GB2086686A - Chroma keying - Google Patents

Abstract

Chroma keying, or colour separation overlay apparatus, incorporates hue suppression of the foreground video signal by subtraction (22) of the keying signal from the foreground signal blue component. To avoid the need for non-linear clipping of the keying signal prior to such hue suppression, the keying signal is formed as the product of two signals each derived from the foreground video signal, one of these two signals being zero when the key colour component of the foreground video signal is zero. The preferred keying signal is: (B-L) {1 - (H-L)} where B is the key colour component, H is the greatest of the other two components, and L is the least of the components. <IMAGE>

Description

SPECIFICATION Chroma keying This invention relates to chroma keying or colour separation overlay (CSO) of television signals, and in particular to the generation of a keying signal for use therein.

In colour separation overlay, two scenes are electrically combined into a single composite picture in the following manner. A first scene consists of the elements of a foreground picture, typically including actors, in front of a background of a specified colour, usually blue, though in principle other colours can be used. The second scene consists of a background picture typically of a distant location.

By use of colour separation overlay the foreground picture is made to replace the corresponding parts of the background picture, thus the actors can be made to appear to be at the distant location. To do this a camera views the first scene and provides a first video signal, and a second video signal is generated representative of the second scene by any suitable means such as a camera, telecine machine, or video tape recorder, synchronised with the camera viewing the first scene. From the first scene there is also produced a waveform, known as the keying signal, which indiates whether the first video signal instantaneously corresponds to a back-drop area of the first scene or a foreground area of the scene.This keying signal can then be used to operate a fast selector switch so as to select the first video signal when this represents foreground information, but at other times to select the second, background video signal, to provide the composite signal.

The keying signal or a signal related to it can also be used to modify the first video signal by reducing the signal amplitude in those areas corresponding to back-drop information. In this way any tendency for the final composite picture to have fringes of the backdrop colour round the foreground picture elements is at least substantially reduced.

Originally the keying signal consisted of the blue component of the first camera output after passing through a threshold circuit. In accordance with our British Patent No.

1,189,402 it was proposed to use either the B-Y signal or preferably B-Q(R + G), where B, R and G are the three camera signals representative of the blue, red and green signal components, and Y represents the luminance component.

Other proposals have been B-M, where M is the mean or (R + G + B)/3, and a so-called 'excess hue' key where the key is the difference between blue and the greater of red and green. Thus if H is the greater of red and green, then the keying signal is B-H.

This keying signal B-H is particularly suitable for 'hue suppression', where the key signal itself is subtracted from the blue signal to remove the excess. This is useful in that it reduces the blue fringes around the edges of the foreground subject, which otherwise often spoil the illusion. This suppresion does not affect any neutral (grey scale) colours but only those to the blue side of magenta and cyan.

Unfortunately, before doing this hue suppression subtraction it is necessary to clip the key signal at zero so as to remove the negative excursions which occur with some colours. This clipping is a non-linear operation and changes the shape of the edges of the key signal, which can result in poor suppression on the edges of foreground objects.

Thus there is a need for a hue suppression system for use in chroma keying which avoids such clipping. The key should be zero for areas with no key colour and, as usual, at a maximum for areas rich in the key colour.

The invention will be described by way of example with reference to the drawings, in which: Figure 1 is a block circuit diagram of a chroma key circuit embodying the invention; Figure 2 is a PAL vector diagram illustrating how colours are affected by hue suppression; Figure 3 shows the key waveform for various colour transitions; Figure 4 shows graphically the vector response to hue suppression.

The problem of a negative key signal which arises by use of B-H could be avoided by using: K = B-BH = B(1-H) where the maximum value of B and H is taken to be unity. This cannot become negative but provides a key waveform which increases as both the red and green components decrease.

This key signal, however, has poor luminance independence, and it is therefore preferred to discount any luminance components.

The luminance component can be equated to the lowest of the three red, green and blue signals and will be denoted as L. If L is subtracted from all three components then what remains can' be regarded as the colour and is relatively independent of luminance.

A preferred key signal is thus: K = (B-L) [I-(H-L)] This will always be zero for neutral or greyscale colours (as B then equals L). The signal is at a maximum for areas rich in key colour, for which H approaches L. The signal has the further advantage that the shape of the key waveform corresponding to a transition from a first colour outside the key region to the key colour is independent of the hue of the first colour. Thus the waveform resulting from a red to blue transition is the same as that from green to blue or from yellow to blue, as is described below.

