--- /dev/null
+#include "chroma_subsampler.h"
+
+#include <movit/util.h>
+#include <string>
+
+#include "embedded_files.h"
+
+#define BUFFER_OFFSET(i) ((char *)nullptr + (i))
+
+using namespace std;
+
+string read_file(const string &filename, const unsigned char *start = nullptr, const size_t size = 0);
+GLuint compile_shader(const string &shader_src, GLenum type);
+GLuint link_program(GLuint vs_obj, GLuint fs_obj);
+void bind_sampler(GLuint program, GLint location, GLuint texture_unit, GLuint tex, GLuint sampler);
+
+extern GLuint linear_sampler;
+
+ChromaSubsampler::ChromaSubsampler()
+{
+ // Set up stuff for 4:2:2 conversion.
+ //
+ // Note: Due to the horizontally co-sited chroma/luma samples in H.264
+ // (chroma position is left for horizontal),
+ // we need to be a bit careful in our subsampling. A diagram will make
+ // this clearer, showing some luma and chroma samples:
+ //
+ // a b c d
+ // +---+---+---+---+
+ // | | | | |
+ // | Y | Y | Y | Y |
+ // | | | | |
+ // +---+---+---+---+
+ //
+ // +-------+-------+
+ // | | |
+ // | C | C |
+ // | | |
+ // +-------+-------+
+ //
+ // Clearly, the rightmost chroma sample here needs to be equivalent to
+ // b/4 + c/2 + d/4. (We could also implement more sophisticated filters,
+ // of course, but as long as the upsampling is not going to be equally
+ // sophisticated, it's probably not worth it.) If we sample once with
+ // no mipmapping, we get just c, ie., no actual filtering in the
+ // horizontal direction. (For the vertical direction, we can just
+ // sample in the middle to get the right filtering.) One could imagine
+ // we could use mipmapping (assuming we can create mipmaps cheaply),
+ // but then, what we'd get is this:
+ //
+ // (a+b)/2 (c+d)/2
+ // +-------+-------+
+ // | | |
+ // | Y | Y |
+ // | | |
+ // +-------+-------+
+ //
+ // +-------+-------+
+ // | | |
+ // | C | C |
+ // | | |
+ // +-------+-------+
+ //
+ // which ends up sampling equally from a and b, which clearly isn't right. Instead,
+ // we need to do two (non-mipmapped) chroma samples, both hitting exactly in-between
+ // source pixels.
+ //
+ // Sampling in-between b and c gives us the sample (b+c)/2, and similarly for c and d.
+ // Taking the average of these gives of (b+c)/4 + (c+d)/4 = b/4 + c/2 + d/4, which is
+ // exactly what we want.
+ //
+ // See also http://www.poynton.com/PDFs/Merging_RGB_and_422.pdf, pages 6–7.
+
+ cbcr_vs_obj = compile_shader(read_file("chroma_subsample.vert", _binary_chroma_subsample_vert_data, _binary_chroma_subsample_vert_size), GL_VERTEX_SHADER);
+ cbcr_fs_obj = compile_shader(read_file("chroma_subsample.frag", _binary_chroma_subsample_frag_data, _binary_chroma_subsample_frag_size), GL_FRAGMENT_SHADER);
+ cbcr_program = link_program(cbcr_vs_obj, cbcr_fs_obj);
+
+ // Set up the VAO containing all the required position data.
+ glCreateVertexArrays(1, &vao);
+ glBindVertexArray(vao);
+
+ float vertices[] = {
+ 0.0f, 2.0f,
+ 0.0f, 0.0f,
+ 2.0f, 0.0f
+ };
+ glCreateBuffers(1, &vbo);
+ glNamedBufferData(vbo, sizeof(vertices), vertices, GL_STATIC_DRAW);
+ glBindBuffer(GL_ARRAY_BUFFER, vbo);
+
+ GLint position_attrib = 0; // Hard-coded in every vertex shader.
+ glEnableVertexArrayAttrib(vao, position_attrib);
+ glVertexAttribPointer(position_attrib, 2, GL_FLOAT, GL_FALSE, 0, BUFFER_OFFSET(0));
+
+ uniform_cbcr_tex = glGetUniformLocation(cbcr_program, "cbcr_tex");
+ uniform_chroma_offset_0 = glGetUniformLocation(cbcr_program, "chroma_offset_0");
+ uniform_chroma_offset_1 = glGetUniformLocation(cbcr_program, "chroma_offset_1");
+}
+
+ChromaSubsampler::~ChromaSubsampler()
+{
+ glDeleteProgram(cbcr_program);
+ check_error();
+ glDeleteBuffers(1, &vbo);
+ check_error();
+ glDeleteVertexArrays(1, &vao);
+ check_error();
+}
+
+void ChromaSubsampler::subsample_chroma(GLuint cbcr_tex, unsigned width, unsigned height, GLuint cb_tex, GLuint cr_tex)
+{
+ glUseProgram(cbcr_program);
+ bind_sampler(cbcr_program, uniform_cbcr_tex, 0, cbcr_tex, linear_sampler);
+ glProgramUniform2f(cbcr_program, uniform_chroma_offset_0, -1.0f / width, 0.0f);
+ glProgramUniform2f(cbcr_program, uniform_chroma_offset_1, -0.0f / width, 0.0f);
+
+ glViewport(0, 0, width / 2, height);
+ fbos.render_to(cb_tex, cr_tex);
+
+ glBindVertexArray(vao);
+ glDrawArrays(GL_TRIANGLES, 0, 3);
+}