 . 
     The application of the “faster” speed effect corresponds to the stage  505  of  FIG. 5 . At the stage  505 , keyframes  515 ,  513  and  511  have all shifted to the left (from t 5 , t′ 3  and t′ 1  to t′ 5 , t″ 3  and CO, corresponding to the shorter duration of the section  465 . The shorter duration of the section  465  also corresponds to a faster playback speed (200%) between t 4  and t′ 5 , as the same duration of media time is being compressed for shorter playback time. The “faster” speed effect has created two additional speed discontinuities: one at t 4  between 50% and 200% playback speeds anchored by the keyframe  514 , and another at t′ 5  between 200% and 50% playback speeds anchored by the keyframe  515 . Some embodiments then perform a curve smoothing operation for all discontinuities in the playback curve. 
     The stage  505   a  of  FIG. 5  shows the curve smooth operation on all of the discontinuities after the stage  505 . At this stage, the playback curve  500  is anchored by keyframes  512 ,  514 ,  515 ,  513 ,  511 , which represents discontinuities at t 2 , t 4 , t′ 5 , t″ 3 , and ending time t″ 1 , respectively. The curve smooth operation introduces sections of smoothed curves based on the transition intervals around the discontinuities at t 2 , t 4 , t′ 5 , and t″ 3 . Specifically the smoothed curve  522  is for the discontinuity at t 2  (the keyframe  512 ), the smoothed curve, the smoothed curve  524  is for the discontinuity at t 4  (the keyframe  514 ), the smoothed curve  525  is for the discontinuity at t′ 5  (the keyframe  515 ), and the smoothed curve  523  is for the discontinuity at t″ 3  (the keyframe  513 ). 
     In some embodiments, media clips do not necessarily start at time 0 (e.g., a media clip can start at 1 minute after time 0). In these instances, the retiming operations that change playback speeds of specific sections (such as the “faster” and “slower” speed effect presets discussed above) perform the retiming operation by pivoting on time 0 rather than the actual start time of the media clip. For example, if a media clip starts at 5 s and ends at 15 s, a retiming operation that slows the entire media clip to 50% playback speed would change the start time of the media clip to 10 s and the end time to 30 s (instead of leaving the start time at 5 s and changing the end time to 25 s.) 
     In addition to “slower” and “faster” preset retiming operations, some embodiments includes other preset retiming operations such as “ramp”, “conform speed”, “hold”, “instant replay”, and “rewind”. Each of these preset retiming operations changes the playback speed on a selected range or section of a media clip. Since an operation that changes the playback speed of a section of a media clip almost always create one or more discontinuities in playback speeds, some embodiments perform curve smoothing operation by introducing transition intervals on some or all of the created discontinuities. 
     These various other retiming presets will now be described. A “ramp” operation is a retiming operation that automatically divides a selected range of a clip or a set of clips in a media clip of a timeline into multiple sections of increasing or decreasing playback speed. For some embodiments,  FIG. 6  illustrates an example ramp retiming operation in a timeline  600  that is similar to the timeline  430  of the GUI  400  in  FIG. 4 . The timeline  600  includes a media clip  610  (clip A) that has an effects bar  620 . 
       FIG. 6  illustrates the ramp retiming operation in four stages  601 - 604 . The first stage  601  of  FIG. 6  shows the selection of a range of media content. A cursor  690  is used to graphically select a range  630  of media content in the media clip  610  from playback time t0 to playback time t1. 
     The second stage  602  illustrates the selection of a ramp retiming operation from an effects menu  640 . Specifically, the selection specifies that the ramp operation gradually decreases the speed of the selected range  630  toward 0% of normal playback speed. The effects menu  640  also includes other options for the ramp retiming operation. For example, the user can select to gradually increase playback speed toward 200% of normal playback speed. 
     The third stage  603  illustrates the result of the ramp retiming operation. The effects bar  620  and the media clip  610  have been partitioned into seven different sections  621 - 627 . Sections  621  and  627  correspond to portions of the media clip  610  that falls outside of the range  630  and thus remain at 100% of normal playback speed. Section  627  starts at a playback time t′1 instead of t1 because the selected ramp retiming operation slows down playback speed and increases playback time. Sections  622 - 626  are assigned playback speed at 87%, 62%, 38%, 13% and 0% respectively. To complete the speed ramp toward 0%, some embodiments include the 0% playback speed section  626 . In some of these embodiments, the 0% playback speed portion of the speed ramp is shorter than other sections ( 622 - 625 ) in the ramp. 
     The fourth stage  604  shows the introduction of transition intervals on discontinuities created by the ramp retiming operation. The stage  604  shows the graphical representations of the transition intervals over the effects bar  620 . Specifically, the transition interval  631  is introduced around the discontinuity between 100% and 87% playback speeds, the transition interval  632  is introduced around the discontinuity between 87% and 62% playback speeds, the transition interval  633  is introduced around the discontinuity between 62% and 38% playback speeds, the transition interval  634  is introduced around the discontinuity between 38% and 13% playback speeds, and the transition interval  635  is introduced around the discontinuity between 13% and 0% playback speeds. In this example, the transition intervals are illustrated as being transparently superimposed over the effects bar. However, one of ordinary skill would realize that are many other possible ways of graphically illustrating transition intervals in the GUI of a media editing application. 
     One of ordinary skill in the art would also recognize that many different possible implementations of the ramp retiming operation is possible than what is illustrated in  FIG. 6 . For example, instead of having five speed ramp sections, the media editing application can provides six or more speed ramp sections. 
       FIG. 7  illustrates a playback curve  700  that is maintained by the media editing application while performing the ramp retiming operation of  FIG. 6 .  FIG. 7  also illustrates a smoothed playback curve  750  that is the result of a curve smoothing operation being performed on the playback curve  700 . 
     As illustrated, the playback curve  700  is partitioned into sections of different playback speed (i.e., different slopes) corresponding to the sections of the effects bar  620 . Keyframes  711 - 716  are inserted along the playback curve  700  corresponding to the speed transitions. The keyframes  711 - 715  are equally spaced in playback time such that the different sections of the speed ramp have nearly equal durations. The keyframe  716  is inserted for one short duration of playback time before t′1 during which the playback speed is 0%. 
     The smoothed playback curve  750  includes sections of smoothed curves  721 - 725  in place of speed discontinuities. Specifically, the discontinuities anchored by the keyframes  711 - 715  are replaced by the smooth curve sections  721 - 725 . The sections of the smoothed curves  721 - 725  are created based on the transition intervals  631 - 635 , respectively. With these sections of the smoothed curves, the playback speed of the clip changes smoothly and gradually without any discontinuity from 100% to 0%. 
     In some embodiments, the video playback of a composite presentation being composited by the media editing application is conducted at a particular frame rate. However, the source media content (e.g., source video clips in the media library) that is used to construct the composite presentation may not have the same frame rate. In such cases, some embodiments construct interpolated frames in order to convert frames from a native frame rate to the particular frame rate of the composite presentation. The interpolation of video frames will be discussed further below in Section IV. 
     In some embodiments, the media editing application provides a retiming speed effect preset that plays every frame of a video clip at a rate that conforms with the particular frame rate of the media editing application. For example, a media editing application in some embodiments plays at a standard frame rate of 24 frames per second, while a piece of high resolution media content produced by a high speed camera may have 60 frames per second. Playing such high resolution piece of media at 100% normal speed requires down sampling of the frames (e.g., playing back only two frames for every five available.) Some embodiments provide a “conform speed” preset that plays every frame of the piece of high resolution media within a selected range at the standard 24 frames per second. The result is a section that plays every frame of the high resolution media content, albeit at a slower rate of 40% of normal speed (i.e., 24/60). 
     One of ordinary skill in the art would recognize that the “conform speed” retiming operation is applicable regardless of the source frame rates and the playback frame rates. For some embodiments,  FIG. 8  illustrates an example “conform speed” retiming operation performed on a piece of media content that has a source frame rate that is three times the playback frame rate of the composite presentation. 
       FIG. 8  illustrates a timeline  800  in a GUI of a media editing application that is similar to the GUI  400 . Within the timeline  800  is a media clip  850 . The media clip displays a series of thumbnail images sampled at regular intervals of the playback time from the media clip  850 . Each thumbnail image is labeled with an index that reflects the position of the thumbnail image in media time for purpose of illustration. The media clip  850  also displays an effects bar  860  that indicates the playback speed of the media content in the container. 
       FIG. 8  illustrates the example “conform speed” retiming operation in four stages  801 - 804 . At the first stage  801 , the effects bar  860  of the content media clip  850  indicates that the entire media clip is at 100% of normal speed. The thumbnails  851 - 858  are indexed at increments of 3 (0, 3, 6, 9 . . . ) at the current zoom level, which correspond to playing back the media content at the standard frame rate. The first stage  801  also illustrates the selection of a range  870  that starts at playback time t0 and ends at playback time t 1 . In some embodiments, the range selection function is activated after the user has chosen to activate a retiming tool such as by selecting the retiming activation item  424  of the GUI  400 . 
     The second stage  802  shows the selection of the “conform speed” retiming operation preset from an effects menu  840 . The “conform speed” retiming operation will be applied to the media content in the selected range  870  between the playback times t0 and t 1 . 
     The third stage  803  illustrates the result of the retiming operation. A new section  862  has appeared in the effects bar  860  that correspond to the selected range  870 . The new section  862  ends at t′ 1  instead of t 1  because its duration is three times as long as the selected range  870 . This section corresponds to a portion of media content that is to be played back at 33.3% normal speed because every frame of the source media content is being played back. Since the source frame rate of the media content is three times the playback frame rate, the playback speed is effectively reduced to 33.3% of normal. The thumbnail images under the section  862  reflect the reduced playback speed, as they are thumbnails indexed at increments of 1 (6, 7, 8, 9, 10, 11 . . . ). 
     By changing the playback speed of section  862  from 100% to 33%, the “conform speed” retiming operation preset has created two discontinuities at time t 0  and at time t′ 1 . The fourth stage  804  shows the introduction of transition intervals on the discontinuities created by the “conform speed” operation. The stage  804  shows the graphical representations of the transition intervals over the effects bar  860 . Specifically, the transition interval  864  is introduced around the discontinuity at t 0  between 100% and 33% playback speeds, and the transition interval  866  is introduced around the discontinuity at t′ 1  between 33% and 100% playback speeds. In this example, the transition intervals are illustrated as being transparently superimposed over the effects bar  860 . However, one of ordinary skill would realize that are many other possible ways of graphically illustrating transition intervals in the GUI of a media editing application. 
       FIG. 9  illustrates a playback curve  900  that corresponds to the “conform speed” retiming operation of  FIG. 8 .  FIG. 9  also illustrates a smoothed playback curve  950  that is the result of a curve smoothing operation being performed on the playback curve  900 . 
     The playback curve  900  has two keyframes  911  at t 0  and  912  at t′ 1 . The section of the playback curve  900  before the first keyframe  911  has a playback speed of 100% normal, meaning that the playback time is elapsing at the same rate as media time. During this section of the curve, only one out of every three frames from the media content is being played back. The section of the curve  900  after t 0  and before t′ 1  corresponds to the section with the “conform speed” retiming speed effect. During this section of the playback curve  900 , every frame of the media content is played, but the media time is elapsing at only ⅓ of previous rate, showing a 33% playback speed. 
