APPARATUS FOR COMBINING VIDEO SIGNALS

Abstract: A first shaped video signal V₁K₁ and a second shaped video signal V₂V₂, are combined so as to generate a third shaped video signal V₃′ given by V₃′ = V₁ K₁ [l - K₂(l - P₁₂)] + V₂K₂(l - K₁P₁₂) where P₁₂ is a priority signal. a key signal K₃ given by K₃ = l - (l - K₁)(l - K₂) is also generated. An output processor receives the shaped video signal V₃′, the key signal K₃ and a matte signal M₃. In a first mode of operation, the output video signal V₃″ of the output processor is given by V₃′′ = V₃ + M₃(l - K₃) and the key signal K₃′ that is generated by the output processor has a constant value, and in a second made of operation of the output processor the output video signal is given by V₃′′ = V₃′/K₃ and the key signal K₃′ is directly proportional to K₃.

Description: This invention relates to apparatus for combining video signals. In a video mixer, two video signals V₁ and V₂ (Fig. 1) are multiplied by a key signal K₁ (having a range of values from 0 to 1) and its complement (1 - K₁) respectively, and the two signals V₁K₁ and V₂(1 - K₁) are additively combined in a summer to produce a composite output signal V q having the form V₁K₁+ V₂(1 - K₁). When the key signal K₁ is zero, the input signal V₁ makes no contribution to the signal V q , regardless of the value of V₁. Similarly, if K₁ is one, the signal V₂ makes no contribution to the signal V q . The proportion of the signal V q that is contributed by V₁ determines the opacity with which the scene represented by the signal V₁ is perceived in the composite picture. If K₁ is one, i.e., V₁ represents 100% of the signal V q , then the V₁ scene (the scene represented by the signal V₁) completely obscures the V₂ scene, regardless of the value of V₂. As K₁ decreases, the extent to which the V₂ scene is obscured in the composite picture is reduced until, when K₁ reaches zero, the V₂ scene is opaque and completely obscures the V₁ scene. Thus, the coefficients K₁ and (1 - K₁) determine the relative opacity of the two component scenes: if the coefficient K₁ is greater than (1 - K₁), then the V₁ scene at least partially obscures the V₂ scene and appears, to a viewer of the composite scene, to be in front of the V₂ scene. The multiplication of the signals V₁ and V₂ by the key signal K₁ and its complement (1 - K₁) is shown in FIG. 1, in which it is assumed that all signals have five discrete values in the range from zero to unity and have sharp transitions between levels. It will, of course, be appreciated that FIG. 1 is in fact very much simplified, and that in the case of analog signals the range of possible values is continuous, and that transitions for either analog or digital signals would have a finite slew rate. A video signal V₁' is said to be a "shaped" video signal when it is the multiplication product of an unshaped video signal V₁ and an associated key signal K₁. In general, there is no necessary relationship between the video signal and its associated key signal. A production switcher normally receives unshaped video signals and their associated key signals and provides a full screen video signal at its output. No key output is produced. Shaping has two aspects, namely spatial or X - Y shaping (only the X-dimension is shown in FIG. 1), which determines the area of the composite picture to which the component signal makes a contribution (when K₁ = 0, the signal V₁ makes no contribution to the signal V q ), and opacity or Z shaping, which determines, for K₁ greater than zero, the magnitude of the contribution that is made by the component signal to the composite signal V q . The shaping of the component signals is discussed in terms of "coverage" in Porter, T. and Duff, T., "Compositing Digital Images", Computer Graphics, Vol. 18, No. 3 (1984), pages 253 to 259. The foregoing discussion of the manner of production of the signal V q is based on the assumption that the signal V₂ is a full field signal, i. e. that the key signal K₂ associated with the video signal V₂ is one for all locations. In the general case, K₂ is not one for all locations and V q = V₁K₁ + V₂K₂ (1 - K₁) It will be seen from this more general expression that the video signal V q , being the weighted sum of two shaped video signals V₁K₁ and V₂K₂, is itself a shaped video signal. For the sake of consistency in notation, the shaped signal that has previously