GB2281834A - Generating a key signal for a digital video mixer - Google Patents

Abstract

To generate a modified key signal from a key signal associated with a series of samples of a video image, the value of the portion of the key signal (K) corresponding to a particular sample (D) is compared with the value of the portion of the key signal (K) corresponding to another sample (B) spaced in said video image a selected number of samples from said particular sample, and the value of the portion of said modified key signal (K*) corresponding to a sample (D*) is determined on the basis of the comparison, e.g. by a NAM method. The process may be applied to HDTV signals which are demultiplexed into four parallel data streams, and is useful for generating border key signals. <IMAGE>

Description

A METHOD OF. AND APPARATUS FOR. GENERATING A KEY SIGNAL FOR A DIGITAL VIDEO MIXER The present invention relates to the generation of a key signal for a digital video mixer, and particularly to the generation of a modified key signal for use in creating a border around an area and to the generation of a modified key signal from the key signal of a demultiplexed video signal.

It is often desired to generate a video signal in which one image is placed in another, for example to place a person, the foreground object, on a different background than that against which he/she was originally filmed.

To do this, a digital mixer is supplied with the foreground picture, the background picture and a key signal indicating how the images are to be mixed. The key signal varies in value between 0 and 1 over the area of the picture, 1 representing all foreground and 0 representing all background.

If an object is to be simply superimposed over the top of a background with no mixing, the key signal will be 1 over the area of the object and 0 elsewhere. Such a key signal is sometimes known as an area key signal.

In HDTV, video signals are transmitted at rates which are too high for normal processing and, hence, the video signals are demultiplexed four ways into four parallel data streams to enable "comfortable" processing. Each of the four "phases" is processed independently and this results in a problem when previous or following samples are important, for instance when a key signal exists in relation to a particular sample.

The aim of the present invention is to generate modified key signals from an area key signal to provide a variety of effects and more particularly to do so for demultiplexed video signals such as are used in HDTV.

According to the present invention there is provided a method of generating a modified key signal from a key signal associated with a series of samples of a video image, the method comprising the steps of: comparing the value of the portion of the key signal corresponding to a particular sample with the value of the portion of the key signal corresponding to another sample spaced in said video image a selected number of samples from said particular sample; and setting the value of the portion of said modified key signal corresponding to a sample on the basis of said comparison.

According to the present invention there is also provided an apparatus for generating a modified key signal from a key signal associated with a series of samples of a video image, the apparatus comprising: comparing means for comparing the value of the portion of the key signal corresponding to a particular sample with the value of the portion of the key signal corresponding to another sample spaced in said video image a selected number of samples from said particular sample; and setting means for setting the value of the portion of said modified key signal corresponding to a sample on the basis of said comparison.

Said series of samples may comprise demultiplexed video data formed as a plurality of, for instance, four parallel data streams and, where appropriate, the portions of the key signal corresponding to samples from different data streams are compared.

Thus, it is possible to produce the appropriate modified key signal in the demultiplexed format without converting the demultiplexed data into a single full rate stream for processing.

The process of generating border keys in this application preferably employs the method of a NAM (non-additive mix), which produces a logical 'OR' when considering a key signal which is either 0 or 1. That is to say a comparator technique where the largest input is passed through.

The invention may also use key signals having values between 0 and 1 so as to provide a smooth image transition.

The present invention will be further described hereinafter with reference to the following description of exemplary embodiment and the accompanying drawings, in which: Figure 1 shows a border to be generated by the invention; Figures 2 - 4 show data and key signals produced therefor by the invention; Figures 5 and 6 are schematic views of parts of the invention; Figure 7 shows alternative types of border generated by the invention; and Figures 8 - 10 show data and key signals produced therefrom; and Figure 11 is a schematic view of a part of a second embodiment adapted to produce the borders of Figure 7.

A normal border, see Figure 1, is a border 1 that encompasses a fill video area 2 equally horizontally and vertically. For full rate processing, this may be produced by a) delaying the key signal appropriately, b) comparing the delayed key signal with the original key signal and c) on the basis of the comparison, generating a border key signal defining the outer edge of the border. This process may use sample delays in the horizontal direction and line delays in the vertical direction.