     The smoothed playback curve  950  includes sections of smoothed curves  921  and  922  in place of speed discontinuities. Specifically, the discontinuities anchored by the keyframes  911  and  912  are replaced by the smooth curves  921  and  922 . The sections of the smoothed curves  921  and  922  are created based on the transition intervals  864  and  866 , respectively. With these sections of the smoothed curves, the playback speed of the clip changes smoothly and gradually without any discontinuity from 100% to 33% and from 33% back to 100%. 
     Some embodiments of the media editing application include other retiming operation presets. For example, some embodiments of the media editing application include “hold” operation that cause playback to pause or hold at a particular video frame. This preset retiming operations creates a 0% playback speed section in the playback curve and results in speed discontinuities between the 0% section and other sections of the playback curve. 
     In some embodiments, the media editing application includes preset operations that repeat a selected section of a media clip or a composite presentation. An “instant replay” preset retiming operation repeats the media content in the selected range in a forward direction. A “rewind” preset retiming operation repeats the media content in the selected range in a reverse direction. Some of these preset retiming operations repeat the selected sections in the playback curve at either the same playback speed or reverse playback speed. The playback curve after these repeat retiming operations would have speed discontinuities at the start and at the end of the repeated section. The description of “instant replay”, “rewind”, and “hold” retiming operation presets can be found in U.S. Patent Application Publication No. 20120210228. U.S. Patent Application Publication No. 20120210228 is hereby incorporated by reference. 
     In some embodiments, the media editing application will not introduce transitional intervals and will not perform curve smoothing for some of the discontinuities. Specifically, some embodiments do not perform curve smoothing operations on discontinuities created by rewind and instant replay operations. Some other embodiments apply curve smoothing on all speed discontinuities unless the media content is discontinuous. For example, some discontinuities created by instant replay operations are also discontinuous in content; some embodiments therefore would not apply curve smoothing on these discontinuities. On the other hand, some discontinuities created by hold or rewind operations are continuous in content; some embodiments therefore apply curve smoothing on these discontinuities even though these speed discontinuities are between positive playback speed and negative (or zero) playback speeds. 
     II. Transitions Between Different Playback Speeds 
     As mentioned earlier by reference to stage  506  of  FIG. 5 , some embodiments perform curve smoothing operation on the playback curve in order to prevent abrupt changes in playback speed. Any of the speed effect operations described above in Section I can create one or more speed discontinuities in the playback curve. For example, a “ramp” retiming operation as described above by reference to  FIG. 6  creates several different sections  621 - 627  with different playback speeds. The intersection joining two of these different sections with different playback speeds necessarily creates a speed discontinuity. The curve smoothing operation adjusts the playback curve near the playback speed discontinuities such that the slope or tangent (i.e., playback speed) of the smoothed playback curve is continuous throughout the curve. In some embodiments, the curve smoothing operation is entirely handled by the media editing application and is not visible to a user nor is it accessible by the user. 
     Some embodiments introduce transition intervals between different playback speeds. Such transition intervals has a specified start time and a specified end time in order to further constrain the smoothed playback curve. In some embodiments, the smoothed playback curve is constrained to curve (i.e., change speed) only within the transition intervals. This allows the smooth curve to immediately settle into a straight line remain a straight line before the start of the transition interval and after the end of the transition interval. Such a smoothed curve goes through keyframes that anchor the start and the end of the transition intervals, but would not necessarily go through the discontinuity keyframes that were created by the retiming operations (e.g., the keyframes  211  or  212  that marks the speed discontinuities between the sections  221 ,  222 , and  223  in  FIG. 2 ). 
       FIG. 10  illustrates a curve smoothing operation that uses transition intervals between different playback speeds in some embodiments. The smoothed curve created by this curve smoothing operation goes through keyframes that anchor the start and the end of the transition intervals but not the discontinuity keyframes that were created by retiming operations to mark the start or end of a particular playback speed.  FIG. 10  illustrates the curve smooth operation in four stages  1001 - 1004  for a playback curve  1000 . 
     At the first stage  1001 , the media clip associated with the playback curve  1000  has not experienced any retiming operations. A section of the media clip has been selected by a range selector for a retiming operation (conceptually illustrated by a box  1010 ). The selected range  1010  (or section) of the media clip starts at playback time t 1  and ends at playback time t 2 . 
     The second stage  1002  shows the creation of a new playback curve  1052  that includes sections  1021 - 1023 . The retiming operation creates the two keyframes  1011  and  1012 . The keyframe  1011  anchors at t 1  the end of the 100% playback speed section  1021  and the start of the 50% playback section  1022 . The keyframe  1012  anchors at t′ 2  the end of the 50% playback speed section  1022  and the start of the 100% playback section  1023 . 
     The third stage  1003  shows the introduction of transition intervals  1041 - 1042  and the keyframes that anchors the transition intervals. The transition interval  1041  is around t 1  and serves to transition from 100% playback speed to 50% playback speed. The transition interval  1042  is around t 2 ′ and serves to transition from 50% playback speed to 100% playback speed. In addition, keyframes  1031 - 1032  are added to anchor the start and end of the transition interval  1041 , and keyframes  1033 - 1034  are added to anchor the start and end of the transition interval  1042 . The keyframe  1031  falls on the playback curve  1052  at t 1a  before the discontinuity keyframe  1011  (at t 1 ). The keyframe  1032  falls on the playback curve  1052  at t 1b  after the discontinuity keyframe  1011 . The keyframe  1033  falls on the playback curve  1052  at t ea  before the discontinuity keyframe  1012  (at t 2 ′). The keyframe  1034  falls on the playback curve  1052  at t 2b  after the discontinuity keyframe  1012 . 
     The fourth stage  1004  shows the result of a curve smoothing operation that is based on the transition intervals. The curve smoothing operation produced a new playback curve  1054  that goes through the keyframes  1031 - 1034  but not the keyframes  1011  and  1012 . In other words, the new playback curve is based only on the keyframes that anchors the transition intervals rather than the keyframes that were created by retiming operations to delineate between different playback speeds. Playback speed changes without discontinuity within the transition intervals  1041  and  1042 . Specifically, the smoothed curve within the transition interval  1041  start with the playback speed of the section  1021  (100%), gradually and continuously changes toward the playback speed of the section  1022  (50%), and the smoothed curve within the transition interval  1042  start with the playback speed of the section  1022  (50%), gradually and continuously changes toward the playback speed of the section  1023  (100%). Playback speed settles quickly into constant speed outside of transition intervals without any discontinuity in playback speed. In some embodiments, the sections of the playback curve  1054  that lies outside of the transition intervals remain at constant speed. Thus, the playback speed is at constant 100% before t 1a  and after t 2b . The playback speed is at constant 50% between t 1b  and t 2a . 
       FIG. 11  illustrates a playback speed plot  1100  that is based on the smoothed playback curve  1054  of  FIG. 10 , which is constrained by specified start and end times of the transition intervals. As illustrated, the plot  1100  includes a dashed line  1112  and a solid curve  1115 . The dashed line  1112  starts at 100% playback speed, transitions to 50% playback speed at t 1 , and then transitions back to 100% playback speed at t 2 ′. Instead of specifying only that the 100% playback speed shall switch over to 50% playback speed at t 1  and that 50% playback speed shall switch over to 100% playback speed at t 2 ′, some embodiments use transition interval  1121  to specify that the transition from 100% to 50% is starts at t 1a  and ends at t 1b , and use transition interval  1122  to specify that the transition from 50% back to 100% is starts at t 2a  and ends at t 2b . 
     The solid curve  1115  corresponds to playback speed that is based on a smoothed curve that is constrained by the start and end times of the transition intervals. As shown in the figure, the solid curve  1115  coincides with the dashed line  1112  in regions outside of the transition intervals. The solid curve  1115  starts its transition to 50% playback speed at the start of the first transition interval at t 1a  and complete the transition at the end of the first transition interval at t 1b . The solid curve  1115  starts its transition to 100% playback speed at the start of the second transition interval at t 2a  and complete the transition at the end of the second transition interval at t 2b . 
     The retiming operation illustrated in  FIGS. 10-11  is a retiming operation that slows the playback speed of a portion of the media clip (from 100% to 50% in this instance). In some embodiments, the curve smooth operation operates equally well for any speed transitions, such as speed transitions to/from 0% playback speed (e.g., “hold” retiming operation) or even negative speed (e.g., “rewind retiming operation”). 
     In some embodiments, a smoothed curve within a transition interval is constrained to gradually transition from the first playback speed at the start of the transition interval to the second playback speed at the end of the transition interval, the first speed being the playback speed of the media clip before the speed discontinuity and the second speed being the playback speed of the media clip after the speed discontinuity. In some embodiments, the smoothed curve is a Bezier curve that fulfills these constraints. In some embodiments, the smoothed Bezier curve is a Quadratic Bezier Curve that is produced according to the following equation:
 
 B ( s )=(1 −s ) 2   P   0 +2(1 −s ) sP   1   +s   2   P   2   ,sε[ 0,1]  (1)
 
     where B(s) is the parametric curve based on the parameter s that varies from 0 to 1, P 0  is the start of the transition interval, P 1  is the discontinuity, P 2  is the end of the transition interval.  FIG. 12  illustrates the creation of a smoothed curve that is based on a Bezier curve.  FIG. 12  shows the creation of the Bezier curve in two stages  1201  and  1202 . 
     The first stage  1201  shows a portion of a playback curve  1252 , which has a discontinuity at playback time t 1  between playback speed 100% and playback speed 50% created by an earlier retiming operation. The discontinuity is anchored by a discontinuity keyframe  1211 . The first stage  1201  also shows the introduction of a transition interval  1241 , which is anchored at its start by a keyframe  1231  at the playback time t 1a  and at its end by a keyframe  1232  at the playback time t 1b . Each of the keyframes  1211 ,  1231 , and  1232  specifies a specific point in time in the media content (i.e., media time) that is to be played back at a particular playback time. For the purpose of creating a quadratic Bezier Curve, the start keyframe  1231 , the discontinuity keyframe  1211 , and the end keyframe  1232  correspond to P 0 , P 1 , and P 2  of the equation (1), respectively. 
     The second stage  1202  shows the computed Bezier curve for the transition interval  1241 . The Bezier curve B(s) starts at P 0  (i.e., the start keyframe  1231 ) with the parameter s=0, and ends at P 2  (i.e., the end keyframe  1232 ) with the parameter s=1. The Bezier curve does not go through the discontinuity keyframe  1211 , but it fulfills the constraints by gradually transitioning from the playback speed 100% at the start of the transition interval  1241  to the playback speed 50% at the end of the transition interval  1241 . The smoothed playback curve  1254  is continuous. Its derivative (playback speed) is also continuous. 
     To specify a smoothed playback curve, some embodiments store the start and the end of the transitional intervals along with keyframes that defines the start and the end of the different speed sections of the media clip. The actual smoothed curve is not stored in some embodiments. To playback a media clip according to a smoothed playback curve, some embodiments use the stored information of the transitional intervals (i.e., their start and end) to construct the actual smoothed playback curve for media clip playback. In some other embodiment, the data structure of the playback curve stores the actual smoothed curve in addition to the keyframes and the transition intervals such the playback engine can retrieve the actual smoothed playback curve without having to create the smoothed curve. 