been designated V q will hereafter be designated V q ', and V q will hereafter be used to designate the corresponding unshaped signal. The key signal K q that relates V q ' to V q is given by K q = 1 - (1 - K₁)(1 - K₂) If, for every location, either K₁ or K₂ is one, then K q = 1 for all locations. In particular, if either V₁ or V₂ is a full field signal, the signal V q ' is a full field signal. If, on the other hand, V q ' is not a full field signal it might be desired to form a composite scene from the scenes represented by the signal V q ' and, e.g., a background scene represented by a signal V r having an associated key signal K r . In such a case, the signal (1 - K q ) would be used to process the signal V r in a production switcher, and an output signal V s ' = V q ' + V r K r (1 - K q ) would be produced. Generally, V r would be a full field signal and so K r = 1 and V s ' = V q ' + V r (1 - K q ) . Recalling that V q ' = V₁K₁ + V₂K₂(1 - K₁) , if K₁ = 1, then V q ' = V₁, i.e. the signal V₂ makes no contribution to the signal V q ', regardless of the value of K₂. Therefore, combining of the video signals V₁ and V₂ is under the primary control of the key signal K₁. Similarly, if the signal V q ' were equal to V₁K₁ (1 - K₂) + V₂K₂, the combining would be under the primary control of the signal K₂, and if K₂ = 1, then V q ' = V₂ and V₁ makes no contribution regardless of the value of K₁. The two different situations are equivalent respectively to the V₁ scene and the V₂ scene being in the foreground of the composite scene. However, the conventional mixer does not allow the operator to control on a dynamic basis whether the mixing operation is under the primary control of the signal K₁ or of the signal K₂. According to the present invention there is provided an apparatus for combining a first shaped video signal V₁K₁ and a second shaped video signal V₂K₂ in accordance with a priority signal P₁₂ and with first and second key signals K₁ and K₂, where V₁ is a first unshaped video signal, V₂ is a second unshaped video signal, K₁ is a first key signal associated with the first unshaped video signal and K₂ is a second key signal associated with the second unshaped video signal, comprising: means for generating from said first and second shaped video signals, from said priority signal and from said first and second key signals a third shaped video signal V₃' given by V₃'=V₁K₁(1-K₂(1-P₁₂))+V₂K₂(1-K₁P₁₂) and means for generating from the first and second key signals a third key signal K₃ given by K₃=1-(1-K₁)(1-K₂). The value of the priority signal P₁₂ determines the relative weighting given to the key signals K₁ and K₂ in forming the third video signal V₃', and this in turn determines whether the scenes represented by the component signals V₁ and V₂ are perceived in the composite picture as representing foreground objects or background objects. The priority signal P₁₂ may be varied over several frames of the video signals, so that the background objects appear to pass through the foreground objects and become foreground objects themselves. In a split-screen effect, by having a change in the value of the priority signal P₁₂ occur at the split, objects that appear to be in the foreground on one side of the composite image can be made to appear in the background on the other side of the composite image, and vice-versa. It will be appreciated that in the context of the present invention, references to component video signals are intended to relate to signals that represent different scenes, and that references to a composite video signal are intended to relate to a signal that represents a scene formed by combining two or more scenes, as represented by respective component video signals. 