Refactor YCbCr chroma offset calculation into a separate function.

[movit] / ycbcr_input.cpp

diff --git a/ycbcr_input.cpp b/ycbcr_input.cpp
index 57fe3f56308acad42983002f8ce20240cc400161..374237e1d748aaf8e61fd4c0c8e24891d6d00dce 100644 (file)
--- 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)
@@ -208,50 +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
-       //
-       // 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 chroma_x_local_position =
-               (0.5 + ycbcr_format.chroma_x_position * (ycbcr_format.chroma_subsampling_x - 1)) /
-               ycbcr_format.chroma_subsampling_x;
-       float chroma_y_local_position =
-               (0.5 + ycbcr_format.chroma_y_position * (ycbcr_format.chroma_subsampling_y - 1)) /
-               ycbcr_format.chroma_subsampling_y;
-
-       float chroma_offset_x = (0.5f - chroma_x_local_position) / widths[1];
-       float chroma_offset_y = (0.5f - chroma_y_local_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;