       FIG. 13  conceptually illustrates a process  1300  for specifying a smoothed playback curve following a retiming operation by using transition intervals. In some embodiments, this process is performed by the media editing application as part of the operation  350  in the process  300  as discussed above by reference to  FIG. 3 . 
     The process  1300  starts when the media editing application has just performed a retiming operation on a media clip. The process receives (at  1310 ) the playback curve that controls the timing of the playback of the media clip. The retiming operation has altered the playback curve of the media clip based on the type of retiming operation that was performed and also based on the range of media clip that was selected for the retiming operation. As the retiming operation changes the playback speed in selected sections of the media clip, speed discontinuities will occur between sections designated to be played at different speeds. In addition to the playback curve itself, some embodiments also receive keyframes that anchor the start and the end of the retimed section(s) of the media clip. Some of these keyframes are created or defined by the retiming operation and the range selection. Some of these keyframes are discontinuity keyframes because they mark the speed discontinuities and serve as border between different sections of the media clip. 
     Next, the process identifies (at  1320 ) a speed discontinuity in the playback curve. In some embodiments, the identification of the speed discontinuities is based on discontinuity keyframes stored in the data structure of the playback curve. Instead or in addition to relying on keyframes to identify speed discontinuities in the playback curve, some embodiments process the playback curve to identify locations where discontinuities are (e.g., by taking a first derivative of the playback curve). 
     Next, the process sets (at  1330 ) the start of the transition interval for the identified discontinuity. The process then sets (at  1340 ) the end of the transition interval for the identified discontinuity. In some embodiments, the media editing application sets the start and end of the transition interval based on a predefined fraction of the section before the discontinuity and of the section after the discontinuity. In some other embodiments, the media editing application sets the start of the transition interval based on a fixed time interval. Defining the start and end of transition will be further described below by reference to  FIG. 14 . 
     At  1370 , the process determines if there are more discontinuities in the received playback curve. If yes, the process returns to  1320  to identify another speed discontinuity in the playback curve. Otherwise, the process proceeds to  1380 ; 
     At  1380 , the process stores the transition intervals. In some embodiments, the transition intervals are stored as part of the data structure of the playback curve. In some of these embodiments, the data structure of the playback curve stores the start playback time and the end playback time of the transition intervals. Data structures of playback curves will be further described below by reference to  FIG. 24 . After storing the transition intervals, the process  1300  ends. Having specified the transition intervals for curve smoothing, the user is free to change the start and the end of the transition intervals as discussed above by reference to  FIGS. 1 and 2 . 
     As mentioned, immediately after a retiming operation, some embodiments automatically set a start and an end for each of the transition intervals. The automatically set start and end of the transition intervals can then be adjusted by the user as discussed above by reference to  FIG. 1 .  FIGS. 14-15  illustrate the automatic setting of the start and the end of transition intervals for some embodiments. 
       FIG. 14  illustrates an example media clip for which the start and the end of its transition intervals are initially defined according to a predefined fraction. The figure illustrates the initial definition of the transition interval in two stages  1401  and  1402 . The example media clip is represented by its corresponding effects bar  1400 , which includes indications of the playback speed at different playback times. At the first stage  1401 , the effects bar  1400  is partitioned into three sections (by a retiming operation) into three speed sections  1410 ,  1420 , and  1430  (labeled section A, B, and C respectively) with speed discontinuity at t 1  and t 2 . 
     At the second stage  1402 , the effects bar  1400  shows the introduction of transition intervals  1415  and  1425 . The transition interval  1415  is for transitioning from the playback speed of section A to the playback speed of section B around the playback time t 1 , while the transition interval  1425  is for transitioning from the playback speed of section B to the playback speed of section C around playback time t 2 . 
     The transition interval  1415  is defined to start one quarter of the duration of the original section A ( 1410 ) before t 1 , and to end of one quarter of the duration of the original section B ( 1420 ) after t 1 . Likewise, the transition interval  1425  is defined to start one quarter of the duration of the original section B ( 1420 ) before t 2 , and to end of one quarter of the duration of the original section C ( 1430 ) after t 2 . In other words, the last quarter of section A and the first quarter of section B join together to become the transition interval  1415 , while the last quarter of section B and the first quarter of section C join together to become the transition interval  1425 . This leaves sections A, B, and C shorter than before, but the duration of the media clip when played back would not change. 
       FIG. 15  illustrates an example media clip for which the start and the end of the transition intervals are initially defined based on fixed time intervals. The figure illustrates the initial definition of transition intervals in two stages  1501  and  1502  for the same media clip that is represented by the effects bar  1400 . The first stage  1501  is the same as the stage  1401  of  FIG. 14 , in which the effects bar  1400  is partitioned into three sections (by a retiming operation) into three speed sections  1410 ,  1420 , and  1430  (labeled section A, B, and C respectively) with speed discontinuity at t 1  and t 2 . 
     At the second stage  1502 , the effects bar  1400  shows the introduction of transition intervals  1515  and  1525 . The transition interval  1515  is for transitioning from the playback speed of section A to the playback speed of section B around the playback time t 1 , while the transition interval  1525  is for transitioning from the playback speed of section B to the playback speed of section C around playback time t 2 . 
     The transition interval  1515  is defined to start 0.5 second before t 1  and end 0.5 second after t 1 . Likewise, the transition interval  1525  is defined to start 0.5 second before t 2  and end 0.5 second after t 2 . In other words, the last 0.5 second of section A and the first 0.5 second of section B join together to become the transition interval  1515 , while the last 0.5 second of section B and the first 0.5 second of section C join together to become the transition interval  1525 . This leaves sections A, B, and C shorter than before, but the duration of the media clip when played back would not change. 
     In some embodiments, the media editing application defines the initial start and the end (hence the initial duration) of transition intervals differently. For example, some embodiments define the duration of a transition interval based on a fraction of the entire duration of the media clip rather than of a particular speed section. 
     Once a transition interval has been initially defined, its duration (i.e., its start and end) can be adjusted by the user as discussed above by reference to  FIGS. 1 and 2 . In some embodiments, a transition interval is just like any other speed effect sections that are created by retiming operations and can be likewise manipulated by users. Section III below describes the user manipulation of speed effect sections. 
     For some embodiments,  FIG. 16  conceptually illustrates a process  1600  that uses the stored information of the transitional intervals (i.e., their start and end) to construct the actual smoothed playback curve for media clip playback. The process  1600  is performed by a playback engine of the media editing application in some embodiments. The process  1600  starts when the media editing application has received a user command to playback a particular a media clip associated with a particular playback curve. 
     The process receives (at  1610 ) the playback curve that controls the timing of the playback of the media clip. In some embodiments, the playback curve is in a data structure that includes keyframes that anchor the start or the end of retimed section(s) of the media clip. In addition, the retrieved playback curve also includes specification of the transitional intervals. 
     Next, the process identifies (at  1620 ) a discontinuity in the playback curve, and identifies (at  1630 ) the start and end of the transition interval that is associated with the discontinuity. In some embodiments, the data structure of the playback curve maintains a list of all the discontinuity keyframes, and each transition interval is associated with a discontinuity keyframe. Some embodiments identify the discontinuities and the transition intervals by tracing through the list of keyframes in the data structure. 
     The process then determines (at  1640 ) the playback speed before the discontinuity. In some embodiments, the speed before the discontinuity is determined by calculating the slope of the playback curve before the discontinuity. The process likewise determines (at  1650 ) the playback speed after the discontinuity. In some embodiments, the speed after the discontinuity is determined by calculating the slope of the playback curve after the discontinuity. 
     Based on the transition interval associated with the discontinuity as well as the playback speed before and after the discontinuity, the process creates (at  1660 ) a smoothed curve. The start and the end of the transition interval serve as anchoring keyframes that constrains the smoothed curve. The smoothed curve is further constrained by the playback speeds before and after the discontinuity such that the slope of the smoothed curve transition gradually and continuously from the playback speed before the discontinuity to the playback speed after the discontinuity. The creation of smoothed curve based on Bezier curves is described above by reference to  FIG. 12 . 
     At  1670 , the process determines if there more discontinuities in the received playback curve. If yes, the process returns to  1620  to identify another speed discontinuity in the playback curve. Otherwise, the process proceeds to  1680  to playback the media clip based on the timing specified by the smoothed playback curve. After the playback, the process  1600  ends. 
     III. User Adjustment of Speed Effects 
     As mentioned above, the media editing application of some embodiments provides preset speed effects so a user can quickly achieve an initial retiming effect on a selected range in the composite presentation or media clip. These retiming effects create different sections in the playback curve with different playback speeds. In addition, some embodiments introduce transition intervals between the different speed effects sections created by the retiming effects. These initially created speed effect sections can then be used as a template for further adjustments and refinements by the user for desired result.  FIGS. 1-2  above illustrate examples of such further adjustments and refinements of speed effects. In some embodiments, the adjustment of the speed effect is accomplished by adjusting individual speed effect sections that have been created by the earlier preset speed effects. In some of these embodiments, effects bars associated with each speed effect section have graphical handles that can be manipulated by the user to adjust the speed effect. After each speed effect adjustment, some embodiments examine each transition interval. Based on such examination of the transition intervals, some embodiments eliminate one or more transition intervals, adjust one or more of the transition intervals, and/or perform new curve smoothing operation based on the remaining adjusted transition intervals. 
       FIGS. 17   a - b  illustrate an example speed effect adjustment operation of a media clip that has been partitioned by an earlier preset retiming operation. The example speed effect adjustment operation results in elimination of a discontinuity and a transition interval. 
       FIG. 17   a  illustrates a timeline  1700  of a GUI similar to the GUI  400  of a media editing application. Within the timeline  1700  is a representation of a media clip (or a media container)  1750 . The representation of the media clip displays a series of thumbnail images sampled at regular intervals of the playback time from the images in the media clip  1750 . The representation of the media clip  1750  also displays an effects bar  1760  that indicates the playback speed of the media content in the container. The effects bar is partitioned into three sections  1761 - 1763  by one or more previous preset speed effect operations such as the “slower” operation discussed earlier by reference to  FIGS. 4-5 . Each section of the effect bar  1760  displays the playback speed of that section. Each section also includes an adjustment handle. Specifically, the section  1761  has an adjustment handle  1771 , the section  1762  has an adjustment handle  1772 , and the section  1763  has an adjustment handle  1773 . 
       FIG. 17   a  illustrates the example speed effect adjustment operation by user manipulation in four stages  1701 - 1704  of the timeline  1700 .  FIG. 17   a  will be described by reference to  FIG. 17   b , which illustrates the corresponding stages  1721 - 1724  of a playback curve  1710  that is associated with the media clip  1750 . 
     The first stage  1701  in  FIG. 17   a  shows the media clip  1750  and the speed effects bar  1760  before user adjustment of speed effect. The stage  1701  corresponds to the playback curve stage  1721  of  FIG. 17   b . The display of transition interval  1764  and  1765  in the effects bar  1760  indicates that the playback curve  1710  of the media clip has been smoothed in accordance with the specified transition intervals. 