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 shows waveforms to illustrate combining of video signals, FIG. 2 is a block diagram of a combiner cell for combining first and second component video signals, FIG. 3 is a block diagram of a combiner system comprising several combiner cells connected in a cascade arrangement, and FIG. 4 is a block diagram of an output processor for a combiner cell. The combiner cell shown in FIG. 2 comprises multipliers 4, 6, 8, 10 and 12, complement circuits 14, 16, 18, 20, 22 and 24, and a summer 26. The illustrated combiner cell operates in the digital domain with parallel data, and therefore all the signal lines that are illustrated would in fact be multiple conductor lines. Additional circuitry would be required to assure proper timing relationships among the various signals, but such matters are well within the skill of the art and therefore are not shown and will not be further described. The input signals of the combiner cell comprise two shaped video signals V₁′ and V₂′, associated key signals K₁ and K₂, and a priority signal P₁₂. The levels of the key signals K₁ and K₂ and the level of the priority signal P₁₂ are normalized to have maximum and minimum values that can be represented numerically as 1 and 0. Also, the video signals V₁′ and V₂′ have the same maximum and minimum values. Additional multipliers 2 and 3 are provided upstream of the combiner for generating the shaped video signals V₁′ and V₂′ from unshaped video signals V₁ and V₂ and the associated key signals K₁ and K₂. The combiner cell provides a shaped output video signal V₃′ and an output key signal K₃. It can readily be seen that the output video signal is given by the equation V₃′ = V₁′[1 - K₂(1 - P₁₂)] + V₂′(1 - K₁P₁₂) and that the output key signal is given by K₃ = 1 - (1 - K₁) (1 - K₂). The value of P₁₂ determines the weighting factors that are applied to the two video signals V₁′ and V₂′. If P₁₂ is equal to zero, this implies that the V₂ scene is in the foreground of the composite scene and that the V₁ scene is in the background, and vice versa if P₁₂ is equal to one. For P₁₂=0, then V₃′=V₁′(1-K₂)+V₂′ The value of K₂ defines areas in which the V₂ scene contributes to the composite scene. If K₂=1, the contribution of V₁ to the composite scene is zero and therefore the V₂ scene completely obscures the V₁ scene. If K₂=0, V₂′=0 and therefore there is no contribution from V₂ and V₁ is allowed to pass to V₃′ unaltered For P₁₂=1, then V₃′=V₂′(1-K₁)+V₁′ The value of K₁ defines areas in which the V₁ scene contributes to the composite scene. If K₁=1, the contribution of V₂ to the composite scene is zero and therefore the V₁ scene completely obscures the V₂ scene. If K₁=0, there is no contribution from V₁ and V₂ is allowed to pass to V₃′ unaltered. For P₁₂=0.5, then V₃′=V₁′(1-K₂/2)+V₂′(1-K₁/2) Where K₂=0, V₁′ is passed unaltered; where K₁=0, V₂′ is passed unaltered; and where K₂ > 0 and K₁ > 0, the relative opacities of the V₁ and V₂ scenes are determined by the ratio of K₁ and K₂. As P₁₂ increases from zero the relative depths of the V₁ and V₂ pixels in the composite image change, from the V₂ pixel appearing in front of the V₁ pixel, through the two pixels appearing to be at the same depth (at P₁₂=0.5), to the V₁ pixel appearing in front of the V₂ pixel. It will therefore be seen that the priority signal P₁₂ makes it possible to determine which of the component scenes will appear as the foreground scene in the composite picture. By changing the value of P₁₂, the composite picture can be changed so that a component scene is the foreground scene at one time and is the background scene at another time. Several combiner cells 30, 40 . . . 90 of the kind shown in FIG. 2 may be connected in cascade, as shown in FIG. 3, to form a combiner system. Output processors 32, 42 . . . 92 are associated with the combiner cells respectively, for a reason which will be explained below. The output signals from the output processors are connected to a production switcher. Conventional production switchers are designed to receive unshaped video signals and their associated key signals, and multiply the video signals by their key signals to produce shaped video signals that are combined with other shaped video signals, e.g., a signal generated by a digital video effects unit, to produce a final program video signal representing the desired composite picture. The output video signals provided by the combiner cells are already shaped by their respective key signals. If the signal V₃′, for example, is applied to a conventional production switcher it will be shaped a second time, and the result will be a black halo in the scene represented by the signal V₃′ for values of K₃ greater than zero and less than unity. The output processors are interposed between the combiner cells and the production switcher in order to generate unshaped video signals from the shaped video signals generated by the combiner cells. FIG. 4 shows the output processor associated with the combiner