Video special effects apparatus

Abstract: A wipe-only effects apparatus comprises a video mixer having video input terminals for re­ ceiving respective input video signals. The mixer combines the input video signals in dependence upon a control signal received at a control input termi­ nal of the mixer. A ramp generator generates a repetitive ramp signal and applies it to a control input terminal of the video mixer as the control signal.

Description: This invention relates to video special effects apparatus. Background of the Invention A conventional video production switcher receives several input video signals and combines them under control of one or more control signals to generate an output video signal. For example, the switcher might receive two full-field program video signals and two full-field matte signals and the control functions might enable the output video signal to be formed by placing a portion of the scene represented by video 1 into a selected area of the scene represented by video 2, or replacing the background of the video 1 scene with matte 1 or matte 2. In the conventional production switcher, each mixing function is accomplished using a video mixer which receives two video signals (either or both of which might be a matte signal) and a control signal. If the video signals received by the mixer are video 1 and matte 1 and the control signal is control 1, and the dynamic range of each signal is limited to 1 volt within the active line interval, then the voltage of the mixer's output video signal during the active line interval is video 1 × control 1 + matte 1 (1 - control 1) For the sake of simplicity, a reference hereinafter to a video or matte signal may include not only the signal itself but also the scene represented by the signal. Because a wide range of effects can be carried out using a production switcher, the waveforms of the control signals that are applied to the mixers of a production switcher are potentially very complex. One of the effects that can be accomplished using a production switcher is a vertical wipe. When an input signal representing a first scene is wiped vertically to an input signal representing a second scene, the scene represented by the output signal changes from the first scene to the second scene over several frames, and the transition between scenes is horizontal. A vertical wipe is accomplished using a control signal of which the voltage level remains essentially constant during any one line interval, the voltage level being 1 volt at one edge of the frame, e.g. the upper edge, being 0 volts at the opposite (lower) edge of the frame, and having a linear transition between 1 volt and 0 volts over a few lines at a vertical level that progresses from top to bottom of the screen. In the region within which the control signal changes from 1 volt to 0 volts, the two video signals are mixed in linear fashion. The vertical wipe is a relatively simple effect, but carrying out a vertical wipe using a conventional production switcher uses substantial mixing resources of the switcher, and therefore precludes use of the switcher to carry out more complex and demanding effects. US-A-4,223,351 discloses a video signal processing circuit having a wipe and key switcher controlled by a digital signal, whereby in a first state of the digital signal a first input video signal is passed by the switcher to a switcher output and in a second state of the digital signal a second input video signal is passed by the switcher to the switcher output. A wipe effect is achieved by varying the point within an active video region at which the transition from the first to the second digital state occurs. To achieve a so-called "soft-edge" effect there may be a region of the image where portions of the image from the first video signal and the second video signal alternate. This effect is achieved by switching between the input signals. US-A-4,223,351 also discloses a dissolve switcher controlled by a ramp signal whereby as the ramp signal ramps up an output signal of the dissolve switcher consists of an increasing proportion of one input video signal with a complementarily decreasing proportion of another input video signal. During a dissolve effect only a single ramp signal is used whereby the voltage applied to the dissolve switcher increases or decreases from frame to frame. WO-A-86/05646 discloses apparatus for generating a rotating clock video wipe from two voltage ramps. The ramps are rotated according to rotation data applied by a controller and the rotated ramps are combined and fed to an output signal generator. The output signal generator produces mixer control signals in dependence on an operator control signal and the signal derived from the combined rotated ramps. Summary of the Invention According to the present invention there is provided a video effects apparatus for producing a video effect, comprising a video mixer having first and second video input terminals for receiving first and second input video signals respectively, and a video output terminal for providing an output video signal, said video mixer further having a control input terminal for receiving a control signal, the video mixer providing the output video signal as a combination of the first and second input video signals in dependence upon the control signal, characterized by a ramp generator for generating a ramp signal having a start point which defines a repetitive line within the active interval of the field during said video effect and an end point which is caused to vary for producing said effect, the ramp signal being applied to the video mixer as the control signal, so that the output video signal between said repetitive start point and said varying end point of said ramp has complementary proportions of said first and said second input video signals, said proportions being defined by said ramp signal. Brief Description of the Drawings For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which: FIG. 1 is a block diagram of video special effects equipment including a wipe-only effects apparatus embodying the present invention. FIG. 2 shows waveforms for explaining the operation of the wipe-only effects apparatus, and FIG. 3 shows a waveform for explaining other modes of operation of the apparatus. Detailed Description The illustrated special effects equipment comprises a production switcher 8 and a video mixer 2. The mixer 2 receives two input video signals (video 1 and video 2) from respective signal sources and provides a background wash video signal to the production switcher. Video 1 is provided by matte generator 4 which generates a signal representing a color selected by the operator of the apparatus through an operator interface 10. Depending on the state of a switch 12, video 2 may be a black burst signal, i.e. a signal containing only sync and burst and therefore representing the color black, or a signal generated by a second matte generator 14 and representing a second operator-selected color. Each of the matte generators 4 and 14 may be of the form shown in US-A-4,758,880, the disclosure of which is hereby incorporated by reference herein. The video signals that are applied to the mixer 2 are synchronized with each other in the manner described in the co-pending application. In FIG. 2, the duration of a field of the background signal provided by the mixer 2 is represented by T F , while T A represents the active picture interval of the field. The production switcher receives the background wash signal and other video signals from various other sources (not shown) and combines its input video signals under control of signals provided by the operator interface 10 to generate an output video signal at a terminal 16. The output video signal may be applied to a videotape recorder or other utilization device (not shown). The mixer 2 combines the video 1 and video 2 signals in response to a wipe control signal provided by a ramp generator 20. A ramp rate signal is provided by an angle encoder 26 in dependence upon the angular position of a control knob 28 and defines a line within a field of the background signal. As the control knob is rotated from a start position to an end position, the line defined by the ramp rate signal sweeps through the field from one vertical blanking interval to the next. The ramp generator receives a ramp start signal from a switch 36. For the purpose of the following description, it will be assumed that the wipe is from 100% video 1 to 100% video 2, and that the wipe starts at the top of the scene represented by the output signal of the mixer 2. It will also be assumed that in order to increase the contribution of video 2 to the output signal, the voltage of the wipe control signal received by the mixer 2 must decrease. Based on the foregoing assumptions, before the wipe is initiated the line defined by the ramp rate signal is within the blanking interval of the current field and the voltage of the wipe control signal is 1 v over the entire field (FIG. 2(a)). In order to accomplish the desired wipe, control knob 28 is rotated so that the ramp rate signal defines a line that is within the active interval of the field. The voltage level of the wipe control signal is brought to 0 v during the vertical interval. The switch 36 is triggered by the vertical sync signal and starts the ramp generator at the start of the active interval. The voltage of the wipe control signal increases monotonically from 0 v and reaches 1 v on the line defined by the ramp rate signal (FIG. 2(b)). As the voltage of the wipe control signal increases, the relative contributions of video 1 and video 2 to the output signal provided by the mixer 2 change. By smoothly rotating the control knob through its range of movement, the operator is able to cause the line defined by the ramp rate signal to sweep smoothly forwards from the first line of the field. As the line advances relative to the beginning of the current field, the slope of the ramp decreases (FIG. 2(c)-(f)) to zero, at which point the output signal of the mixer 2 is 100% video 2. In order to wipe back to video 1, the video inputs of the mixer 2 may be switched or the wipe control signal may be inverted (FIG. 3). If mixing of two video signals is controlled using a ramp signal having a simple linear waveform, with a discontinuity in slope at the end that occurs later in time, i.e. at 1 v in the case of a ramp increasing from 0 v to 1 v and at 0 v in the case of a ramp decreasing from 1 v to 0 v, objectionable visual effects may be observed. It is therefore desirable that the ramp generator not generate a simple linear ramp but a ramp having smooth transitions to zero slope at the end that occurs later in time if that end of the ramp occurs within the active interval of the field. The ramp circuit may be of the form shown in US-A-4,859,955 the disclosure of which is hereby incorporated by reference herein. By use of the illustrated wipe-only effects apparatus, it is possible to generate at the output of the mixer a video signal representing a fade from the color represented by the signal provided by the source 4 to black or to another color, represented by the signal provided by the matte generator 14. This commonly-used video background signal is generated without using the mixing resources of the switcher 8. The invention is not restricted to wipe-only apparatus that is only capable of performing a vertical wipe, and may be applied also to wipe-only apparatus that is capable of performing only a horizontal wipe or both a horizontal wipe and a vertical wipe, either simultaneously or sequentially. If the apparatus is capable of performing both a horizontal wipe and a vertical wipe, separate vertical and horizontal ramp generators may be required.