     The section  1762  is at 60% speed, while the sections  1761  and  1763  remain at 100% speed. The section  1761  ends at playback time t 1 , the section  1762  ends at playback time t 2  while the section  1763  ends at playback time t 3 . The speed effects bar also includes sections for transition intervals  1764  and  1765 . The transition interval  1764  is located around (or over) t 1  and is between the 100% speed section  1761  and the 60% speed section  1762 . The transition interval  1765  is located around (or over) t 2  and is between the 60% speed section  1762  and the 100% speed section  1763 . As illustrated in the corresponding stage  1721  of  FIG. 17   b , which illustrates the playback curve  1710  before user adjustments, the playback curve  1710  has been smoothed, particularly within the transition intervals  1764  and  1765 . 
     The graphical representations of the transition intervals  1764  and  1765  are transparently superimposed over the effects bar  1760  such that the boundaries between the speed display sections as well as the adjustment handles of each speed section are still visible beneath the transition intervals. In this example, the graphical transition interval  1765  is transparently superimposed over the boundary between the 60% speed section  1762  and the 100% speed section  1763  such that the boundary between the two speed sections is visible and so is the adjustment handle  1772  for the section  1762 . As a result, the user is still able to access the adjustment handle  1772  to adjust the speed section  1762 . 
     The second stage  1702  in  FIG. 17   a  shows the speed effects bar  1760  at the start of the user adjustment process. The stage  1702  corresponds to the playback curve stage  1722  of  FIG. 17   b . The second stage  1702  also shows a cursor placed over the adjustment handle  1773  of the section  1763  for performing a click and drag operation. 
     The corresponding playback curve stage  1722  of  FIG. 17   b  shows the four keyframes  1711 - 1714  at playback times 0, t 1 , t 2  and t 3 . The keyframe  1711  anchors the start of the playback curve, while the keyframe  1714  anchors the end of the playback curve. The keyframes  1712  and  1713  are discontinuity keyframes that mark the change in speed from 100% speed, to 60% speed, and back to 100% speed. The manipulation of an adjustment handle (such as  1773 ) corresponds to the movement of one of the keyframes (such as  1714 ) of the playback curve  1710 . 
     The third stage  1723  in  FIG. 17   a  shows the user adjustment of speed effect by the click and drag operation. The stage  1703  correspond to the playback curve stage  1723  of  FIG. 17   b . As illustrated, the user has clicked and dragged the adjustment handle  1773  toward the right by using the cursor, which causes the section  1763  to expand from t 3  to t′ 3 . The expansion of the section  1763  in playback time, as illustrated in stage  1723  of  FIG. 17   b , causes the keyframe  1714  to move to the right (later in time), which also slows down the playback speed of the section  1763 . In this case, the playback speed of the section  1763  slows down to 60% because of the movement of the keyframe  1714 . 
     Since the click and drag operation expand the playback time of the section  1763 , some embodiments visually animate the expansion of the section of the media clip under the effects bar section  1763 . In some embodiments, the graphical expansion of a speed effect section is accompanied by graphical stretching of thumbnail images in that section. As illustrated, the thumbnails  1751 - 1753  have been graphically stretched along with the section  1763 . 
     The fourth stage  1704  in  FIG. 17   a  shows the media clip after the speed effect adjustment. The stage  1704  correspond to the playback curve stage  1724  of  FIG. 17   b . The playback speed of section  1763  has been reduced to 60% of normal speed. In place of the three stretched thumbnails  1751 - 1753 , there are five new thumbnails  1754 - 1758  that display a sequence of images corresponding to the new 60% playback speed. Since the section  1763  is now at the same playback speed as the section  1764 , some embodiments merge the two sections as one single section  1766  that starts from the playback time t 1  to the playback time t 3 ′ at playback speed 60%. Consequently, the discontinuity keyframe  1713  is deleted and so is the transition interval  1765 . Since the speed discontinuity at t 1  remains, the discontinuity keyframe  1712  remains (though not shown), and the transition interval  1764  also remains. 
     In some embodiments, the start and end of transition intervals would not change after a user adjustment of a section of the media clip. In some embodiments, the user adjustment of a speed section triggers an automatic adjustment of the start and end of the transition intervals, because some embodiments determine initial start and end of the transition intervals based on the durations of the adjoining speed sections as discussed above by reference to  FIG. 14 . 
     Some of these embodiments perform a new curve smoothing operation that is constrained by the adjusted transition intervals and the adjusted speed sections. In this example, for embodiments that automatically adjust the start and end of transition intervals based on the duration of adjoining speed sections, a new curve smoothing operation will be performed based on the new duration of the transition interval  1764 . However, since the speed sections adjoining the transition interval  1764  have not changed speed (the playback section  1761  is still at 100% and the playback section  1766  is still at 60%), some embodiments will not perform a new curve smoothing operation if the start and end of the transition interval  1764  does not change. 
       FIG. 17   a - b  illustrates an example of a user adjustment of speed effects that expands a section of the effects bar (and the playback curve).  FIGS. 18   a - b , on the other hand, illustrates an example of a user adjustment of speed effect that shrinks a section of the effects bar. 
       FIG. 18   a  illustrates a timeline  1800  that is similar to the timeline  1700 . Within the timeline  1800  is a representation of a media clip (or a media container)  1850 . The representation of the media clip  1850  displays a series of thumbnail images sampled at regular intervals of the playback time from the images in the media clip  1850 . The media clip  1850  also displays an effects bar  1860  that indicates the playback speed of the media content in the media container. The effects bar is partitioned into three sections  1861 - 1863  by one or more previous preset speed effect operations such as the “slower” operation discussed earlier by reference to  FIGS. 4-5 . Each section of the effect bar  1860  displays the playback speed of that section. Each section also includes an adjustment handle. Specifically, the section  1861  has an adjustment handle  1871 , the section  1862  has an adjustment handle  1872 , and the section  1863  has an adjustment handle  1873 . 
       FIG. 18   a  illustrates the example speed effect adjustment operation by user manipulation in four stages  1801 - 1804  of the timeline  1800 .  FIG. 18   a  will be described by reference to  FIG. 18   b , which illustrates corresponding stages  1821 - 1824  of a playback curve  1810  for the media clip. 
     The first stage  1801  in  FIG. 18   a  shows the media clip  1850  and the speed effects bar  1860  before user adjustment of speed effect. The stage  1801  corresponds to the playback curve stage  1821  of  FIG. 18   b , which illustrates the playback curve  1810  before user adjustments. The display of transition interval  1864  and  1865  in the effects bar  1860  indicates that the playback curve  1810  of the media clip has been smoothed in accordance with the specified transition intervals. 
     The section  1862  is at 50% speed while sections  1861  and  1863  remain at 100% speed. The section  1861  ends at playback time t 1 , the section  1862  ends at playback time t 2  while the section  1863  ends at playback time t 3 . The speed effects bar also includes sections for transition intervals  1864  and  1865 . The transition interval  1864  is located around (or over) t 1  and is between the 100% speed section  1861  and the 50% speed section  1862 . The transition interval  1865  is located around (or over) t 2  and is between the 50% speed section  1862  and the 100% speed section  1863 . As illustrated in corresponding stage  1821  of  FIG. 18   b , the playback curve  1810  has been smoothed, particularly within the transition intervals  1864  and  1865 . 
     The graphical representations of the transition intervals  1864  and  1865  are transparently superimposed over the effects bar  1860  such that the boundaries between the speed display sections as well as the adjustment handles of each speed section are still visible beneath the transition intervals. For example, the transition interval  1865  is transparently superimposed over the boundary between the 50% speed section  1862  and the 100% speed section  1863  such that the boundary between the two speed sections is visible and so is the adjustment handle  1872  for the section  1862 . As a result, the user is still able to access the adjustment handle  1872  to adjust the speed section  1862 . 
     The second stage  1802  in  FIG. 18   a  shows the speed effects bar  1860  at the start of the user adjustment process. The stage  1802  corresponds to the playback curve stage  1822  of  FIG. 18   b . The second stage  1802  also shows a cursor placed over the handle  1872  of the section  1862  for performing a click and drag operation. As mentioned above, even though the handle  1872  and the right boundary of the sections  1862  are both beneath the representation of the transition interval  1865 , the user can still access the handle  1872  for adjusting the section  1862 . 
     The corresponding playback curve stage  1822  of  FIG. 18   b  shows the four keyframes  1811 - 1814  at playback times 0, t 1 , t 2  and t 3 . The keyframes  1812  and  1813  are discontinuity keyframes that mark the change in speed from 100% speed, to 50% speed, and back to 100% speed. The keyframe  1811  anchors the start of the playback curve, while the keyframe  1814  anchors the end of the playback curve. The adjustment of a handle (such as  1872 ) corresponds to the movement of one of the keyframes (such as  1813 ) of the playback curve  1810 . 
     The third stage  1823  in  FIG. 18   a  shows the user adjustment of speed effect by the click and drag operation. The stage  1803  correspond to the playback curve stage  1823  of  FIG. 18   b . As illustrated, the user has clicked and dragged the adjustment handle  1872  toward the left, which causes the section  1862  to shrink from t 2  to t′ 2 . The shrinking of the section  1862  in playback time, as illustrated in stage  1823  of  FIG. 18   b , causes the keyframe  1813  to move to the left (earlier in time), which also accelerates the playback speed of the section  1862 . In this case, the playback speed of the section  1862  accelerates to 125% because of the movement of the keyframe  1813  from user&#39;s click and drag operation. 
     Since the user has used the handle  1872  to shrink the playback time of the section  1862 , some embodiments visually animate the compression of the section of the media clip under the effects bar section  1862 . In some embodiments, the graphical expansion of a speed effect section is accompanied by graphical compression of thumbnail images in that section. As illustrated, the thumbnails  1841 - 1844  have been graphically compressed along with the section  1862 . 
     The fourth stage  1804  in  FIG. 18   a  shows the media clip after the speed effect adjustment. The stage  1804  corresponds to the playback curve stage  1824  of  FIG. 18   b . The playback speed of section  1862  has been increased to 125% of normal speed. In place of the four compressed thumbnails  1841 - 1844 , the media clip has been resampled to generate a set of new thumbnails that display a sequence of images corresponding to the new 125% playback speed. 
     Since the user adjustment operation of the section  1862  from 50% speed to 125% speed has not caused the speed section  1862  to be at the same speed with its adjacent neighboring sections ( 1861  and  1863 ), the discontinuity keyframes  1812  and  1813  would not be deleted. In some embodiments, the start and end of the transition intervals would not change after a user adjustment of another section of the media clip. In some embodiments, the user adjustment of the speed sections triggers an automatic adjustment of the start and end of the transition intervals (because some embodiments determines initial start and end of the transition intervals based on the durations of the adjoining speed sections as discussed above by reference to  FIG. 14 .) 
     Some of these embodiments then perform a new curve smoothing operation that is constrained by the adjusted transition intervals and the adjusted speed sections. In this example, the playback curve within the transition intervals  1864  and  1865  will have to be smoothed again based on (i) the new playback speeds of the adjoining speed sections  1861 - 1863 , and (ii) the updated start and end of the transition intervals  1864  and  1865 , which in some embodiments changes according to the durations of the adjoining speed sections. 
