Use exact sRGB matrix values.

[movit] / colorspace_conversion_effect.cpp

diff --git a/colorspace_conversion_effect.cpp b/colorspace_conversion_effect.cpp
index 2b8a0ca329fd34b3d535fca0cbdfb56584b8ff1d..c14a8898f9ab2e10588479dc4601a37e34257ab4 100644 (file)
--- a/colorspace_conversion_effect.cpp
+++ b/colorspace_conversion_effect.cpp
@@ -38,12 +38,28 @@ Matrix3d ColorspaceConversionEffect::get_xyz_matrix(Colorspace space)
        if (space == COLORSPACE_XYZ) {
                return Matrix3d::Identity();
        }
+       if (space == COLORSPACE_sRGB) {
+               // sRGB is not defined by the color primaries, but by concrete
+               // forward and inverse matrices that are rounded-off versions
+               // of the Rec. 709 color space (see
+               // https://photosauce.net/blog/post/what-makes-srgb-a-special-color-space).
+               // We're not compliant with the inverse matrix, since we'd be
+               // too accurate (sRGB is specified for 8-bit only); however,
+               // results should be very close in practice (and even closer to
+               // scRGB's inverse matrix, which is a higher-accuracy inversion of
+               // the same forward matrix).
+               return Matrix3d{
+                       { 0.4124, 0.3576, 0.1805 },
+                       { 0.2126, 0.7152, 0.0722 },
+                       { 0.0193, 0.1192, 0.9505 }
+               };
+       }
 
        double x_R, x_G, x_B;
        double y_R, y_G, y_B;
 
        switch (space) {
-       case COLORSPACE_REC_709:  // And sRGB.
+       case COLORSPACE_REC_709:
                x_R = rec709_x_R; x_G = rec709_x_G; x_B = rec709_x_B;
                y_R = rec709_y_R; y_G = rec709_y_G; y_B = rec709_y_B;
                break;