Claims: A video effects apparatus for producing a video effect, comprising a video mixer (2) having first and second video input terminals for receiving first and second input video signals (VIDEO 1, VIDEO 2) respectively, and a video output terminal for providing an output video signal, said video mixer further having a control input terminal for receiving a control signal, the video mixer providing the output video signal as a combination of the first and second input video signals in dependence upon the control signal, characterized by a ramp generator (20) for generating a ramp signal having a start point which defines a repetitive line within the active interval of the field during said video effect and an end point which is caused to vary for producing said effect, the ramp signal being applied to the video mixer as the control signal, so that the output video signal at any instant of time between said repetitive start point and said varying end point of said ramp has complementary proportions of said first and said second input video signals, said proportions being defined by said ramp signal. An apparatus according to claim 1, wherein the first and second video signals (VIDEO 1, VIDEO 2) each convey picture information during a succession of active picture intervals and the ramp generator (20) is operative to generate a ramp signal without slope discontinuities within the active picture intervals of the first and second video signals. Apparatus according to claim 1, wherein the first and second video signals (VIDEO 1, VIDEO 2) are each composed of a succession of fields, each field being composed of a succession of line intervals, and wherein the ramp generator (20) is operable to generate a ramp signal having a transition from a first voltage level to a second voltage level over an interval having the duration of several line intervals of the video signals. Apparatus according to claim 1, wherein the first and second video signals (VIDEO 1, VIDEO 2) are each composed of a succession of fields, each field being composed of a succession of line intervals and each line interval having a blanking interval and an active picture interval, and wherein the ramp generator (20) is operable to generate a ramp signal having a transition from a first voltage level to a second voltage level over an interval that is substantially shorter than the active picture interval of the video signals. Apparatus according to any preceding claim further comprising a production switcher (8) having a plurality of input terminals (VIDEO IN), one of the input terminals being connected to the video output terminal of the video mixer and at least one other of the input terminals receiving another video signal, having a video output terminal for providing video output signal (VIDEO OUT), and having at least one control input terminal (CONTROL IN) for receiving an external control signal.

Inventors: HALL RANDALL K, MCFETRIDGE GRANT T, HALL, RANDALL K., MCFETRIDGE, GRANT T.
Applicants: GRASS VALLEY GROUP, THE GRASS VALLEY GROUP, INC.
Application year: 1988
Main classification: H04N5
IPC codes: H04N5