     In the examples of  FIGS. 17-18 , the transition intervals are illustrated as being transparently superimposed over the effects bar such that the boundaries of the speed sections are still visible to the user and the user still have the access to the adjustment handles of each speed sections. One of ordinary skill would realize that are many other possible ways of graphically representing transition intervals in the effects bar that allows the user to discern the boundaries of speed sections as well as to adjust the speed sections for performing retiming operations. For example, in some embodiments, the transition intervals are not transparent and the true boundaries between the different speed sections are not immediately visible to the user (such in the effects bar  160  of  FIG. 1 ). However, the user in some embodiments can select an adjustment UI item for opening a contextual menu item that provide user access to the true boundaries of the different speed sections. Or alternatively, the selection of the adjustment UI item would bring the true boundaries between the different speed sections to the foreground over the transition intervals so to allow the user to adjust each speed effect section at the sections&#39; true boundaries. 
     In addition to adjusting playback speed of individual sections of the effects bar of a media clip, a user can also adjust the range of each individual section.  FIGS. 19   a - b  illustrate an example range adjustment operation of a media clip that has been partitioned by an earlier preset retiming operation. 
       FIG. 19   a  illustrates a timeline  1900  of a GUI that is similar to the GUI  400  of a media editing application. Within the timeline  1900  is a representation of a media clip (or media container)  1950 . The representation of the media clip  1950  displays a series of thumbnail images sampled at regular intervals of the playback time from the media clip  1950 . The representation of the media clip  1950  also displays an effects bar  1960  that indicates the playback speed of the media content in the media container. The effects bar is partitioned into three sections  1961 - 1963  by one or more previous preset speed effect operations such as the “slower” or “faster” retiming operations discussed earlier by reference to  FIGS. 4-5 . In addition to displaying the playback speed, each of the speed effect bar sections  1961 - 1963  also includes an adjustment UI item (UI items  1981 - 1983  for the sections  1961 - 1963  respectively). Each of the adjustment UI items is for opening a contextual menu item. 
       FIG. 19   a  illustrates the example range adjustment operation by user manipulation in five stages  1901 - 1905 .  FIG. 19   a  will be described by reference to  FIG. 19   b , which illustrates corresponding stages  1921 - 1924  of a playback curve  1910  for the media clip  1950 . 
     The first stage  1901  shows the media clip  1950  before the user range adjustment. The stage  1901  corresponds to the playback curve stage  1921  of  FIG. 19   b . The effects bar  1960  shows that the section  1961  is at 100% speed, the section  1962  is at 50% speed, and the section  1963  is at 150% of normal speed. The section  1961  ends at playback time t 1 , the section  1962  ends at playback time t 2  while the section  1963  ends at playback time t 3 . The speed effects bar  1960  also displays transition intervals  1964  and  1965 . The transition interval  1964  is located around (or over) t 1  and is between the 100% speed section  1961  and the 50% speed section  1962 . The transition interval  1965  is located around (or over) t 2  and is between the 50% speed section  1962  and the 150% speed section  1963 . The stage  1901  also shows a cursor being placed over an adjustment UI item  1982  in the effects bar section  1962  to allow user adjustment of speed effects. As illustrated in corresponding playback curve stage  1921  of  FIG. 19   b , which illustrates the playback curve  1910  before user adjustments, the playback curve  1910  has been smoothed, particularly within the transition intervals  1964  and  1965 . 
     The second stage  1902  in  FIG. 19   a  shows the speed effects bar  1960  at the start of the user adjustment process. The stage  1902  corresponds to the playback curve stage  1922  of  FIG. 19   b . As illustrated, the user has selected the adjustment UI item  1982 , which opens up a contextual menu  1940  that allows user access to other commands that are available for adjusting the section  1962 . As illustrated, the contextual menu  1940  includes commands such as “slow”, “fast”, “normal” and “change end” that are specific to the section  1961 . The command “slow” slows the playback speed of the section  1961 . The command “faster” accelerates the playback speed of the section  1961 . The command “normal” reset the playback speed of the section  1961  to 100% of normal speed. One of ordinary skill in the art would recognize that these commands are similar to the preset “slower” and “faster” operations as illustrate above by reference to  FIGS. 4-5 . However, unlike the preset commands which apply to a selected range as discussed above in section I, these contextual commands apply only to the specific section from which the contextual command is invoked. The “change end” command is also a contextual command applicable only to the section  1961 . It changes the position of the end of the section  1961  and the start of the section  1962  in media time. In other words, the “change end” command shifts the border between the sections  1961  and  1962  such that some media content that were once in section  1962  becomes part of the section  1961  (or vice versa). 
     The different speed sections of the media clip  1950  are anchored by keyframes on the playback curve  1910 . As illustrated in the corresponding stage  1922  of  FIG. 19   b , there are four such keyframes  1911 - 1914  for the playback curve  1910 . The keyframes  1912  and  1913  are discontinuity keyframes that mark the change in speed from 100% speed to 50% speed at times t 1 , and from 50% speed to 150% speed at time t 2 . The keyframe  1911  anchors the start of the playback curve  1910  at time 0, while the keyframe  1914  anchors the end of the playback curve  1910  at times t 3 . 
     The third stage  1903  shows the appearance of a “change end” tool UI item  1995  after the invocation of the “change end command”. The “change end” tool UI item  1995  is situated at the border (at t 1 ) between the sections  1961  and  1962  so the user can manipulate the UI item  1995  to shift the border between the two sections. In some embodiments, the UI item  1995  includes a miniature preview display area for displaying a video image that is being pointed to by the UI item  1995 . This is so the user can precisely place the border by observing the content being pointed to by the UI item  1995 . In some embodiments, the GUI includes a separate preview display area for displaying a video image that is being pointed to by the UI item  1995  (such as the preview display area  420  of GUI  400  in  FIG. 4 ). The third stage  1903  also shows the cursor  1990  placed over the “change end” tool UI item  1995  and ready to perform a range adjustment operation. 
     The fourth stage  1904  shows the effect of the manipulation of the “change end” tool UI item  1995 . The stage  1904  corresponds to the playback curve stage  1923  of  FIG. 19   b . As illustrated, the user has dragged the UI item  1995  from t 1  to t′ 1 , which correspond to a movement of the keyframe  1912  to the right (later in time). Unlike the adjustment for speed in which a keyframe moves horizontally to change the slope of one of the sections and thereby changing its speed, the keyframe  1912  here moves along the existing playback curve so to preserve the playback speed of each of the sections. 
     However, since the section  1961  has a different playback speed (100%) than the section  1962  (50%), the shift of the border between the section  1961  and the section  1962  made by the “change end” operation causes all media content after t′ 1  to shift. Consequently, the keyframe  1912  has shifted from t 1  to t′ 1  by extending the 100% speed section (section  1961 ), which in turn causes the 50% section of the playback curve  1910  to start later at t′ 1 . The end of the 50% section (section  1962 ) must still end at the same media time as before (i.e., 5), which occurs at an earlier playback time t′ 2 , causing the 150% section (section  1963 ) and the keyframes  1913  and  1914  to shift earlier in playback time (from t 2  and t 3  to t′ 2  and t′ 3 , respectively). 
     The fifth and final stage  1905  in  FIG. 19   a  shows the curve smoothing operation after the range adjustment operation. The stage  1905  corresponds to the playback curve stage  1924  of  FIG. 19   b . The effects bar  1960  once again shows the transition intervals  1964  and  1965 . Since the range adjustment operation does not actually change playback speeds but instead changes only the range and duration of individual sections, some embodiments would not perform a new curve smoothing operation unless the start and the end of the transition intervals have also changed. In some embodiments, the start and end of the transition intervals would not change after a user adjustment of another section of the media clip. In some embodiments, the user adjustment of the speed sections triggers an automatic adjustment of the start and end of the transition intervals (because some embodiments determines initial start and end of the transition intervals based on the durations of the adjoining speed sections as discussed above by reference to  FIG. 14 .) 
       FIGS. 19   a - b  illustrates an example range adjustment operation by invoking a command that adjusts the end of a particular section of a media clip (or the media clip&#39;s effects bar) thus shifting the border between the particular section and an immediately subsequent sections. One of ordinary skill in the art would recognize that such a command is functionally equivalent to a command that changes the start of the particular section, which adjusts the border between the particular section and an immediately preceding section. Some of these embodiments include both “change end” and “change start” commands in the contextual menu of the section. 
     For some embodiments of the invention,  FIG. 20  conceptually illustrates a process  2000  for performing a user adjustment of the media clip. The process  2000  also performs a curve smoothing operation after the user adjustment has been made. The process  2000  starts after the media editing application has performed a retiming operation that partitions the media clip into two or more sections with different playback speeds. 
     The process receives (at  2010 ) a specification for the user adjustment. The specification of the adjustment can be a user manipulation in a GUI that changes the playback speed of a particular section of media clip as discussed above by reference to  FIGS. 17-18 , a user manipulation that changes the range of a particular section of the media clip as discussed above by reference to  FIGS. 19   a - b , or a user manipulation that changes the start and/or the end of a transition interval as discussed above by reference to  FIGS. 1-2 . 
     Next, the process receives (at  2020 ) the playback curve of the media clip. This playback curve is the result of previous retiming operations, which can include both present retiming operations and user adjustment operations. In some embodiments, a playback curve includes specifications for start and end of the media clip, as well as one or more keyframes that mark the borders between different sections. In some embodiments, the playback curve also includes specifications for transition intervals. These transition intervals are used to generate smoothed curves when the media clip is being played back according to the playback curve. In some embodiments, the playback curve is retrieved from storage, which stores the playback curve in data structures that will be further discussed by reference to  FIG. 24  below. 
     The process next determines (at  2030 ) whether the adjustment is for a transition interval or for a section of the media clip that has a specified playback speed. For example, section  1962  is a section of the media clip that has a specified playback speed of 50%, while the section  1964  is a transition interval between playback speeds of 100% and 50%. If the adjustment is for a section of the media clip with a specified playback speed such as described above by reference to  FIGS. 17-19 , the process proceeds to  2040 . If the adjustment is for a transition interval as described above by reference to  FIGS. 1-2 , the process proceeds to  2065 . 
     At  2040 , the process moves key frames according to the received user adjustment. In some embodiments, the adjustment of a playback curve is accomplished by moving keyframes that anchors the start and/or end of different sections of the media clip to different positions in the media time/playback time space.  FIGS. 17   b  and  18   b  above illustrates the movement of keyframes when adjusting the playback speed of a section of the media clip, while  FIG. 19   b  above illustrates the movement of keyframes when adjusting the range of a section of the media clip. 
     The process next removes (at  2050 ) obsolete discontinuity keyframes and transition intervals. As discussed above by reference to  FIGS. 17   a - b , user adjustment of the playback curve can sometimes cause the playback speed of adjacent sections divided by a discontinuity keyframe to become the same. In these instances, the speed discontinuity marked by the discontinuity keyframe no longer exists, and the discontinuity keyframe is no longer necessary. Some embodiments hence remove the unnecessary discontinuity keyframes. Since each transition interval is associated with a discontinuity in some embodiments, a transition interval that is associated with a removed discontinuity is also removed from the playback curve. 