cell 30. The other output processors are identical. As shown in FIG. 4, the output processor 32 receives the shaped video signal V₃′ and the key signal K₃ from the associated combiner cell 30, and also receives a video matte signal M₃. The signal M₃ represents a background for the V₃ scene. The background may be, for example, a plain, solid color. The output processor comprises a summer 102, a multiplier 104, a divider 106 (implemented as a reciprocal look-up table 108 and a multiplier 110) and a complement circuit 112. In addition, the processor comprises a switch 114 for selecting one of two operating modes for the processor. The processor provides a composite output video signal V₃″ and a composite output key signal K₃′, which are applied as input signals to the production switcher. In the first mode of operation of the processor (background on), the signal V₃″ is given by V₃″ = V₃′ + M₃(1 - K₃). Thus, for any pixel for which K₃ is zero, V₃′ = 0 and V₃″ = M₃. If K₃ is not zero, indicating that V₃′ is non-zero, the relative contribution of V₃′ to V₃″ depends on the value of K₃, and it can be seen that for K₃ = 1, M₃ is not permitted to make a contribution and V₃″ = V₃′. The output key signal K₃′ is set to unity for the entire frame in order to inhibit the production switcher from attempting to add another background. In the second mode of operation (background off), the output key signal K₃′ is equal to K₃, and the production switcher will then add background to pixels for which K₃′ is not 1.0 in proportion to the value of (1 - K₃). The production switcher will multiply V₃″ by K₃′. Since K₃′ is equal to K₃, it is desired for the reason indicated above that V₃″ not be equal to V₃′. Therefore, in the second mode the output signal V₃′ from the summer 102 is divided by the key signal K₃, so that V₃″ = V₃′/K₃. Therefore, the signal V₃″ is equal to V₃ (which does not actually exist), and the switcher may multiply the signal V₃″ by K₃′ (which is equal to K₃) and produce the desired signal V₃′ at the switcher output. For pixels at which K₃ is close to zero, V₃″ is indeterminate, but this is not important to the final program video signal because these pixels make no contribution to that signal. It will be appreciated that the invention is not restricted to the particular apparatus that has been described and illustrated, and that variations may be made therein without departing from the scope of the invention as defined in the appended claims, and equivalents thereof. For example, instead of using interdependent processing circuits for generating, from K₁, K₂ and P₁₂, the multiplication factors for the video signals V₁ and V₂, these multiplication factors may be generated independently, as mix constants that are separately applied to the multipliers of a signal mixer that is of otherwise conventional form. It is not necessary that each combiner cell of the system shown in FIG. 3 should have its own output processor, since if the output is always to be taken from the same combiner cell, it is necessary only that an output processor be associated with that combiner cell. In a preferred implementation of the invention, the key and video output signals of the last combiner cell in the cascade are connected as inputs to the first combiner cell, so as to establish a closed ring. This provides additional flexibility, in that the ring can be logically broken at any point, allowing additional priority combinations. For example, in the arrangement shown in FIG. 3 the signal V₄′ can only be combined with the composite signal V₁′/V₂′, and the component signal V₉′ can only be combined with the composite signal V₁′/ . . ./V₈′. The scene represented by the signal V₉′ cannot be made to appear behind the V₁ scene but in front of the V₄ scene. However, if the switches 94 are operated so that the output signals V₉' and K₉ are applied to the combiner cell 30 in lieu of the input signals V₂′ and K₂, and P₁₂ and P₃₄ are set to unity, the desired composite signal would be provided at the output of the output processor 42 (V₅″, K₅′). The key signal K₈ must be forced to zero inside the combiner cell 90. It will also be appreciated that although the preferred embodiment of the invention, described above with reference to the drawings, is implemented using parallel digital data, it could also be implemented using serial digital data or analog data.

