Colour signal processing
First Claim
1. A method of adjusting the N components of an N component colour signal (such as the 3 components of an RGB colour signal) so as to produce component values which are independent of the colour temperature of the illuminating light, comprising the step of computing for each component a new value using the original component value and one other component value and an algorithm incorporating a mathematical operator whose value is related to the spectral sensitivity of the imaging means to different colour temperature illuminants.
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Abstract
A method of adjusting the N components of an N component colour signal (such as the 3 components of an RGB colour signal) with respect to colour temperature of illuminating light when imaging a coloured subject, comprising the step of computing the difference between each of the components of the colour signal for different temperatures of illuminating light, using an algorithm based on the spectral sensitivity of the imaging means and Planck'"'"'s equation defining black body illumination to obtain N components whose values are independent of the colour temperature of the illuminating light. Normalisation of the adjusted values renders the values independent of intensity of illumination and can also reduce the processing time for further processing of the signals. A technique for compensating for gamma correction which is commonly built in cameras, is disclosed.
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Citations
25 Claims
- 1. A method of adjusting the N components of an N component colour signal (such as the 3 components of an RGB colour signal) so as to produce component values which are independent of the colour temperature of the illuminating light, comprising the step of computing for each component a new value using the original component value and one other component value and an algorithm incorporating a mathematical operator whose value is related to the spectral sensitivity of the imaging means to different colour temperature illuminants.
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2. 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:
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(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.
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9. 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:
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(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.
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- 11. A method of processing a colour signal from a colour signal source so as to eliminate dependency on colour temperature and/or intensity variation by using a mathematical algorithm derived from the source sensitivities to different wavelengths of light in a signal processing computer supplied with the colour signal, to provide new (corrected) values for the components making up the colour signal which are independent of the colour temperature of the light illuminating (or deemed to illuminate) the subject described by the colour signal.
- 20. A method of image enhancement when processing a colour signal in a system in which the log of the colour response is composed of three parts, a vector due to the intensity of the illumination, a vector due to the illumination colour temperature, and a vector due to reflective properties of the surface illumiated for the purpose of producing the image, in which the magnitude of the intensity vector, or the magnitude of the illumination colour temperature vector is adjusted for each pixel so as to change the effective illumination for each pixel in a display of the image using the processed signal.
Specification