     The process next determines (at  2060 ) whether the user adjustment of the media clip makes it is necessary to adjust the start and end of transition intervals. In some embodiments, the automatic setting of the start and end of a transition interval takes place only when the transition interval is first created. All subsequent adjustment of the transition interval is done by the user, even if the duration and/or the playback speed of the sections before and after the discontinuity has been changed by the user. On the other hand, some embodiments do perform automatic adjustment of transition intervals after each user adjustment. As mentioned earlier, in some embodiments, the start and end of the transition intervals are initially set according to the duration of the sections before and after the discontinuity. Some of these embodiments would reset the start and end of the transition intervals based on the new durations of the sections adjoining the discontinuity (e.g., by setting the start (and end) of the transition interval to a particular fraction of the duration of the section before (and after) the discontinuity.) If the process  2000  determines that it is not necessary to adjust start and end of transition intervals (e.g., because the duration of adjoining section has not changed), the process proceeds to  2070 . Otherwise, the process proceeds to  2065  to move the start and end of the transition interval. 
     At  2065 , the process  2000  moves the start and the end of the transition interval. The new destination of the start and end of the transition interval can be directly specified by the user as described above by reference to  FIGS. 1-2 . The new destination can also be specified by the process  2000 , which in some embodiments specifies new positions for the start and end for the transition interval based on changes that were made to the sections before and after the discontinuity. Having moved the start and end of the transition interval, the process proceeds to  2075  to perform curve smoothing based on the transition interval. 
     At  2070 , the process determines whether the playback speeds before and after the discontinuity has changed. Since the smoothed curve is constrained by the transition intervals as well as by the playback speeds of section adjoining the discontinuity, the process  2000  will have to perform new curve smoothing operation if any of these constraints has changed. Therefore, if the playback speed of the adjoining sections has been altered by the user adjustment, the process proceeds to  2075  to create a new smoothed curve that fits the new constraints. Otherwise, the process proceeds to  2080  to produce the smoothed playback curve. 
     At  2075 , the process performs the curve smoothing operation to create a new smoothed curve for the transition interval. The new curve is constrained by the keyframes anchoring the start and end of transition intervals. The new curve is also constrained by the playback speed of the section before and after the discontinuity. After creating the smoothed curve for the transition interval, the process proceeds to  2080 . 
     The process produces (at  2080 ) the smoothed playback curve for either storage or playback. For storage, the process  2000  in some embodiments stores the playback curve according to a data structure described below by reference to  FIG. 24 . After producing the smoothed playback curve, the process  2000  ends. 
     IV. Mapping of Playback Curves 
     A. Interpolation 
     The playback curves as discussed above in Sections I-III map instants in playback time to instants in media time. In some embodiments, the mapped media time is then used to fetch a frame from the source media clip. However, not all media time instants mapped by the playback curve has a video frame that is specified to be displayed at that precise moment. For example, a playback curve may specify a media time that is temporally half way between two video frames in the original media clip. In these instances, it can be unclear as to which video frame should the media editing application fetch for display. In such cases, some embodiments produce an interpolated frame based on other frames in the video clip that are in the vicinity of the media time. 
     For some embodiments,  FIG. 21  illustrates the mapping of a playback curve from playback time to media time and then to actual video frames in the source of the media content.  FIG. 21  illustrates an example playback curve  2100  that maps playback time (x-axis) to media time (y-axis).  FIG. 21  also illustrates frame count scale  2110  that shows the mapping from media time to an actual video frame in a source media clip. In some embodiments, each integer unit of playback time corresponds to one playback video frame, such that the playback time N corresponds to the Nth playback video frame. 
     As illustrated, the playback curve  2100  maps the playback time instant P1 (at playback time 18 or playback video frame 18) to the media time instant M1 (media time 21), which precisely or very nearly maps to frame count 14 on the frame count scale  2110 . Not all playback video frames (or integer playback times) map precisely onto an actual video frame in the source media clip. For example, the playback time instant P2 (at playback time 15) maps to media time instant M2 (media time 10) and then to a position 6.7 in the frame count scale  2100  that is positioned between source video frames 6 and 7. Likewise the playback time instant P3 (at playback time 7) maps to media time instant M3 (media time 6.3), which maps to a position 4.2 in the frame count scale  2100  which is positioned between the source video frame 4 and 5. 
     In some embodiments, a playback time instant that maps to a media time instant sufficiently close to an actual video frame (on the frame count scale) does not require an interpolated frame. In some of these embodiments, the difference in media time between the position of the actual video frame and the mapped playback position is compared against a threshold. If the difference is within such a threshold, interpolation will not be performed and the actual video frame (the nearest frame to the mapped playback position) is fetched directly for playback. 
     For a playback time instant that does not map to a media time instant sufficiently close to an actual video frame, some embodiments generate an interpolated frame. In some other embodiments, interpolation is always performed, even if a playback time instant maps exactly on to an actual video frame in the source media content. 
     Some embodiments perform interpolation by using the nearest neighboring video frame in the source media content as the interpolated frame. For example, for playback frame 7 (P3) that maps on to position 4.2 in the frame count scale (M3), the actual frame 4 in the source media content will be used as the interpolated frame and be displayed during playback. 
     Some embodiments perform interpolation by blending video frames. In some of these embodiments, frames neighboring the mapped playback position are blended together to produce the interpolated frame. In some embodiments, frame blending is performed by applying a weighting factor to each of the blended source frames according to the temporal distance between the blended source frame and the mapped playback position. In the example of  FIG. 21 , for playback frame 15 (P2) that maps to position 6.7, some embodiments produce the interpolated frame by blending source frames 6 and 7 and by applying a weighting factor of 0.7 to frame 7 and a weighting factor of 0.3 to frame 6. 
     Some embodiments perform interpolation by optical flow. Optic flow is the pattern of apparent motion of objects, surfaces, and edges in a visual scene caused by the relative motion between an observer (an eye or a camera) and the scene. Sequences of ordered images allow the estimation of motion as either instantaneous image velocities or discrete image displacements. Some embodiments create the interpolated frame by estimating motions of pixels using ordered images of frames neighboring the mapped playback position. 
     For some embodiments,  FIG. 22  conceptually illustrates a process  2200  for mapping playback times to actual or interpolated video frames. The process receives (at  2210 ) a playback curve associated with a media clip that maps instants of playback time to instants in media time. The process next receives (at  2220 ) a playback time instant to be mapped. In some embodiments, the process  2200  receives such a playback time instant whenever a video frame needs to be fetched from a source media clip, such as when the GUI  400  of  FIG. 4  is producing a preview image on the preview display area  420 , or when the GUI  400  of  FIG. 4  is producing a thumbnail image in a media clip, or when the media editing application is directed by the user to output the product of the composite presentation being constructed in the timeline of the GUI. 
     The process  2200  next uses (at  2230 ) the playback curve to look up a media time that corresponds to the received playback time. The process determines (at  2235 ) whether the media time is sufficiently aligned with an actual frame. If the media time being looked up is sufficiently aligned with an actual frame, the process  2200  proceeds to  2260  to retrieve the actual frame for display. Otherwise the process proceeds to  2240 . In some embodiments, the process always proceeds to  2240  and create an interpolated frame regardless of whether the media time is sufficiently aligned with the actual frame. 
     At  2240 , the process creates an interpolated frame based on the media time. The mapping (or look up) of media time and the creation of interpolated frame are described above by reference to  FIG. 21 . After creating the interpolated frame, the process displays (at  2250 ) or delivers the interpolated frame. After delivering or displaying the interpolated frame, the process  2200  ends. 
     At  2260 , the process retrieves an actual frame based on the mapped media time. The process  2200  then displays (at  2270 ) or deliver the retrieved frame. After displaying or delivering the retrieved frame, the process  2200  ends. 
     B. Audio Mapping 
     In some embodiments, each speed effect or retiming operation performed on video content is accompanied by a corresponding speed effect or retiming operation on audio content. A preset retiming operation performed on a selected section of a media clip applies to both video and audio of the selected section of the media clip. In some embodiments that use a same playback curve for both audio and video, every alteration or adjustment to the playback curve (such as retiming or curve smoothing) applies to both video and audio in order to keep audio and video in sync. Each playback time instant is mapped to a media time instant using the playback curve for both video and audio. The slope of the playback curve at each of the playback time instants is used to determine the instantaneous audio playback speed. 
     In some embodiments, retiming operation affects the playback speed of audio but not the pitch of audio. Some embodiments use common audio pitch preservation techniques to ensure that changes in playback speed would not affect the pitch of audio during playback. 
     V. Software Architecture 
     In some embodiments, the processes described above are implemented as software running on a particular machine, such as a computer or a handheld device, or stored in a computer readable medium.  FIG. 23  conceptually illustrates the software architecture of a media editing application  2300  of some embodiments. In some embodiments, the media editing application is a stand-alone application or is integrated into another application, while in other embodiments the application might be implemented within an operating system. Furthermore, in some embodiments, the application is provided as part of a server-based solution. In some of these embodiments, the application is provided via a thin client. That is, the application runs on a server while a user interacts with the application via a separate machine that is remote from the server. In other such embodiments, the application is provided via a thick client. That is, the application is distributed from the server to the client machine and runs on the client machine. 
     The media editing application  2300  includes a user interface (UI) interaction module  2305 , a retiming module  2330 , and a playback control module  2320 . The media editing application  2300  also includes playback curve storage  2345 , video source storage  2355 , and audio source storage  2365 . In some embodiments, storages  2345 ,  2355 , and  2365  are all stored in one physical storage  2390 . In other embodiments, the storages are in separate physical storages, or two of the storages are in one physical storage, while the third storage is in a different physical storage. For instance, the video source storage  2355  and the audio source storage  2365  will often not be separated in different physical storages. 
       FIG. 23  also illustrates an operating system  2370  that includes input device driver(s)  2372 , a network connection interface(s)  2374 , a display module  2380  and an audio module  2385 . In some embodiments, as illustrated, the input device drivers  2372 , the network connection interfaces  2374 , the display module  2380  and the audio module  2385  are part of the operating system  2370 , even when the media editing application  2300  is an application separate from the operating system. 
     The input device drivers  2372  may include drivers for translating signals from a keyboard, mouse, touchpad, drawing tablet, touchscreen, etc. A user interacts with one or more of these input devices, which send signals to their corresponding device driver. The device driver then translates the signals into user input data that is provided to the UI interaction module  2305 . 
     The media editing application  2300  of some embodiments includes a graphical user interface that provides users with numerous ways to perform different sets of operations and functionalities. In some embodiments, these operations and functionalities are performed based on different commands that are received from users through different input devices (e.g., keyboard, trackpad, touchpad, mouse, etc.). For example, the present application illustrates the use of a cursor in the graphical user interface to control (e.g., select, move) objects in the graphical user interface. However, in some embodiments, objects in the graphical user interface can also be controlled or manipulated through other controls, such as touch control. In some embodiments, touch control is implemented through an input device that can detect the presence and location of touch on a display of the input device. An example of a device with such functionality is a touch screen device (e.g., as incorporated into a smart phone, a tablet computer, etc.). In some embodiments with touch control, a user directly manipulates objects by interacting with the graphical user interface that is displayed on the display of the touch screen device. For instance, a user can select a particular object in the graphical user interface by simply touching that particular object on the display of the touch screen device. As such, when touch control is utilized, a cursor may not even be provided for enabling selection of an object of a graphical user interface in some embodiments. However, when a cursor is provided in a graphical user interface, touch control can be used to control the cursor in some embodiments. 
