Abstract:
In a video camera the effects of global motion of a subject being photographed are accommodated by varying the pixel definition or focus of a digitally transmitted video image in response to frame-to-frame changes falling outside of a predetermined range. Specifically this is accomplished by sensing changes in the image content between successive images, determining whether the extent of that change lies within the predetermined range, and adjusting the pixel definition when the extent of change lies outside the range, and maintaining a given pixel definition when the extent of change remains within a predetermined range.

Description:
FIELD OF THE INVENTION 
     This invention relates to dynamic video focusing controls for video cameras, and particularly to accommodating rapid changes in scenes being taken by video-telephone cameras operating on transmission lines with predetermined channel capacity. 
     BACKGROUND OF THE INVENTION 
     Video-telephone cameras have substantial capacity to capture the illumination over a number of pixel sites, and to divide the analog voltage representing the illumination amplitude at each pixel into a multiplicity of values represented digitally by a number of bits. They can also produce a large number of frames representing each image captured. The total number of bits per unit time then equals at least to the total number of bits per pixel times the number of pixels per frame times the number of frames per unit time. 
     The degree to which the amplitude of the analog signal appearing at each pixel site may be subdivided determines the pixel definition. For example, the amplitude may be divided into 128 values with 7 bits. This produces a fine gray scale gradation which a viewer may perceive as high definition and “fine” focus. On the other hand, dividing the analog amplitude at a pixel site into only two values with a single on off bit produces a high contrast that may be perceived as poor or “soft” focus. 
     Similarly, the frame rate can also be changed. A high frame rate assures smooth motion while a low frame rate may produce flicker or perception of jerky motion. 
     The capacity of a video-telephone camera to generate high bit rates from the product of the number of bits per pixel site, the number of pixels per frame, and the frame rate, exceeds the capacity of most transmission channels to carry the bit rate. Accordingly, a coder is used to reduce the number of pixel levels which the amplitude at any pixel site is divided, i.e. the number of bits per pixel sites, and/or the frame rate. The number of bits per pixel site is generally referred to herein as the pixel definition or focus. The coder selects a suitable pixel definition or focus relative to a particular frame rate on the basis of the channel capacity. 
     A typical coder operates according to a conventional CCITT standard. In that environment, the coder uses a predetermined relationship between pixel definition and frame rate. It is possible for the camera user to preset the relationship between the pixel definition or focus and the frame rate on the basis of the user&#39;s perception. However, such arrangement cannot accommodate rapid changes in scenes being taken without overloading the channel capacity or producing undesirable picture quality. 
     SUMMARY OF THE INVENTION 
     According to an embodiment of the invention video images are digitally transmitted by setting a pixel definition and a frame rate to a data stream of video images on the basis of available channel capacity, sensing changes in the content between successive image frames, determining whether the changes lie within a predetermined range, and adjusting the pixel definition if the changes lie outside the range. 
     The various features of novelty which characterize the invention are pointed out in the claims. Objects and advantages of the invention will become evident following the detail description when read in light of the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a system which constitutes an embodiment of the invention. 
     FIG. 2 is a flow chart of the operation of the system in FIG.  1 . 
     FIG. 3 is a diagram of the building blocks of an algorithm used in the system of FIG.  1 . 
     FIG. 4 is a flow chart of the operation of a block in FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In FIG. 1, a video-telephone VT 1  includes a camera CA 1 , a coder CO 1  and a receiver RE 1 . The camera CA 1  captures images on an internal photosensing device SD 1  such as a CCD which produces a predetermined number of bits per pixel of information in the captured image. This results in a data stream composed of successive frames. At the camera output, the total number of bits per pixel is maximum and the frame rate is the maximum capable of being produced by the camera. The bit stream passes through the coder C 01  and then a channel CH 1  which may for example be a telephone link, to a video-telephone VT 2  composed of a receiver RE 2  a camera CA 2  and a coder C 02 . The receivers RE 1  and RE 2  may take the form of a monitor or screen. In the video-telephone VT 2 , a camera CA 2  and a coder C 02  on the end of the receiver RE 2  transmits through a channel CH 2  to the second receiver RE 1  at the end of the camera CA 1 . 
     The following description refers to the operation of the members CA 1 , C 01 , CH 1  and RE 2 . The members CA 2 , C 02 , CH 2 , and RE 1  operate in the same manner. Also the channels CH 1  and CH 2  may exist in a common connection or link. 
     The total bits per unit time, i.e. the total number of bits, times the total number of pixels, times the frame rate usually exceeds the capacity of the channel, i.e. the channel capacity, of the channel CH 1 . Accordingly, the coder CO 1 , which may be made a part of the camera or separate therefrom, is set to reduce the number of bits per pixel, or to reduce the frame rate, or to reduce both, so that the total bitrate lies within the channel capacity. 
