}
// Set up stuff for NV12 conversion.
+ //
+ // Note: Due to the horizontally co-sited chroma/luma samples in H.264
+ // (chrome position is left for horizontal and center for vertical),
+ // 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.
// Cb/Cr shader.
string cbcr_vert_shader =
" \n"
"in vec2 position; \n"
"in vec2 texcoord; \n"
- "out vec2 tc0; \n"
+ "out vec2 tc0, tc1; \n"
"uniform vec2 foo_chroma_offset_0; \n"
+ "uniform vec2 foo_chroma_offset_1; \n"
" \n"
"void main() \n"
"{ \n"
" gl_Position = vec4(2.0 * position.x - 1.0, 2.0 * position.y - 1.0, -1.0, 1.0); \n"
" vec2 flipped_tc = texcoord; \n"
" tc0 = flipped_tc + foo_chroma_offset_0; \n"
+ " tc1 = flipped_tc + foo_chroma_offset_1; \n"
"} \n";
string cbcr_frag_shader =
"#version 130 \n"
- "in vec2 tc0; \n"
+ "in vec2 tc0, tc1; \n"
"uniform sampler2D cbcr_tex; \n"
"out vec4 FragColor; \n"
"void main() { \n"
- " FragColor = texture(cbcr_tex, tc0); \n"
+ " FragColor = 0.5 * (texture(cbcr_tex, tc0) + texture(cbcr_tex, tc1)); \n"
"} \n";
vector<string> frag_shader_outputs;
cbcr_program_num = resource_pool->compile_glsl_program(cbcr_vert_shader, cbcr_frag_shader, frag_shader_outputs);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
check_error();
- float chroma_offset_0[] = { -0.5f / global_flags.width, 0.0f };
+ float chroma_offset_0[] = { -1.0f / global_flags.width, 0.0f };
+ float chroma_offset_1[] = { -0.0f / global_flags.width, 0.0f };
set_uniform_vec2(cbcr_program_num, "foo", "chroma_offset_0", chroma_offset_0);
+ set_uniform_vec2(cbcr_program_num, "foo", "chroma_offset_1", chroma_offset_1);
glUniform1i(cbcr_texture_sampler_uniform, 0);