     The display module  2380  translates the output of a user interface for a display device. That is, the display module  2380  receives signals (e.g., from the UI interaction module  2305 ) describing what should be displayed and translates these signals into pixel information that is sent to the display device. The display device may be an LCD, plasma screen, CRT monitor, touchscreen, etc. In some embodiments, the display module  2380  also receives signals from the playback control module  2320  for displaying video images from a composite presentation that the media editing application is composing. 
     The audio module  2385  translates the output of a user interface for a sound producing device that translates digital audio signals into actual sounds. In some embodiment, the audio module  2385  also receives digital audio signals from the playback control module for playing sound produced from a composite presentation that the media editing application is composing. 
     The network connection interface  2374  enable the device on which the media editing application  2300  operates to communicate with other devices (e.g., a storage device located elsewhere in the network that stores the raw audio data) through one or more networks. The networks may include wireless voice and data networks such as GSM and UMTS, 802.11 networks, wired networks such as Ethernet connections, etc. 
     The UI interaction module  2305  of the media editing application  2300  interprets the user input data received from the input device drivers  2372  and passes it to various modules, including the retiming module  2330  and the playback control module  2320 . The UI interaction module also manages the display of the UI, and outputs this display information to the display module  2380 . This UI display information may be based on information from the playback control module  2320  or directly from the video source storage  2355  and audio source storage  2365 . In some embodiments, the UI interaction module  2305  includes a range selector module  2315  for processing user selection of a range in a media clip for retiming operation. 
     The playback curve storage  2345  receives and stores playback curves generated and adjusted by the retiming module  2330 . In some embodiments, the playback curves are stored in data structures that include specification for transition intervals.  FIG. 24  below provides an example data structure of a playback curve. The playback curves stored can be accessed for further adjustment by the retiming module  2330 , or be retrieved for controlling the timing of media clip playback by the playback control module  2320 . The video source storage  2355  receives and stores video data from the UI interaction module  2305  or the operating system  2370 . The audio source storage  2365  likewise receives and stores audio data from the UI interaction module and the operating system  2370 . 
     The retiming module (or retiming engine)  2330  generates and adjusts playback curves. In some embodiments, the retiming module generates a new playback curve and stores it in the playback curve storage  2345  whenever a new media clip or composite presentation is created. The retiming module also receives retiming commands and associated parameters from the UI interaction module  2305 . The retiming module includes a playback curve generator  2332 , a playback curve adjuster  2334 , and a transition handler  2336 . The playback curve generator  2332  generates the new playback curves for controlling the timing of new media clips. The playback curve adjuster  2334  manipulates keyframes in the playback curve based on user command received from the UI interaction module  2305 . These received user commands can be for retiming operations applied to a particular selected range, or an adjustment operations that changes the speed or range of individual sections of the media clips. The playback curve adjuster  2334  can move, insert, or remove keyframes in the playback curves. The transition handler  2336  inserts transition intervals into the playback curves. It also sets the start and end of the transition intervals by analyzing keyframes in the playback curve (e.g., to determine the duration of different sections of the media clip). 
     The playback control module  2320  includes an image fetcher  2322 , a frame interpolator  2324 , an audio processing module  2326 , and a curve smoother  2328 . The curve smoother  2328  retrieves data structures of playback curves from the playback curve storage  2345  and creates smoothed curves based on speeds around discontinuities and on the transition intervals. The frame interpolator  2324  uses the smoothed curve supplied by the curve smoother  2328  to determines which video frame to fetch from the video source  2355  at any given point in playback time. The image fetcher  2322  retrieves the images required by the frame interpolator  2324  from the video source storage  2355 . The frame interpolator  2324  then produces the interpolated frame for display based on the smoothed curve and the retrieved images as described above by reference to  FIG. 21 . The audio processing module  2326  produces audio for the audio module  2385  in the operating system  2370  based on audio data retrieved from the audio source storage  2365  and the smoothed playback curve from the curve smoother  2328 . 
     While many of the features have been described as being performed by one module, one of ordinary skill in the art will recognize that the functions described herein might be split up into multiple modules. Similarly, functions described as being performed by multiple different modules might be performed by a single module in some embodiments. For example, the functions of the playback curve generator  2332 , the playback curve adjuster  2334 , and the transition handler  2336  can be performed by one larger retiming module  2330 , and the functions performed by the curve smoother  2328 , the frame interpolator  2324 , and the image fetcher  2322  can be performed by one larger playback control module  2320 . 
     For some embodiments,  FIG. 24  conceptually illustrates a data structure for a playback curve. The data structure is stored in a storage such as the playback curve storage  2345 . As illustrated, the playback curve data structure  2400  includes an ID of media content  2410  and a list of keyframes  2420 . 
     The ID of media content  2410  identifies or provides links to the media content (media clip or the composite presentation) whose playback timing is controlled by the playback curve represented by the data structure  2400 . 
     The list of keyframes  2420  includes several keyframes, each keyframe includes a playback-time/media-time pair for specifying a particular point in the media clip (i.e., a specific media time) that is to be played back at a particular playback time. In some embodiments, the list of keyframe  2420  is a linked list that allows the insertion into and deletion from the list. 
     In some embodiments, the list of keyframes includes at least a start key frame  2430  (keyframe 0) and end key frame  2439  (keyframe n). The start keyframe  2430  anchors the start of the playback curve, while the end keyframe  2439  anchors the end of the playback curve. Between the keyframe 0 and the keyframe n are discontinuity keyframes  2431 - 2433  (key frames 1-3). Each discontinuity keyframe specifies a discontinuity in playback speed. In other words, each discontinuity keyframe anchors the end of one section of playback curve at a first speed and the also the start of another section of the playback curve at a second, different speed. 
     The playback curve data structure  2400  further includes several transition intervals  2441 - 2442 . Each transition interval is linked to a discontinuity keyframe. As illustrated, the transition interval  2441  is linked to the discontinuity keyframe  2431 , and the transition interval  2442  is linked to the discontinuity keyframe  2432 . Each transition interval specifies the start and the end of the transition interval. In some embodiments, the start and end of the transition interval is specified in terms of playback time. 
     VI. Media Editing Application 
     A more detailed view of a media editing application with features discussed above in Sections I is illustrated in  FIG. 25 .  FIG. 25  illustrates a graphical user interface (GUI)  2500  of a media-editing application of some embodiments. One of ordinary skill will recognize that the graphical user interface  2500  is only one of many possible GUIs for such a media-editing application. In fact, the GUI  2500  includes several display areas which may be adjusted in size, opened or closed, replaced with other display areas, etc. The GUI  2500  includes a clip library  2505 , a clip browser  2510 , a timeline  2515 , a preview display area  2520 , an inspector display area  2525 , an additional media display area  2530 , and a toolbar  2535 . 
     The clip library  2505  includes a set of folders through which a user accesses media clips that have been imported into the media-editing application. Some embodiments organize the media clips according to the device (e.g., physical storage device such as an internal or external hard drive, virtual storage device such as a hard drive partition, etc.) on which the media represented by the clips are stored. Some embodiments also enable the user to organize the media clips based on the date the media represented by the clips was created (e.g., recorded by a camera). As shown, the clip library  2505  includes media clips from both 2009 and 2011. 
     Within a storage device and/or date, users may group the media clips into “events”, or organized folders of media clips. For instance, a user might give the events descriptive names that indicate what media is stored in the event (e.g., the “New Event 2-8-09” event shown in clip library  2505  might be renamed “European Vacation” as a descriptor of the content). In some embodiments, the media files corresponding to these clips are stored in a file storage structure that mirrors the folders shown in the clip library. 
     Within the clip library, some embodiments enable a user to perform various clip management actions. These clip management actions may include moving clips between events, creating new events, merging two events together, duplicating events (which, in some embodiments, creates a duplicate copy of the media to which the clips in the event correspond), deleting events, etc. In addition, some embodiments allow a user to create sub-folders of an event. These sub-folders may include media clips filtered based on tags (e.g., keyword tags). For instance, in the “New Event 2-8-09” event, all media clips showing children might be tagged by the user with a “kids” keyword, and then these particular media clips could be displayed in a sub-folder of the wedding event that filters clips in this event to only display media clips tagged with the “kids” keyword. 
     The clip browser  2510  allows the user to view clips from a selected folder (e.g., an event, a sub-folder, etc.) of the clip library  2505 . As shown in this example, the folder “New Event 2-8-11 3” is selected in the clip library  2505 , and the clips belonging to that folder are displayed in the clip browser  2510 . Some embodiments display the clips as thumbnail filmstrips, as shown in this example. By moving a cursor (or a finger on a touchscreen) over one of the thumbnails (e.g., with a mouse, a touchpad, a touchscreen, etc.), the user can skim through the clip. That is, when the user places the cursor at a particular horizontal location within the thumbnail filmstrip, the media-editing application associates that horizontal location with a time in the associated media file, and displays the image from the media file for that time. In addition, the user can command the application to play back the media file in the thumbnail filmstrip. 
     In addition, the thumbnails for the clips in the browser display an audio waveform underneath the clip that represents the audio of the media file. In some embodiments, as a user skims through or plays back the thumbnail filmstrip, the audio plays as well. 
     Many of the features of the clip browser are user-modifiable. For instance, in some embodiments, the user can modify one or more of the thumbnail size, the percentage of the thumbnail occupied by the audio waveform, whether audio plays back when the user skims through the media files, etc. In addition, some embodiments enable the user to view the clips in the clip browser in a list view. In this view, the clips are presented as a list (e.g., with clip name, duration, etc.). Some embodiments also display a selected clip from the list in a filmstrip view at the top of the browser so that the user can skim through or playback the selected clip. 
     The timeline  2515  provides a visual representation of a composite presentation (or project) being created by the user of the media-editing application. Specifically, it displays one or more geometric shapes that represent one or more media clips that are part of the composite presentation. The timeline  2515  of some embodiments includes a primary lane  2540  (also called a “spine”, “primary compositing lane”, or “central compositing lane”) as well as one or more secondary lanes  2545  (also called “anchor lanes”). The spine represents a primary sequence of media which, in some embodiments, does not have any gaps. The clips in the anchor lanes are anchored to a particular position along the spine (or along a different anchor lane). Anchor lanes may be used for compositing (e.g., removing portions of one video and showing a different video in those portions), B-roll cuts (i.e., cutting away from the primary video to a different video whose clip is in the anchor lane), audio clips, or other composite presentation techniques. 
     The user can add media clips from the clip browser  2510  into the timeline  2515  in order to add the clip to a presentation represented in the timeline. Within the timeline, the user can perform further edits to the media clips (e.g., move the clips around, split the clips, trim the clips, apply effects to the clips, etc.). The length (i.e., horizontal expanse) of a clip in the timeline is a function of the length of media represented by the clip. As the timeline is broken into increments of time, a media clip occupies a particular length of time in the timeline. As shown, in some embodiments the clips within the timeline are shown as a series of images. The number of images displayed for a clip varies depending on the length of the clip in the timeline, as well as the size of the clips (as the aspect ratio of each image will stay constant). 
     As with the clips in the clip browser, the user can skim through the timeline or play back the timeline (either a portion of the timeline or the entire timeline). In some embodiments, the playback (or skimming) is not shown in the timeline clips, but rather in the preview display area  2520 . 