Claims: Apparatus for combining a first shaped video signal V₁K₁ and a second shaped video signal V₂K₂ in accordance with a priority signal P₁₂ and with first and second key signals K₁ and K₂, where V₁ is a first unshaped video signal, V₂ is a second unshaped video signal, K₁ is a first key signal associated with the first unshaped video signal and K₂ is a second key signal associated with the second unshaped video signal, comprising: means (6-10, 14-18, 26) for generating from said first and second shaped video signals, from said priority signal and from said first and second key signals a third shaped video signal V₃' given by V₃'=V₁K₁(1-K₂(1-P₁₂))+V₂K₂(1-K₁P₁₂) and means (12, 20-24) for generating from the first and second key signals a third key signal K₃ given by K₃=1-(1-K₁)(1-K₂). Apparatus according to claim 1, further comprising an output processor (32) for receiving the third shaped video signal V₃' and the third key signal K₃, said output processor being operative in a first mode to generate an output signal V₃'' given by V₃''=V₃'+A where A is a function of the third key signal and is independent of V₃', and a key signal K₃' having a constant value. Apparatus according to claim 2, wherein the output processor is operative in a second mode to provide the output signal V₃'' as a function that is directly proportional to the ratio of V₃'/K₃, and the key signal K₃'as a function that is directly proportional to the third key signal K₃. Apparatus according to claim 2, wherein the output processor has an input terminal for receiving a matte signal M₃ and is operative in the first mode to generate the output video signal V₃''where V₃''=V₃'+M₃(1-K₃). A combiner system for combining a plurality of shaped video signals in accordance with associated respective key signals and priority signals, comprising first and second apparatus according to claim 1, with the third video signal and the third key signal of the first apparatus connected as the first video signal and the first key signal of the second apparatus. Apparatus for combining more than two shaped video signals, each shaped video signal having an associated key signal, as a function of a plurality of priority signals characterized by: a plurality of combiner cells (30, 40, 90), each combiner cell comprising apparatus according to claim 1 to combine two shaped video signals and associated key signals as a function of one of the plurality of priority signals, the combiner cells being coupled in cascade so that a shaped video output signal and associated key signal from one cell is one of the shaped video signals and associated key signals input to a succeeding cell, the other input of each combiner cell being one of the shaped video signals and associated key signal to be combined; and a plurality of output processors (32, 42, 92), one for each of the combiner cells, having as inputs the output shaped video signal and associated key signal from the respective combiner cells and having as outputs an unshaped video signal and associated key signal. A combiner system as recited in claim 6 further comprising means (94) for applying the outputs from the last combiner cell in the cascade to the inputs of the first combiner cell in the cascade in lieu of one of the shaped video signals and associated key signal to establish a closed ring. Apparatus for combining more than two shaped video signals, each shaped video signal having an associated key signal, as a function of a plurality of priority signals characterized by: a plurality of combiner cells (30, 40, 90), each combiner cell comprising apparatus according to claim 1 to combine two shaped video signals and associated key signals as a function of one of the plurality of priority signals, the combiner cells being coupled in cascade so that a shaped video output signal and associated key signal from one cell is one of the shaped video signals and associated key signals input to a succeeding cell, the other input of each combiner cell being one of the shaped video signals and associated key signal to be combined; and an output processor having as inputs the output shaped video signal and associated key signal from a predetermined one of the combiner cells and having as outputs an unshaped video signal and associated key signal. A combiner system as recited in claim 8 further comprising means for applying the outputs from the last combiner cell in the cascade to the inputs of the first combiner cell in the cascade in lieu of one of the shaped video signals and its associated key signal to establish a closed ring.

Inventors: JACKSON, RICHARD A., WINDREM, KEVIN D.
Applicants: THE GRASS VALLEY GROUP, INC.
Application year: 1987
Main classification: H04N5
IPC codes: H04N5