From 181fca60b28290c92207cfb40e27113e4f5f021c Mon Sep 17 00:00:00 2001 From: "Steinar H. Gunderson" Date: Sat, 3 Nov 2012 17:39:11 +0100 Subject: [PATCH] When correcting for white balance, move the D65 normalization into compute_lms_scaling_factors() instead of folding it into the LMS matrix. This makes much more sense, and should be equivalent. --- white_balance_effect.cpp | 45 +++++++++++++++++----------------------- 1 file changed, 19 insertions(+), 26 deletions(-) diff --git a/white_balance_effect.cpp b/white_balance_effect.cpp index f185228..8a495fb 100644 --- a/white_balance_effect.cpp +++ b/white_balance_effect.cpp @@ -67,41 +67,34 @@ double rgb_to_xyz_matrix[9] = { * The actual perceptual differences were found to be minor, though. * We use the Bradford tranformation matrix from that page, and compute the * inverse ourselves. (The Bradford matrix is also used in CMCCAT97.) - * - * We normalize the Bradford fundamentals to D65, which means that the standard - * D65 illuminant (x=0.31271, y=0.32902, z=1-y-x) gives L=M=S under this - * transformation. This makes sense because sRGB (which is used to derive - * those XYZ values in the first place) assumes the D65 illuminant, and so the - * D65 illuminant also gives R=G=B in sRGB. (We could also have done this - * step separately in XYZ space, but we'd have to do it to all colors we - * wanted scaled to LMS.) */ const double xyz_to_lms_matrix[9] = { - 0.8951 / d65_X, -0.7502 / d65_X, 0.0389 / d65_X, - 0.2664, 1.7135, -0.0685, - -0.1614 / d65_Z, 0.0367 / d65_Z, 1.0296 / d65_Z, + 0.7328, -0.7036, 0.0030, + 0.4296, 1.6975, 0.0136, + -0.1624, 0.0061, 0.9834, }; /* - * For a given reference color (given in XYZ space), - * compute scaling factors for L, M and S. What we want at the output is equal L, M and S - * for the reference color (making it a neutral illuminant), or sL ref_L = sM ref_M = sS ref_S. - * This removes two degrees of freedom for our system, and we only need to find fL. + * For a given reference color (given in XYZ space), compute scaling factors + * for L, M and S. What we want at the output is turning the reference color + * into a scaled version of the D65 illuminant (giving it R=G=B in sRGB), or + * + * (sL ref_L, sM ref_M, sS ref_S) = (s d65_L, s d65_M, s d65_S) * + * This removes two degrees of freedom from our system, and we only need to find s. * A reasonable last constraint would be to preserve Y, approximately the brightness, - * for the reference color. Since L'=M'=S' and the Y row of the LMS-to-XYZ matrix - * sums to unity, we know that Y'=L', and it's easy to find the fL that sets Y'=Y. + * for the reference color. Thus, we choose our D65 illuminant's Y such that it is + * equal to the reference color's Y, and the rest is easy. */ -Vector3d compute_lms_scaling_factors(const Vector3d &xyz) +Vector3d compute_lms_scaling_factors(const Vector3d &ref_xyz) { - Vector3d lms = Map(xyz_to_lms_matrix) * xyz; - double l = lms[0]; - double m = lms[1]; - double s = lms[2]; - - double scale_l = xyz[1] / l; - double scale_m = scale_l * (l / m); - double scale_s = scale_l * (l / s); + Vector3d ref_lms = Map(xyz_to_lms_matrix) * ref_xyz; + Vector3d d65_lms = Map(xyz_to_lms_matrix) * + (ref_xyz[1] * Vector3d(d65_X, d65_Y, d65_Z)); // d65_Y = 1.0. + + double scale_l = d65_lms[0] / ref_lms[0]; + double scale_m = d65_lms[1] / ref_lms[1]; + double scale_s = d65_lms[2] / ref_lms[2]; return Vector3d(scale_l, scale_m, scale_s); } -- 2.20.1