     Reducing the pixel definition softens the focus. Increasing the pixel definition sharpens the focus. The frame rate may also be changed. A high frame rate produces the perception in the viewer of smooth motion on the screen. On the other hand, a low frame rate may result in the perception of jerkiness in the image when portions of the images are moving. The embodiments of the invention are based upon the understanding that a viewer will accept soft focus in a scene having substantial movement, but would require sharper focus for a still scene. 
     In the embodiment of FIG. 1, the coder CO 1  determines the pixel definition and the frame rate on the basis of changes in the image content from image to image. In order to reduce the number of bits per frame, thereby allowing an increase in the bit rate or frame rate for a particular channel capacity, the camera CA 1  or coder CO 1  divides each frame into blocks. Each block typically represents an area of 8 by 8 pixels. The coder CO 1  then determines if any change has occurred in any one block from one frame to the next frame. If no change takes place from one frame to the next, as may occur in blocks sensing a stationary background, the coder CO 1  simply sends a no-change signal for that block. This avoids the need, in that frame to retransmit the bits representing the unchanged luminance level for the 8×8 or 64 pixel sites in that block. 
     If most or all blocks in a frame have unchanged luminous levels, the number of bits per pixel, i.e. pixel definition, or focus, can be made high for a particular frame rate. In one embodiment the coder CO 1  sets the focus for each frame and sends a signal to the receiver RE 1  indicating the focus, i.e. pixel definition, and frame rate. In another embodiment the coder CO 1  divides the frame into sections, such as three sections, and sets a new focus for each section. The coder CO 1  signals the receiver RE 1  the focus for each section and the frame rate for each frame. 
     The coder CO 1  includes a microprocessor which achieves its ends as shown in the flow chart of FIG. 2 illustrating the operation of the coder CO 1 . In step RD 1  the coder receives the data stream for the camera CA 1 . In step SP 1 , the coder CO 1  sets a pixel definition or focus. At a start of an operation this could be preset at any value. In step SF 1 , the coder CO 1  sets a frame rate on the basis of a pixel definition which conforms the frame rate to the channel capacity of the channel CH 1 . At the outset, the steps CO 1  and SF 1  may be reversed, i.e, the frame rate set first, or coincidental. In step ST 1  the coder CO 1  stores the contents of the blocks in the last frame generated. In step SC 1 , the coder CO 1  senses the changes in the image content of corresponding blocks in succeeding frames. In step TR 1  the coder CO 1  transmits the data concerning the images. If there the image content of corresponding blocks in succeeding frames remains constant, that is, the image remains static, the coder CO 1  in step ST 1  merely transmits a signal to indicate no change. It does not resend the image content of the block. In this manner, it saves channel space. If there are changes in the block the coder CO 1  transmits the changes in the block or retransmits the block. In step ST 1  the coder CO 1  also transmits details of how the image is quantized. 
     After transmission of signals representing all the blocks in a frame, the coder CO 1  determines if the number of changed blocks lie within a predetermined range such as 20% to 80%. It does this by comparing the number of changed blocks with a high threshold and a low threshold that define the range. The high threshold may for example be 80% of all blocks, and the low threshold 20% of all blocks. In step CA 1  the coder CO 1  asks whether the number of changed blocks exceeds the high threshold. If yes, the coder CO 1  proceeds to step RP which sends a reduce signal to step SP 1  and resets the pixel definition or focus at a reduced value. If the answer is no, the coder proceeds to step CB 1  where it asks whether the number of change blocks lies below the low threshold. If yes, it proceeds to step IP 1  which sends an increase signal to step SP 1  therefore sharpening the focus or increasing pixel definition. If the answer is no in step CB 1 , the combination of step CA 1  and CB 2  indicate that the number of changed blocks is within the predetermined range, i.e. between the thresholds, and the operation returns to step SC 1  for sensing succeeding changes. 
     In yet another embodiment of the invention, the process is the same as in FIG. 2 except that the frames are divided into thirds as occurs in many video-telephone operations. In this embodiment, the pixel definition and focus may be changed after a third of each frame. 
     Because the changes in image content are determined on the basis of blocks of 8×8 pixels, the comparison is made of corresponding blocks of succeeding frames. Thus the coder CO 1  maintains an entire frame in memory for comparing the blocks of the succeeding frame with the previous frame. As stated, if appropriate, in the transmit step TR 1 , the coder CO 1  transmits a no change signal in place of the image content for each block at a time. In step SF 1  the frame rate is determined by a known averaging or quantizing method based upon the numbers of changes of each block. 