     The preview display area  2520  (also referred to as a “viewer” displays images from media files that the user is skimming through, playing back, or editing. These images may be from a composite presentation in the timeline  2515  or from a media clip in the clip browser  2510 . In this example, the user has been skimming through the beginning of clip  2540 , and therefore an image from the start of this media file is displayed in the preview display area  2520 . As shown, some embodiments will display the images as large as possible within the display area while maintaining the aspect ratio of the image. 
     The inspector display area  2525  displays detailed properties about a selected item and allows a user to modify some or all of these properties. The selected item might be a clip, a composite presentation, an effect, etc. In this case, the clip that is shown in the preview display area  2520  is also selected, and thus the inspector displays information about media clip  2540 . This information includes duration, file format, file location, frame rate, date created, audio information, etc. about the selected media clip. In some embodiments, different information is displayed depending on the type of item selected. 
     The additional media display area  2530  displays various types of additional media, such as video effects, transitions, still images, titles, audio effects, standard audio clips, etc. In some embodiments, the set of effects is represented by a set of selectable UI items, each selectable UI item representing a particular effect. In some embodiments, each selectable UI item also includes a thumbnail image with the particular effect applied. The display area  2530  is currently displaying a set of effects for the user to apply to a clip. In this example, only two effects are shown in the display area (the keyer effect and the luma keyer effect, because the user has typed the word “keyer” into a search box for the effects display area). 
     The toolbar  2535  includes various selectable items for editing, modifying what is displayed in one or more display areas, etc. The right side of the toolbar includes various selectable items for modifying what type of media is displayed in the additional media display area  2530 . The illustrated toolbar  2535  includes items for video effects, visual transitions between media clips, photos, titles, generators and backgrounds, etc. In addition, the toolbar  2535  includes an inspector selectable item that causes the display of the inspector display area  2525  as well as items for applying a retiming operation to a portion of the timeline, adjusting color, and other functions. In some embodiments, selecting the retiming tool activation item  2575  invokes a speed effects menu that includes one or more selectable retiming operation presets. 
     The left side of the toolbar  2535  includes selectable items for media management and editing. Selectable items are provided for adding clips from the clip browser  2510  to the timeline  2515 . In some embodiments, different selectable items may be used to add a clip to the end of the spine, add a clip at a selected point in the spine (e.g., at the location of a playhead), add an anchored clip at the selected point, perform various trim operations on the media clips in the timeline, etc. The media management tools of some embodiments allow a user to mark selected clips as favorites, among other options. 
     One or ordinary skill will also recognize that the set of display areas shown in the GUI  2500  is one of many possible configurations for the GUI of some embodiments. For instance, in some embodiments, the presence or absence of many of the display areas can be toggled through the GUI (e.g., the inspector display area  2525 , additional media display area  2530 , and clip library  2505 ). In addition, some embodiments allow the user to modify the size of the various display areas within the UI. For instance, when the additional media display area  2530  is removed, the timeline  2515  can increase in size to include that area. Similarly, the preview display area  2520  increases in size when the inspector display area  2525  is removed. 
     VII. Electronic System 
     Many of the above-described features and applications are implemented as software processes that are specified as a set of instructions recorded on a computer readable storage medium (also referred to as computer readable medium). When these instructions are executed by one or more computational or processing unit(s) (e.g., one or more processors, cores of processors, or other processing units), they cause the processing unit(s) to perform the actions indicated in the instructions. Examples of computer readable media include, but are not limited to, CD-ROMs, flash drives, random access memory (RAM) chips, hard drives, erasable programmable read only memories (EPROMs), electrically erasable programmable read-only memories (EEPROMs), etc. The computer readable media does not include carrier waves and electronic signals passing wirelessly or over wired connections. 
     In this specification, the term “software” is meant to include firmware residing in read-only memory or applications stored in magnetic storage which can be read into memory for processing by a processor. Also, in some embodiments, multiple software inventions can be implemented as sub-parts of a larger program while remaining distinct software inventions. In some embodiments, multiple software inventions can also be implemented as separate programs. Finally, any combination of separate programs that together implement a software invention described here is within the scope of the invention. In some embodiments, the software programs, when installed to operate on one or more electronic systems, define one or more specific machine implementations that execute and perform the operations of the software programs. 
       FIG. 26  conceptually illustrates an electronic system  2600  with which some embodiments of the invention are implemented. The electronic system  2600  may be a computer (e.g., a desktop computer, personal computer, tablet computer, etc.), phone, PDA, or any other sort of electronic device. Such an electronic system includes various types of computer readable media and interfaces for various other types of computer readable media. Electronic system  2600  includes a bus  2605 , processing unit(s)  2610 , a graphics processing unit (GPU)  2615 , a system memory  2620 , a network  2625 , a read-only memory  2630 , a permanent storage device  2635 , input devices  2640 , and output devices  2645 . 
     The bus  2605  collectively represents all system, peripheral, and chipset buses that communicatively connect the numerous internal devices of the electronic system  2600 . For instance, the bus  2605  communicatively connects the processing unit(s)  2610  with the read-only memory  2630 , the GPU  2615 , the system memory  2620 , and the permanent storage device  2635 . 
     From these various memory units, the processing unit(s)  2610  retrieves instructions to execute and data to process in order to execute the processes of the invention. The processing unit(s) may be a single processor or a multi-core processor in different embodiments. Some instructions are passed to and executed by the GPU  2615 . The GPU  2615  can offload various computations or complement the image processing provided by the processing unit(s)  2610 . In some embodiments, such functionality can be provided using CoreImage&#39;s kernel shading language. 
     The read-only-memory (ROM)  2630  stores static data and instructions that are needed by the processing unit(s)  2610  and other modules of the electronic system. The permanent storage device  2635 , on the other hand, is a read-and-write memory device. This device is a non-volatile memory unit that stores instructions and data even when the electronic system  2600  is off. Some embodiments of the invention use a mass-storage device (such as a magnetic or optical disk and its corresponding disk drive) as the permanent storage device  2635 . 
     Other embodiments use a removable storage device (such as a floppy disk, flash memory device, etc., and its corresponding disk drive) as the permanent storage device. Like the permanent storage device  2635 , the system memory  2620  is a read-and-write memory device. However, unlike storage device  2635 , the system memory  2620  is a volatile read-and-write memory, such a random access memory. The system memory  2620  stores some of the instructions and data that the processor needs at runtime. In some embodiments, the invention&#39;s processes are stored in the system memory  2620 , the permanent storage device  2635 , and/or the read-only memory  2630 . For example, the various memory units include instructions for processing multimedia clips in accordance with some embodiments. From these various memory units, the processing unit(s)  2610  retrieves instructions to execute and data to process in order to execute the processes of some embodiments. 
     The bus  2605  also connects to the input and output devices  2640  and  2645 . The input devices  2640  enable the user to communicate information and select commands to the electronic system. The input devices  2640  include alphanumeric keyboards and pointing devices (also called “cursor control devices”), cameras (e.g., webcams), microphones or similar devices for receiving voice commands, etc. The output devices  2645  display images generated by the electronic system or otherwise output data. The output devices  2645  include printers and display devices, such as cathode ray tubes (CRT) or liquid crystal displays (LCD), as well as speakers or similar audio output devices. Some embodiments include devices such as a touchscreen that function as both input and output devices. 
     Finally, as shown in  FIG. 26 , bus  2605  also couples electronic system  2600  to a network  2625  through a network adapter (not shown). In this manner, the computer can be a part of a network of computers (such as a local area network (“LAN”), a wide area network (“WAN”), or an Intranet, or a network of networks, such as the Internet. Any or all components of electronic system  2600  may be used in conjunction with the invention. 
     Some embodiments include electronic components, such as microprocessors, storage and memory that store computer program instructions in a machine-readable or computer-readable medium (alternatively referred to as computer-readable storage media, machine-readable media, or machine-readable storage media). Some examples of such computer-readable media include RAM, ROM, read-only compact discs (CD-ROM), recordable compact discs (CD-R), rewritable compact discs (CD-RW), read-only digital versatile discs (e.g., DVD-ROM, dual-layer DVD-ROM), a variety of recordable/rewritable DVDs (e.g., DVD-RAM, DVD-RW, DVD+RW, etc.), flash memory (e.g., SD cards, mini-SD cards, micro-SD cards, etc.), magnetic and/or solid state hard drives, read-only and recordable Blu-Ray® discs, ultra density optical discs, any other optical or magnetic media, and floppy disks. The computer-readable media may store a computer program that is executable by at least one processing unit and includes sets of instructions for performing various operations. Examples of computer programs or computer code include machine code, such as is produced by a compiler, and files including higher-level code that are executed by a computer, an electronic component, or a microprocessor using an interpreter. 
     While the above discussion primarily refers to microprocessor or multi-core processors that execute software, some embodiments are performed by one or more integrated circuits, such as application specific integrated circuits (ASICs) or field programmable gate arrays (FPGAs). In some embodiments, such integrated circuits execute instructions that are stored on the circuit itself. In addition, some embodiments execute software stored in programmable logic devices (PLDs), ROM, or RAM devices. 
     As used in this specification and any claims of this application, the terms “computer”, “server”, “processor”, and “memory” all refer to electronic or other technological devices. These terms exclude people or groups of people. For the purposes of the specification, the terms display or displaying means displaying on an electronic device. As used in this specification and any claims of this application, the terms “computer readable medium,” “computer readable media,” and “machine readable medium” are entirely restricted to tangible, physical objects that store information in a form that is readable by a computer. These terms exclude any wireless signals, wired download signals, and any other ephemeral signals. 
     While the invention has been described with reference to numerous specific details, one of ordinary skill in the art will recognize that the invention can be embodied in other specific forms without departing from the spirit of the invention. In addition, a number of the figures (including  FIGS. 3 ,  13 ,  16 ,  20  and  22 ) conceptually illustrate processes. The specific operations of these processes may not be performed in the exact order shown and described. The specific operations may not be performed in one continuous series of operations, and different specific operations may be performed in different embodiments. Furthermore, the process could be implemented using several sub-processes, or as part of a larger macro process. Thus, one of ordinary skill in the art would understand that the invention is not to be limited by the foregoing illustrative details, but rather is to be defined by the appended claims.

Metadata:
Filing Date: 20121219
Publication Date: 20150421
Grant Date: 20150421
Priority Date: 20121219
Inventors: WANG XIAOHUAN C.
MEANEY BRIAN
STERN MICHAEL P.
SUNDARAM VIJAY
Assignee: APPLE INC
CPC Classifications: [{"code": "H04N5/76", "inventive": true, "first": false, "tree": "[]"}, {"code": "G11B27/34", "inventive": true, "first": false, "tree": "[]"}, {"code": "G11B27/031", "inventive": true, "first": false, "tree": "[]"}, {"code": "G11B27/005", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N9/79", "inventive": true, "first": true, "tree": "[]"}, {"code": "G11B27/005", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N9/79", "inventive": true, "first": true, "tree": "[]"}, {"code": "G11B27/34", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N5/76", "inventive": true, "first": true, "tree": "[]"}, {"code": "G11B27/005", "inventive": true, "first": false, "tree": "[]"}, {"code": "G11B27/34", "inventive": true, "first": false, "tree": "[]"}, {"code": "G11B27/031", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N5/76", "inventive": true, "first": false, "tree": "[]"}, {"code": "G11B27/031", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 50930991