GB2176671A - Separation overlay - Google Patents

Abstract

Foreground and background colour video signals are processed to provide separation overlay employing a background and a foreground of different brightness (luminance) levels, the keying signal being derived by clipping through peak luminance signals obtained by summing the R, G and B signals. <IMAGE>

Description

SPECIFICATION Separation overlay This invention relates to a method and apparatus for separation overlay in the field of video signal proces sing, also known as "chromakey" or"travelling matte". The term "separation overlay" will be used herein to refer to all such techniques of processing video signals in the television and video fields.

In the conventional chromakey process the subject is positioned in front of a backcloth or cyclorama which is highly saturated in a particular colour. This colour is chosen to be as far removed as possible from those hues present in the foreground subject, blue being commonly used since it is readily distinguish able from flesh tones. Thefunction of chromakey apparatus is then to detectthose portions ofthe signal which are highly saturated in the keying colour and to replace those portions electronically by substitute background scenery.

For example th-e derived keying signal from a blue background may be obtained by clipping through a B-Ysignal orthrough a Bvectorsignal produced bya non-additive mixer (NAM): i.e. Bv = B - (RNAMG).

This technique has disadvantages such as a) colourfringeing oftheforeground objects b) careful lighting oftheforeground and coloured background is required to ensure even clipping levels c) normally, the technique can only be used indoors because ofthe difficulty of constructing screens etc.

outdoors d)theforeground object must notcontainthe keying colour chosen since this would generate a keying signal and hence cause printthrough ofthe keyed background picture.

It is an object of the present invention to provide a separation overlay method and apparatus which can obviate or mitigate the aforementioned disadvantages.

According to a first aspect ofthe present invention there is provided a method of processing foreg round and backgroundcolourvideo signals to provide separation overlay which method comprises employing a background and a foreground of different brightness (luminance) levels, the keying signal being derived by clipping through peak luminance signals obtained by summing the R, G and B signals.

Preferably equal R, G and B signals are used, i.e. a white or near white background, having a luminance greaterthan any white ornearwhite in the for ground. The keying signal would thus be derived by clipping through the peak white obtained bysumming the R, G and B signals. It is not however essential that equal proportions of R, G and B be employed. For exampletheskycould be used as the background or, if itwas wished to reduce noise then a background having an increased proportion of G could be used.

Moreover pure R. G and B signals need not be used; an encoded e.g. (PAL, NTSC or SECAM) signal may be employed with keying dependent upon the luminance signal Y. It is not to be noted that the luminance signal Y is itself a summation of R, G and B signals although in proportions R:G:B of approximately 30:60:10.

Preferably the background signal has a greater luminance than the foreground signal; it is possible howeverfortheforeground to have a greater lumi nancethanthe background e.g.forthebackgroundto be black. It is not however the intention ofthe present invention to include within its scope the brightness separation overlay techniques for monochrome tele- vision purposes which are described by A. J. Mitchell in "A personal history of Video Effects in the BBC", International Broadcast Engineer, September 1980, pp 6,7.

The method ofthe present invention eliminates colourfringeing when white is used. There may be some white-fringeing (which is of itself less objectionablethancolourfringeing) butsuchfringeingcanif desired befurther minimized by use of appropriate signal delay circuitry to narrow the width ofthe transition zone from foreground to background and/or vice versa.

The invention also has the advantage that less careful lighting offoreground and background is required, it being possible merely to relyupone.g.the background always being brighterthan the foreground. Moreover the method can be used outdoors with e.g. natural objects such as the sky as the background, e.g. by ensuring that the background sum of R, G and B is always greaterthan the foreground sum of R, G and B. Additionally no print through will occur ifthe same difference criterion is adopted; also, the sig nal to noise ratio ofthe keying signal derived from the summed R, G, B signals will be better than that derived from eg a single blue colour, thus resulting in cleaner keying (switching) edges.

According to a further aspect of the present invention there is provided apparatus for processing foreground and background colour video signals which apparatus includes means for additively mixing the R, G and B signals and meansforclipping said mixed signal as a function of its brightness to produce a keying signal.

An embodiment of apparatus according to the present invention will now be described, by way of example only, by reference to the accompanying fragmentary circuit diagram wherein separate R, G and B signals Rf, Gland Bf are amplified by respective amplifiers A and after attenuation through respective resistances KRI KG and KB are mixed in equal proportions and passed to a clipper unit provided with an adjustable key clipping level.

The signal outputfrom the clipper unit is passed to keying mixer circuitry such as is conventionally used in chromakeying orto such circuitry as is shown and described in our copending Patent Application 8523128, filed 19th September 1985 (Serial No 2168564A) claiming priority from Patent Application 8423654, filed 19th September 1984.

It will be appreciated thatthe method and apparatus of this invention may be embodied other than as has been described and hence the invention includes within its scope all such changes amd modifications thereto as would beapparentto one skilled in the art.

Claims (10)

1. A method of processing foreground and background colour video signals to provide separation overlay which method comprises employing a background and a foreground of different brightness (luminance) levels, the keying signal being derived by clipping through peak luminance signals obtained by summing the R, G and B signals.

2. A method according to claim 1, wherein there areusedassaid background colourvideosignals R, G and B signals which aresubstantially equal such asto have a luminancegreaterthan any white ornearwhite in the foreground.

3. A method according to claim 2, wherein said keying signal is derived by clipping through the peak white obtained by summing the B, G and B signa[s.

4. A method according to claim 1, wherein said background signals R, G and B are unequal.

5. A method according to claim 1 wherein said keying signal is dependent upon the luminance signal Y.

6. A method according to any of claims 1 to 5, wherein said background signal has a greater lumi nancethan theforeground signal.

7. Amethod according to any of claims 1 to 6, wherein said foreground signal has a greater lumi nance than the backgrnund signal.

8. A method of processing foreground and background colourvideo signals to provide separation overlay, substantially as hereinbefore described with reference to the accompanying drawing.

9. Apparatus for processing foreground and background colour video signals which apparatus includes means for additively mixing the R, G and B signals and means for clipping said mixed signal as a function of its brightnessto produce a keying signal.-

10. The features hereinbefore described, or their equivalents, in any novel selection.