X-Git-Url: https://git.sesse.net/?p=movit;a=blobdiff_plain;f=ycbcr_input.cpp;h=374237e1d748aaf8e61fd4c0c8e24891d6d00dce;hp=65103713df923aa59d38317e183564d04e58e446;hb=dbdbdd8f13c846ce751dc455ce4d19cbe5dbb232;hpb=b0532d475da098fd6d8a0475e4b106cac6502163

diff --git a/ycbcr_input.cpp b/ycbcr_input.cpp
index 6510371..374237e 100644
--- a/ycbcr_input.cpp
+++ b/ycbcr_input.cpp
@@ -9,6 +9,51 @@
 
 using namespace Eigen;
 
+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;
+}
+
+}  // namespace
+
 YCbCrInput::YCbCrInput(const ImageFormat &image_format,
                        const YCbCrFormat &ycbcr_format,
                        unsigned width, unsigned height)
@@ -208,27 +253,10 @@ std::string YCbCrInput::output_fragment_shader()
 		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];
+	float chroma_offset_x = compute_chroma_offset(
+		ycbcr_format.chroma_x_position, ycbcr_format.chroma_subsampling_x, widths[1]);
+	float chroma_offset_y = compute_chroma_offset(
+		ycbcr_format.chroma_y_position, ycbcr_format.chroma_subsampling_y, heights[1]);
 	sprintf(buf, "const vec2 PREFIX(chroma_offset) = vec2(%.8f, %.8f);\n",
 		chroma_offset_x, chroma_offset_y);
 	frag_shader += buf;