5 #include "colorspace_conversion_effect.h"
14 // Color coordinates from Rec. 709; sRGB uses the same primaries.
15 static const double rec709_x_R = 0.640, rec709_x_G = 0.300, rec709_x_B = 0.150;
16 static const double rec709_y_R = 0.330, rec709_y_G = 0.600, rec709_y_B = 0.060;
18 // Color coordinates from Rec. 601. (Separate for 525- and 625-line systems.)
19 static const double rec601_525_x_R = 0.630, rec601_525_x_G = 0.310, rec601_525_x_B = 0.155;
20 static const double rec601_525_y_R = 0.340, rec601_525_y_G = 0.595, rec601_525_y_B = 0.070;
21 static const double rec601_625_x_R = 0.640, rec601_625_x_G = 0.290, rec601_625_x_B = 0.150;
22 static const double rec601_625_y_R = 0.330, rec601_625_y_G = 0.600, rec601_625_y_B = 0.060;
24 // Color coordinates from Rec. 2020.
25 static const double rec2020_x_R = 0.708, rec2020_x_G = 0.170, rec2020_x_B = 0.131;
26 static const double rec2020_y_R = 0.292, rec2020_y_G = 0.797, rec2020_y_B = 0.046;
28 ColorspaceConversionEffect::ColorspaceConversionEffect()
29 : source_space(COLORSPACE_sRGB),
30 destination_space(COLORSPACE_sRGB)
32 register_int("source_space", (int *)&source_space);
33 register_int("destination_space", (int *)&destination_space);
36 Matrix3d ColorspaceConversionEffect::get_xyz_matrix(Colorspace space)
38 if (space == COLORSPACE_XYZ) {
39 return Matrix3d::Identity();
41 if (space == COLORSPACE_sRGB) {
42 // sRGB is not defined by the color primaries, but by concrete
43 // forward and inverse matrices that are rounded-off versions
44 // of the Rec. 709 color space (see
45 // https://photosauce.net/blog/post/what-makes-srgb-a-special-color-space).
46 // We're not compliant with the inverse matrix, since we'd be
47 // too accurate (sRGB is specified for 8-bit only); however,
48 // results should be very close in practice (and even closer to
49 // scRGB's inverse matrix, which is a higher-accuracy inversion of
50 // the same forward matrix).
52 { 0.4124, 0.3576, 0.1805 },
53 { 0.2126, 0.7152, 0.0722 },
54 { 0.0193, 0.1192, 0.9505 }
62 case COLORSPACE_REC_709:
63 x_R = rec709_x_R; x_G = rec709_x_G; x_B = rec709_x_B;
64 y_R = rec709_y_R; y_G = rec709_y_G; y_B = rec709_y_B;
66 case COLORSPACE_REC_601_525:
67 x_R = rec601_525_x_R; x_G = rec601_525_x_G; x_B = rec601_525_x_B;
68 y_R = rec601_525_y_R; y_G = rec601_525_y_G; y_B = rec601_525_y_B;
70 case COLORSPACE_REC_601_625:
71 x_R = rec601_625_x_R; x_G = rec601_625_x_G; x_B = rec601_625_x_B;
72 y_R = rec601_625_y_R; y_G = rec601_625_y_G; y_B = rec601_625_y_B;
74 case COLORSPACE_REC_2020:
75 x_R = rec2020_x_R; x_G = rec2020_x_G; x_B = rec2020_x_B;
76 y_R = rec2020_y_R; y_G = rec2020_y_G; y_B = rec2020_y_B;
82 // Recover z = 1 - x - y.
83 double z_R = 1.0 - x_R - y_R;
84 double z_G = 1.0 - x_G - y_G;
85 double z_B = 1.0 - x_B - y_B;
87 // We have, for each primary (example is with red):
89 // X_R / (X_R + Y_R + Z_R) = x_R
90 // Y_R / (X_R + Y_R + Z_R) = y_R
91 // Z_R / (X_R + Y_R + Z_R) = z_R
93 // Some algebraic fiddling yields (unsurprisingly):
95 // X_R = (x_R / y_R) Y_R (so define k1 = x_R / y_R)
96 // Z_R = (z_R / y_R) Y_R (so define k4 = z_R / y_R)
98 // We also know that since RGB=(1,1,1) should give us the
99 // D65 illuminant, we must have
101 // X_R + X_G + X_B = D65_X
102 // Y_R + Y_G + Y_B = D65_Y
103 // Z_R + Z_G + Z_B = D65_Z
105 // But since we already know how to express X and Z by
106 // some constant multiple of Y, this reduces to
108 // k1 Y_R + k2 Y_G + k3 Y_B = D65_X
109 // Y_R + Y_G + Y_B = D65_Y
110 // k4 Y_R + k5 Y_G + k6 Y_B = D65_Z
112 // Which we can solve for (Y_R, Y_G, Y_B) by inverting a 3x3 matrix.
115 temp(0,0) = x_R / y_R;
116 temp(0,1) = x_G / y_G;
117 temp(0,2) = x_B / y_B;
123 temp(2,0) = z_R / y_R;
124 temp(2,1) = z_G / y_G;
125 temp(2,2) = z_B / y_B;
127 Vector3d d65_XYZ(d65_X, d65_Y, d65_Z);
128 Vector3d Y_RGB = temp.inverse() * d65_XYZ;
130 // Now convert xyY -> XYZ.
131 double X_R = temp(0,0) * Y_RGB[0];
132 double Z_R = temp(2,0) * Y_RGB[0];
134 double X_G = temp(0,1) * Y_RGB[1];
135 double Z_G = temp(2,1) * Y_RGB[1];
137 double X_B = temp(0,2) * Y_RGB[2];
138 double Z_B = temp(2,2) * Y_RGB[2];
141 m(0,0) = X_R; m(0,1) = X_G; m(0,2) = X_B;
142 m(1,0) = Y_RGB[0]; m(1,1) = Y_RGB[1]; m(1,2) = Y_RGB[2];
143 m(2,0) = Z_R; m(2,1) = Z_G; m(2,2) = Z_B;
148 string ColorspaceConversionEffect::output_fragment_shader()
150 // Create a matrix to convert from source space -> XYZ,
151 // another matrix to convert from XYZ -> destination space,
152 // and then concatenate the two.
154 // Since we right-multiply the RGB column vector, the matrix
155 // concatenation order needs to be the opposite of the operation order.
156 Matrix3d source_space_to_xyz = get_xyz_matrix(source_space);
157 Matrix3d xyz_to_destination_space = get_xyz_matrix(destination_space).inverse();
158 Matrix3d m = xyz_to_destination_space * source_space_to_xyz;
160 return output_glsl_mat3("PREFIX(conversion_matrix)", m) +
161 read_file("colorspace_conversion_effect.frag");