High video data rates such as used for HDTV are too high for conventional data processing and are therefore demultiplexed, normally four ways, so as to allow "comfortable" processing of four parallel data streams. Figure 2 illustrates the full rate data FD with samples A, B, C and D in comparison with one of the four samples in one of the four parallel data streams.

In the case of four way demultiplexed data, by delaying a key signal of one phase by its sample period, effectively, the key signal is compared with that of 4 full rate samples earlier and hence only 4 pixel resolution is possible for horizontal borders. In order to achieve up to single pixel border resolution, it is proposed that horizontal borders are generated by comparing samples separated by a quarter rate clock i.e. by comparing samples of different "phases". Thus, to derive such resolution, outputs for each phase must be available and, more particularly, when borders of width greater than 3 pixels are required, for each phase, all outputs of the corresponding register file must be available for comparison.

Hardware limitations may be helped by using a lower resolution. For example, it may be appropriate only to have two pixel horizontal border resolution, since vertical interlace results in one line key expansion per field giving two lines on a frame.

The horizontal procedure is best illustrated by way of example. Figure 2 below shows quarter rate data QD, full rate data FD, the original key K, the desired expanded key K* and the output data OD.

The procedure is carried out on the basis of NAMing two samples. The method of a NAM (non additive mix) is a comparison of two samples which produces an output from the logical "or" of the input samples Thus, in the present implementation, only when both input samples do not carry a key signal, is the output zero.

To expand the key by two full rate pixels requires that the four phases, which will arrive at the same time, are NAMed in the following manner A* = A NAM C-1 = 1 C-1 = previous C sample B* = B NAM D-1 = 1 C* = C NAM A = 1 D* = D NAM B = 1 The result is a key expanded by two full rate pixels.

Figure 3 illustrates a second example with the key falling at a different phase.

A* = A NAM C-1 = 1 C-1 = previous C sample B* = 8 NAM D-1 = 1 C* = C NAM A = 1 D* = D NAM B = 0 Expanding the key in the other direction in time requires similar analysis.

A four cycle full rate expansion is shown in Figure 4 and the NAM becomes as follows: = = A NAM A-1 = 1 = B NAM B-1 = 1 C* = C NAN C-1 r 1 D* = D NAN D-1 = 1 and in the next cycle A* = A+1 NAN A = 1 B* = B+1 NAM B = O C* = C+1 NAM C = 0 D* = D+1 NAM D = 0 Vertically, of course, in the case of a normal border the analysis is similar, but other "phases" in the vertical NAM process are not necessary, since the resolution is line based whatever the horizontal sampling structure.

To summarise the above, Figures 5 and 6 show the architecture for Phase A horizontal and Phases A & C vertical NAMing for the generation of a normal border.

In Figure 5, Z is a sample register and, in Figure 6, H is a vertical register.

The normal border is derived by the combination of the horizontal and vertical NAM processes.

Figure 7 shows a drop border 3 and a drop shadow 4 for which the NAMing process requires a horizontal and vertical shift of the expanded phases. The NAMing process for a two sample drop shadow or border, is illustrated in Figure 8.

For phases A and B A* = A NAN C (H-1 + Z-1) = 1 = B NAM D (H-1 + Z-1) = 1 For Phases C and D: C* = C NAN A (H-1) = 1 D* = D NAM B (H-1) = 1 where H-1 represents the line register and Z-1 represents the sample register.

This generates a two full rate sample drop border/shadow.

For a four full rate sample NAM, as shown in Figure 9, it is only necessary to have a one sample shift (four full rate samples) and a two line delay (equivalent to 4 lines for a frame).

The output equations are: = A NAM A (H-2 + Z-1) = 1 B* = B NAN B (H-2 + Z-1) = 1 C* = C NAN C (H2 + Z-1) = 1 D* = D NAM D (H-2 + Z-1) = 1 For a six pixel wide drop Border/shadow as shown in Figure 10, the output equations are for cycle n+l: = A NAM C (H-3 + Z-2) = 1 = B NAM D (H-3 + Z-2) = 1 C* = C NAM A (H3 + Z-1) = 0 D* = D NAM B (H3 + Z-1) = 0 The general pattern for NAMing stages for drop border and drop shadow is shown in Figure 11, in which this is a vertical register.

