Colour signal processing

A method of processing component signals making up an N component colour signal (such as RGB signals of a 3 component colour signal) defining an image of an illuminated coloured subject to produce modified signal components defining an image of the same subject as if illuminated under light of a different colour temperature, comprising the steps of:

• (1) generating the N-(RGB etc.) components of a colour signal corresponding to an illumination of the subject using illumination of colour temperature T1;
• (2) forming the N log values (R′ G′ B′ etc.) of the N-RGB etc. components of the colour signal so obtained;
• and
• (3) computing the N log values (R*, G*, B* etc.) of the N-RGB etc. components of the colour signal for a different illumination colour temperature T2 using the equation:
• (R* G* B* . . . )=(R′ G′ B′ . . . )+X (u, v, w . . . )
• where X is proportional to (T1−T2) and u, v, w etc. are constants that depend only on the spectral sensitivities of the signal source and Planck's equation defining black body illumination;
• and
• (4) optionally processing and/or storing and/or controlling a display device using the N-(R*, G*, B* . . . ) values or signals derived therefrom, or converting the log values (R*, G*, B* . . . ) to N appropriate RGB etc. values, for processing and/or display and/or storing.

A method of processing the N component signals making up a colour signal (such as RGB signals of a 3 component colour signal) defining an image of an illuminated coloured subject, to produce modified signal components defining an image of the same subject as if illuminated by a different colour temperature (T), comprising the steps of:

• (1) generating the N-(RGB etc) components of a colour signal corresponding to an illumination of the subject using illumination of colour temperature T;
• (2) normalising the N components of the colour signal with respect to one of the components (e.g. G) to provide (N−1) co-ordinates (e.g. R/G, B/G in a 3 component RGB signal) so that each of the resulting components is independent of illumination intensity,
• (3) computing the log values of these (N−1) co-ordinates, (such that log(R/G)=R′ and log (B/G)=B′ in the case of an RGB signal),
• (4) adjusting the values of (R′, B′, etc.) to produce new values (R*, B*, etc.) corresponding to the values which would apply if the colour temperature of the illumination is T2 using the equation (R*, B*, etc.)=(R′, B′, etc.)+X(a, b, etc.) where X is proportional to (T1−T2) and a and b etc. are constants that depend only on the spectral sensitivities of the source and Planck's equation defining black-body illumination;
• and
• (5) optionally converting the values (R*, B*, etc.) to new (RGB etc.) values or using the new values, for processing and/or comparison and/or storage and/or display.