     FIGS. 3 and 4 illustrate the operation of the coder CO in more detail. FIG. 4 illustrates the building blocks of the operation of the coder CO 1  showing a video transform coding algorithm. In FIG. 4 all the blocks but the global motion control GMC are those defined in CCITT standard COMX XV-R 37-E and known. Details of the block GMC appear in FIG.  4 . FIG. 3 is composed of a transform T, a quantizer Q, a picture memory P with motion compensated variable delay, a loop filter F, a coding control CC, and inverse quantizer Q −1 , and an inverse transform T −1 . The primary building blocks are the transform T itself and the quantizer Q. This operation is well known and set forth in the aforementioned CCITT standard. Operated in a differential mode, the inverse quantizer Q −1  and inverse transform T −1  place a decoded copy of the previously transmitted image (i.e. frame) in the picture memory P to serve as a reference for coding in the next frame. Only those blocks that have changed significantly from frame to frame are transmitted to conserve bit rate. 
     The coding block CC maintains a constant channel rate amid widely varying scene content and bit rate generated by the coder. It does so by varying the coding quality (i.e. quantizer step size, frame rate) depending on the amount of data awaiting transmission relative to the channel capacity (i.e. buffer fullness). 
     The global motion control GMC serves as a feedback mechanism from which the coding control CC can determine the amount of global motion in the scene and adjust the focus setting accordingly. The global motion is estimated from the number of blocks that have significantly changed from one frame to the next. This is determined by the output p representing the inter/intra flag. Very few occurrences of the intraflag indicate relatively slow motion and the global motion control GMC sets guidelines for sharp focus. On the other hand, a large percentage of block changes indicate global motion and the global motion control GMC sets the guidelines for soft focus. The global motion control feedback mechanism removes all burdens of focus control from a user and provides a means for controlling the bit rate more consistently with scene content. 
     FIG. 4 illustrates the details of the global motion control GMC. Here, in step SF 2  the global motion control GMC goes to the start of the frame. In step EB 1  it encodes a block and in step IF, it asks whether the output p produces an intraflag i.e. an indication that the block image has changed significantly from the corresponding image in the last frame. If the answer is yes, the control GMC goes to step IC 1  and increments the count. It then goes to step EF 1  to ask if this is the end of the frame. If the answer in step IF 1  is no, the control GMC also goes to step EF 1  to ask if this is the end of the frame. If the answer is no, the steps EB 1 , IF 1  and IC 1  are repeated. If the answer is yes, that it is the end of the frame, the control GMC asks in step GC 1  whether the intraflag count atp is greater than a predetermined value T H . If yes, the control GC 1  goes to step DF 1  to decrease the focus. If the answer to step GC 1  is no, the control GMC proceeds to step CL 1 , to ask whether the count is less than a threshold T L . If the answer is yes, the control GMC increases the focus in step IN 1 . If the answer is no, the control GMC proceeds to the start of the frame. After step IN 1  the control GMC also proceeds to start of the frame. The threshold T H  may for example be 80%, that is 80% of the blocks in the image have changed. The focus is then decreased toward soft. If on the other hand the number of blocks falls below on a lower threshold such as 10% or 20% the image, the focus setting is increased toward sharp. The thresholds T H  and T L  define the range with which the number of significant changes are acceptable without need to alter the operator. Normally the scene content will change in 30% to 50% of the image area depending on the application, and the focus setting remains the same. 
     The predetermined range is set in the in the coder C 01 . The invention is not limited to the predetermined ranges and percentages mentioned. According to various embodiments of the invention, the ranges are set for particular circumstances dictated by the expected change in the scene, the channel capacity, and possibly the subjective values of the system designers or the user. The lower threshold may be set at values as low as one or two percent and as high as 60% or even higher. The higher threshold may be set as high as 90% and low as 30% or even lower. Of course the lower threshold must always be less than the higher threshold. According to an embodiment of the invention the coder contains manual setting knobs or rings that allow a user to change the thresholds that define the ranges. 
     The previous description refers to the operation of the members CA 1 , CO 1 , CH 1  and RE 2 . As stated, the members CA 2 , C 02 , CH 2 , and RE 1  operate in the same manner. Also the channels CH 1  and CH 2  may exist in a common connection or link. 
     The invention avoids the difficulties that arise in instances when the scene complexity does not match the focus setting at the coding control CC. This focus setting may be set by a user or may be preset. The invention avoids updating the entire scene at sharp focus during global motion and thus prevents consuming all of the available bit rate which would slow down the frame rate to the point of still images. The invention takes advantage of the loss of spatial fidelity is less evident during global motion. It sets the bit rate spent at soft focus and updates the images more frequently for better motion rendition. 
     The embodiments of the invention, during a relatively still scene, update the images at sharp focus and utilize the available bit rate by rendering fine spatial fidelity until motion resumes. 
     While embodiments of the invention have been described in detail, it will be evident to those skilled in the art that the invention may be embodied otherwise without departing from its spirit and scope.