The key signal can be generated by using a linear multiplier as illustrated in Fig. 1.

Referring to Fig. 1, the foreground camera signals are shown as applied to inputs 10 for the red, green and blue signals respectively. A diode circuit 1 2 is connected to all three inputs to output the lowest input L and a diode circuit 14 is connected to the red and green inputs to output the higher H of the red and green inputs. A combining circuit 1 6 subtracts the signal L from the blue signal B to provide B-L. A combining circuit 1 8 subtracts the signal L from the H signal to provide H-L. This signal is applied to a complementing input of a multiplier 20, the other input of which receives the B-L signal.Thus the multiplier output is (B-L)[l-(H-L)]. This is used to generate the key (by use of a suitable threshold) and is applied to the subtractive input of a combining circuit 22 connected in the blue video signal path. In this way the blue signal suffers hue suppression.

The key signal does not have to be the same as the signal used for hue suppression though in the example illustrated this is conveniently the case.

In the circuit illustrated the key used for hue suppression has the desirable property that the proportion of suppression depends on the amount of blue contributing to the coloured component of the picture, and not just the amount by which it exceeds red and green. That is to say, it is quite common for a foreground scene intended for CSO to have an overall blue cast. With conventional blue suppression it has been assumed that this blue cast should be removed only in areas where blue is the dominant primary component.

However, there is no reason why the blue cast does not affect all colours except, of course, those without any blue. In other words it is wrong to assume that a colour on the blue side of magenta has a small cast while a colour on the red side has none. Suppression with the circuit of Fig. 1, however, will reduce the blue in both cases, although to a lesser extent in the latter.

As indicated in Fig. 2, considering a conventional PAL vector diagram, any colours in the red-yellow-green sector are unaffected by suppression, as are neutral (grey-scale) colours. All other are affected to a greater or lesser extent, and no colours will be left in the magenta-blue-cyan sector after suppression.

Reverting to Fig. 1, an interesting feature of the circuit is the use of multipliers to multiply signals dependent upon two different colour components. In a system which aims to avoid non-linearities in the key response this might be thought to be an inappropriate approach.

In fact, the key signal described is found to be extremely satisfactory in practice providing very good subjective hue suppression.

It is preferred to modify the circuit illustrated by the application of a controlled degree of suppression to the red and green video signals. This is achieved by applying the key signal to two adjustable attenuators which are connected to subtractors in the red and green signal paths. These attenuators are then set up so that the background is suppressed to black. The resultant foreground signal can then simply be added to the keyed background system, and the foreground signal does not need to be switched.

Fig. 3 illustrates the shaped of the keying waveform produced as the video signal traverses a number of colours as in the form of bars. It will be seen that the key waveform has no discontinuity in slope such as is formed by clipping, that the waveform is the same for the different transitions noted, and that a degree of hue suppression extends well into the magenta, cyan and grey regions. Fig.

4 shows how the output colour varies compared with the input colour as a result of blue suppression. The colour approaches magenta and cyan assymptotically and never enters the magenta-blue-cyan sector. However colours in the red-yellow-green sector remain unaltered.

While the above example relates to blue as the background colour, similar principles apply if another colour is used with appropriate rotation of the vector diagram.

Claims (7)

1. Colour separation apparatus comprising means for generating a keying signal which is never negative and has a relatively high value when the foreground video signal has a high key colour component, the keying signal being the product of at least two signals derived from the foreground video signal, and a first one of the said two derived signals being zero when the key colour component of the foreground video signal is zero.

2. Apparatus according to claim 1 in which the keying signal is substantially equal to B(l-H), where B represents the key colour component and H represents the greater one of the other two components, B and H each being taken to have a maximum value of unity.

3. Apparatus according to claim 1, in which the said first signal is zero when the key colour component constitutes the smallest component of the foreground video signal.

4. Apparatus according to claim 3, in which the keying signal is substantially equal to (B-L)(1-(H-L)), where B represents the key colour component, H represents the greatest of the other two components, and L represents the least of the three components; B, H and L each being taken to have a maximum value of unity.

5. Apparatus according to any of claims 1 to 4, including hue suppression means responsive to the keying signal for reducing the magnitude of the key colour component of the foreground video signal.

6. Apparatus according to claim 5, in which the hue suppression means comprises means for subtracting the keying signal or a signal derived therefrom from one or more of the components of the foreground video sig nal.

7. Colour separation overlay apparatus substantially as herein described with refer ence to the drawings.