Calculate the coordinates for secondaries on CIE chromacity diagram?

Question

I'm trying to use this solution to calculate the secondary coordinates from primaries: https://photo.stackexchange.com/a/75238/115749

But it won't work for any other primary than those in the solution (red and green). The results are wrong.

Here is my adaption to work with colour 0.4.4:

import colour
import numpy as np
from colour.models import RGB_COLOURSPACE_BT709
Conversion from RGB to chromaticity coordinates.
Defining RGB values for reference.
RGB_r = np.array([1, 0, 0])
RGB_g = np.array([0, 1, 0])
We assume they are encoded in sRGB colourspace.
XYZ_r = colour.RGB_to_XYZ(RGB_r,RGB_COLOURSPACE_BT709)
XYZ_g = colour.RGB_to_XYZ(RGB_g,RGB_COLOURSPACE_BT709)
Conversion to chromaticity coordinates.
xy_r = colour.XYZ_to_xy(XYZ_r)
print(xy_r)
[ 0.64  0.33]
xy_g = colour.XYZ_to_xy(XYZ_g) 
print(xy_g)
[ 0.3  0.6]
Conversion to CIE xyY in order to maintain Luminance ratios.
Using sRGB Luminance ratios, second row of the NPM.
xyY_r = [xy_r[0], xy_r[1], colour.models.RGB_COLOURSPACE_BT709.matrix_RGB_to_XYZ[1, 0]]
xyY_g = [xy_g[0], xy_g[1], colour.models.RGB_COLOURSPACE_BT709.matrix_RGB_to_XYZ[1, 1]]
print(xyY_r)
print(xyY_g)
xy_s = colour.XYZ_to_xy(
    colour.RGB_to_XYZ(
        colour.XYZ_to_RGB(colour.xyY_to_XYZ(xyY_r),RGB_COLOURSPACE_BT709) +
        colour.XYZ_to_RGB(colour.xyY_to_XYZ(xyY_g),RGB_COLOURSPACE_BT709),RGB_COLOURSPACE_BT709))
print(xy_s)

So if I change here to any other primary the coordinates are wrong:

RGB_r = np.array([1, 0, 0])
RGB_g = np.array([0, 1, 0])

Yellow ([1, 0, 0] + [0, 1, 0]) --> [ 0.419306 0.505257] correct
Cyan ([0, 1, 0] + [0, 0, 1]) --> [ 0.15433113 0.07559207] not correct
Magenta ([1, 0, 0] + [0, 0, 1]) --> [ 0.17513059 0.07384747] not correct

I’m voting to close this question because it is not about the photography: https://photo.stackexchange.com/help/on-topic — Romeo Ninov, Mar 27 '24 at 21:13
@RomeoNinov the link to the solution I used is on the same topic ... — FLX, Mar 28 '24 at 09:59
I cannot vote to NOT close this question because apparently more than 15 years are needed to implement this feature. — Euri Pinhollow, Mar 29 '24 at 19:17

Euri Pinhollow · Answer 1 · 2024-03-29T19:24:50.783

0

It seems very likely to me that the problem is with xyY_r = and xyY_g = calculations. It's very obvious that it only works for finding yellow.
For other colours the subscripts of RGB_to_XYZ_matrix must be different.

edited Mar 29 '24 at 19:24

answered Mar 29 '24 at 16:38

Euri Pinhollow

5,071
17
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Calculate the coordinates for secondaries on CIE chromacity diagram?

Conversion from RGB to chromaticity coordinates.

Defining RGB values for reference.

We assume they are encoded in sRGB colourspace.

Conversion to chromaticity coordinates.

[ 0.64 0.33]

[ 0.3 0.6]

Conversion to CIE xyY in order to maintain Luminance ratios.

Using sRGB Luminance ratios, second row of the NPM.

1 Answers1