- // Matrix to convert RGB to YCbCr. See e.g. Rec. 601.
- Matrix3x3 rgb_to_ycbcr;
- rgb_to_ycbcr[0] = coeff[0];
- rgb_to_ycbcr[3] = coeff[1];
- rgb_to_ycbcr[6] = coeff[2];
-
- float cb_fac = (224.0 / 219.0) / (coeff[0] + coeff[1] + 1.0f - coeff[2]);
- rgb_to_ycbcr[1] = -coeff[0] * cb_fac;
- rgb_to_ycbcr[4] = -coeff[1] * cb_fac;
- rgb_to_ycbcr[7] = (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] = (1.0f - coeff[0]) * cr_fac;
- rgb_to_ycbcr[5] = -coeff[1] * cr_fac;
- rgb_to_ycbcr[8] = -coeff[2] * cr_fac;
-
- // Inverting the matrix gives us what we need to go from YCbCr back to RGB.
- Matrix3x3 ycbcr_to_rgb;
- invert_3x3_matrix(rgb_to_ycbcr, ycbcr_to_rgb);
-
- std::string frag_shader;
-
- char buf[1024];
- sprintf(buf,
- "const mat3 PREFIX(inv_ycbcr_matrix) = mat3(\n"
- " %.8f, %.8f, %.8f,\n"
- " %.8f, %.8f, %.8f,\n"
- " %.8f, %.8f, %.8f);\n",
- ycbcr_to_rgb[0], ycbcr_to_rgb[1], ycbcr_to_rgb[2],
- ycbcr_to_rgb[3], ycbcr_to_rgb[4], ycbcr_to_rgb[5],
- ycbcr_to_rgb[6], ycbcr_to_rgb[7], ycbcr_to_rgb[8]);
- frag_shader = buf;
-
- sprintf(buf, "const vec3 PREFIX(offset) = vec3(%.8f, %.8f, %.8f);\n",
- offset[0], offset[1], offset[2]);
- frag_shader += buf;
-
- sprintf(buf, "const vec3 PREFIX(scale) = vec3(%.8f, %.8f, %.8f);\n",
- scale[0], scale[1], scale[2]);
- frag_shader += buf;
-
- // 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
- //
- // Obviously, the chroma plane here needs to be moved to the left,
- // which means _adding_ 0.5 to the texture coordinates when sampling
- // chroma.
- float chroma_offset_x = (0.5f - ycbcr_format.chroma_x_position) / widths[1];
- float chroma_offset_y = (0.5f - ycbcr_format.chroma_y_position) / heights[1];
- sprintf(buf, "const vec2 PREFIX(chroma_offset) = vec2(%.8f, %.8f);\n",
- chroma_offset_x, chroma_offset_y);
- frag_shader += buf;