Gamut mapping using luminance parameters
A colour processor for mapping an image from source to destination colour gamuts has an input for receiving a source image including a plurality of source colour points expressed according to the source gamut; a colour characterizer configured to, for each source colour point in the source image, determine a position of intersection of a curve with the boundary of the destination gamut; and a gamut mapper configured to, for each source colour point in the source image: if the source colour point lies inside the destination gamut, apply a first translation factor to translate the source colour point to a destination colour point within a first range of values; or if the source colour point lies outside the destination gamut, apply a second translation factor, different to the first translation factor, to translate the source colour point to a destination colour point within a second range of values.
1. A computer-implemented method of mapping a source colour point expressed in accordance with a source colour gamut to a destination colour point expressed in accordance with a destination colour gamut in YUV colour space, wherein the luminance of the source colour point is modified in dependence on:
the square of the distance of the source colour point from the axis U=V=0; or
a maximum magnitude of the U and V coordinates of the source colour point in the YUV colour space.
2. The computer-implemented method as claimed in claim 1 , wherein the source colour point is outside the destination colour gamut and the destination colour point is within the destination colour gamut.
3. The computer-implemented method as claimed in claim 1 , wherein the source colour point is comprised by a source image and the destination colour point is comprised by a destination image.
4. The computer-implemented method as claimed in claim 1 , wherein the luminance of the source colour point is modified in accordance with Y′=Y=(1−τ)*k(Y)*(U 2 +V 2 ), where Y′ is a transformed luminance value for the destination colour point, Y is a luminance value of the source colour point, τ is a translation factor, k(Y) is a polynomial function of the Y coordinate of the source colour point, and (U 2 +V 2 ) is the square of the distance of the source colour point from the axis U=V=0.
5. The computer-implemented method as claimed in claim 4 , wherein the polynomial function has form k(Y)=C*(1−2Y), where C is a free parameter.
6. The computer-implemented method as claimed in claim 5 , wherein the size of the free parameter, C, can be tuned so as to optimise a colour mapping of a source image comprising the source colour point.
7. The computer-implemented method as claimed in claim 1 , wherein the luminance of the source colour point is modified in accordance with Y′=Y+k 1 (Y)*max(|U|,|V|), where Y′ is a transformed luminance value for the destination colour point, Y is a luminance value of the source colour point, k 1 (Y) is a polynomial function of the Y coordinate of the source colour point, and max (|U|,|V|) is the maximum magnitude of the U and V coordinates in the YUV colour space of the source colour point.
8. The computer-implemented method as claimed in claim 7 , wherein the polynomial function has the form k 1 (Y)=C*(1−2Y) 3 , where C is a free parameter.
9. The computer-implemented method as claimed in claim 8 , wherein the size of the free parameter, C, can be tuned so as to optimise a colour mapping of a source image comprising the source colour point.
10. The computer-implemented method as claimed in claim 1 , wherein the luminance of the source colour point is modified in dependence on a polynomial function of the Y coordinate of the source colour point, the degree of the polynomial function being odd.
11. The computer-implemented method as claimed in claim 10 , wherein the polynomial function has the form k 2 (Y)=C 2 *(2Y−1) 5 +C 3 *(1−2Y) 3 , where C 2 and C 3 are free parameters.
12. The computer-implemented method as claimed in claim 11 , wherein the size of one or both of the free parameters, C 2 and C 3 can be tuned so as to optimise a colour mapping of a source image comprising the source colour point.
13. The computer-implemented method as claimed in claim 10 , wherein the polynomial function is selected such that its maxima in magnitude lie substantially at the Y values of first and second vertices of a surface defining the boundary of the destination colour gamut in YUV colour space.
14. The computer-implemented method as claimed in claim 13 , wherein the first vertex is the vertex of the surface having the greatest Y value and the second vertex is the vertex of the surface having the smallest Y value.
15. The computer-implemented method as claimed in claim 1 , wherein the luminance of the source colour point is shifted along a curve of constant hue and varying luminance, wherein luminance along that curve varies with distance from the centre of the destination colour gamut.
16. The computer-implemented method as claimed in claim 15 , wherein the centre of the destination colour gamut is:
at the origin of the YUV colour space at U=0, V=0, Y=0.5;
a centroid of the destination colour gamut; or
a weighted average of the position of some or all of the colour points in the destination gamut.
17. The computer-implemented method as claimed in claim 1 , wherein the luminance of the source colour point is shifted along a curve of constant hue and varying luminance towards the luminance value at the centre of the destination colour gamut.
18. The computer-implemented method as claimed in claim 17 , wherein the centre of the destination colour gamut is:
at the origin of the YUV colour space at U=0, V=0, Y=0.5;
a centroid of the destination colour gamut; or
a weighted average of the position of some or all of the colour points in the destination gamut.
19. A colour processor configured to map a source colour point expressed in accordance with a source colour gamut to a destination colour point expressed in accordance with a destination colour gamut in YUV colour space, wherein the luminance of the source colour point is modified in dependence on:
the square of the distance of the source colour point from the axis U=V=0; or
a maximum magnitude of the U and V coordinates of the source colour point in the YUV colour space.
20. A non-transitory machine readable storage medium having stored thereon machine readable instructions that, when processed at a computer system, cause the computer system to implement a method of mapping a source colour point expressed in accordance with a source colour gamut to a destination colour point expressed in accordance with a destination colour gamut in YUV colour space, wherein the luminance of the source colour point is modified in dependence on:
the square of the distance of the source colour point from the axis U=V=0; or
a maximum magnitude of the U and V coordinates of the source colour point in the YUV colour space.