+#include "ycbcr_input.h"
+
+using namespace Eigen;
+using namespace std;
+
+namespace movit {
+
+namespace {
+
+// OpenGL has texel center in (0.5, 0.5), but different formats have
+// chroma in various other places. If luma samples are X, the chroma
+// sample is *, and subsampling is 3x3, the situation with chroma
+// center in (0.5, 0.5) looks approximately like this:
+//
+// X X
+// *
+// X X
+//
+// If, on the other hand, chroma center is in (0.0, 0.5) (common
+// for e.g. MPEG-4), the figure changes to:
+//
+// X X
+// *
+// X X
+//
+// In other words, (0.0, 0.0) means that the chroma sample is exactly
+// co-sited on top of the top-left luma sample. Note, however, that
+// this is _not_ 0.5 texels to the left, since the OpenGL's texel center
+// is in (0.5, 0.5); it is in (0.25, 0.25). In a sense, the four luma samples
+// define a square where chroma position (0.0, 0.0) is in texel position
+// (0.25, 0.25) and chroma position (1.0, 1.0) is in texel position (0.75, 0.75)
+// (the outer border shows the borders of the texel itself, ie. from
+// (0, 0) to (1, 1)):
+//
+// ---------
+// | |
+// | X---X |
+// | | * | |
+// | X---X |
+// | |
+// ---------
+//
+// Also note that if we have no subsampling, the square will have zero
+// area and the chroma position does not matter at all.
+float compute_chroma_offset(float pos, unsigned subsampling_factor, unsigned resolution)
+{
+ float local_chroma_pos = (0.5 + pos * (subsampling_factor - 1)) / subsampling_factor;
+ return (0.5 - local_chroma_pos) / resolution;
+}
+
+// Given <ycbcr_format>, compute the values needed to turn Y'CbCr into R'G'B';
+// first subtract the returned offset, then left-multiply the returned matrix
+// (the scaling is already folded into it).
+void compute_ycbcr_matrix(YCbCrFormat ycbcr_format, float* offset, Matrix3d* ycbcr_to_rgb)
+{
+ double coeff[3], scale[3];
+
+ switch (ycbcr_format.luma_coefficients) {
+ case YCBCR_REC_601:
+ // Rec. 601, page 2.
+ coeff[0] = 0.299;
+ coeff[1] = 0.587;
+ coeff[2] = 0.114;
+ break;
+
+ case YCBCR_REC_709:
+ // Rec. 709, page 19.
+ coeff[0] = 0.2126;
+ coeff[1] = 0.7152;
+ coeff[2] = 0.0722;
+ break;
+
+ case YCBCR_REC_2020:
+ // Rec. 2020, page 4.
+ coeff[0] = 0.2627;
+ coeff[1] = 0.6780;
+ coeff[2] = 0.0593;
+ break;
+
+ default:
+ assert(false);
+ }
+
+ if (ycbcr_format.full_range) {
+ offset[0] = 0.0 / 255.0;
+ offset[1] = 128.0 / 255.0;
+ offset[2] = 128.0 / 255.0;
+
+ scale[0] = 1.0;
+ scale[1] = 1.0;
+ scale[2] = 1.0;
+ } else {
+ // Rec. 601, page 4; Rec. 709, page 19; Rec. 2020, page 4.
+ offset[0] = 16.0 / 255.0;
+ offset[1] = 128.0 / 255.0;
+ offset[2] = 128.0 / 255.0;
+
+ scale[0] = 255.0 / 219.0;
+ scale[1] = 255.0 / 224.0;
+ scale[2] = 255.0 / 224.0;
+ }
+
+ // Matrix to convert RGB to YCbCr. See e.g. Rec. 601.
+ Matrix3d rgb_to_ycbcr;
+ rgb_to_ycbcr(0,0) = coeff[0];
+ rgb_to_ycbcr(0,1) = coeff[1];
+ rgb_to_ycbcr(0,2) = coeff[2];
+
+ float cb_fac = (224.0 / 219.0) / (coeff[0] + coeff[1] + 1.0f - coeff[2]);
+ rgb_to_ycbcr(1,0) = -coeff[0] * cb_fac;
+ rgb_to_ycbcr(1,1) = -coeff[1] * cb_fac;
+ rgb_to_ycbcr(1,2) = (1.0f - coeff[2]) * cb_fac;
+
+ float cr_fac = (224.0 / 219.0) / (1.0f - coeff[0] + coeff[1] + coeff[2]);
+ rgb_to_ycbcr(2,0) = (1.0f - coeff[0]) * cr_fac;
+ rgb_to_ycbcr(2,1) = -coeff[1] * cr_fac;
+ rgb_to_ycbcr(2,2) = -coeff[2] * cr_fac;
+
+ // Inverting the matrix gives us what we need to go from YCbCr back to RGB.
+ *ycbcr_to_rgb = rgb_to_ycbcr.inverse();
+
+ // Fold in the scaling.
+ *ycbcr_to_rgb *= Map<const Vector3d>(scale).asDiagonal();
+}
+
+} // namespace