The length of the line register file depends on the width of the drop border or shadow required. For the drop shadow the intermediate NAM stages are nulled.

To summarise the requirements for drop border/shadow implementations with a minimum resolution of two pixels in the horizontal direction is given in the generic equations below: A* = A NAN C (H-n + Z-1) B* = B NAM D (H-n + Z-1) C* = C NAN A (H-") D* = D NAN B (H-n) where n = border width/2 and drop shadow/border is delayed with respect to fill video.

For the general case of drop shadow/border, the equations have the following form: A* = A NAM C (H+/-n + ....etc.

Furthermore, for the symmetrical case m = n - 1.

For a drop shadow/border in the bottom left corner of Figure 7, the equations are A* = A NAM C (H-n + Z+1) .....etc.

For a drop shadow/border in the top left corner of Figure 7, the equations are A* = A NAM C (H+n + Z+1) ..........etc.

Claims (23)

1. A method of generating a modified key signal from a key signal associated with a series of samples of a video image, the method comprising the steps of: comparing the value of the portion of the key signal corresponding to a particular sample with the value of the portion of the key signal corresponding to another sample spaced in said video image a selected number of samples from said particular sample; and setting the value of the portion of said modified key signal corresponding to a sample on the basis of said comparison.

2. A method according to claim 1 wherein said series of samples comprises video data which has been demultiplexed so as to form a plurality of parallel data streams.

3. A method according to claim 2 wherein said video data is demultiplexed four ways into four parallel data streams.

4. A method according to claim 2 or 3 wherein said comparing step includes comparing portions of the key signal corresponding to samples from different data streams.

5. A method according to any preceding claim wherein the value of said portion of said modified key signal is set to be the logical OR of the compared portions of the key signals.

6. A method according to any preceding claim wherein the value of said portion of said modified key signal is set to be the largest value of the compared portions of the key signals.

7. A method according to claim 5 or 6 wherein the value of said portion of said modified key signal is set by a NAM method.

8. A method of generating a border key signal from an input area key signal, the method comprising the steps of: first generating a modified area key signal related to said area key signal according to the method of any preceding claim; and secondly generating said border key signal by combining said area key signal and said modified area key signal.

9. A method according to claim 8 wherein said modified area key signal is generated by extending said area key signal in at least one direction by a specified amount.

10. A method according to claim 9 wherein said specified amount is a number of pixels.

11. A method according to any preceding claim wherein said key signal may take a value in the range 0 to 1.

12. An apparatus for generating a modified key signal from a key signal associated with a series of samples of a video image, the apparatus comprising: comparing means for comparing the value of the portion of the key signal corresponding to a particular sample with the value of the portion of the key signal corresponding to another sample spaced in said video image a selected number of samples from said particular sample; and setting means for setting the value of the portion of said modified key signal corresponding to a sample on the basis of said comparison.

13. An apparatus according to claim 12 wherein said series of samples comprises video data which has been demultiplexed so as to form a plurality of parallel data streams.

14. An apparatus according to claim 13 wherein said video data is demultiplexed four ways into four parallel data streams.

15. An apparatus according to claim 13 or 14 wherein said comparing means includes means for comparing portions of the key signals corresponding to samples from different data streams.

16. An apparatus according to claim 13, 14 or 15 wherein said comparing means comprises: first input means for one data stream; second input means for another data stream.

17. An apparatus according to claim 16 wherein said comparing means further comprises means for delaying said one data stream relative to said other data stream.

18. An apparatus according to any one of claims 12 to 17 wherein, in use, the setting means sets the value of said portion of said modified key signal to be the logical OR of the compared portions of the key signals.

19. An apparatus according to any one of claims 12 to 18 wherein, in use, the setting means sets the value of said portion of said modified key signal to be the largest value of the compared portions of the key signals.

20. An apparatus according to claim 18 or 19 wherein said setting means comprises a NAM apparatus.

21. An apparatus according to any one of claims 12 to 20 wherein said key signal may take a value in the range of 0 to 1.

22. A method of generating a modified key signal substantially as hereinbefore described with reference to the accompanying drawings.

23. An apparatus for generating a modified key signal arranged substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.