vo_opengl: handle chroma location

Use the video decoder chroma location flags and render chroma locations
other than centered. Until now, we've always used the intuitive and
obvious centered chroma location, but H.264 uses something else.

FFmpeg provides a small overview in libavcodec/avcodec.h:

-----------
/**
 *  X   X      3 4 X      X are luma samples,
 *             1 2        1-6 are possible chroma positions
 *  X   X      5 6 X      0 is undefined/unknown position
 */
enum AVChromaLocation{
    AVCHROMA_LOC_UNSPECIFIED = 0,
    AVCHROMA_LOC_LEFT        = 1, ///< mpeg2/4, h264 default
    AVCHROMA_LOC_CENTER      = 2, ///< mpeg1, jpeg, h263
    AVCHROMA_LOC_TOPLEFT     = 3, ///< DV
    AVCHROMA_LOC_TOP         = 4,
    AVCHROMA_LOC_BOTTOMLEFT  = 5,
    AVCHROMA_LOC_BOTTOM      = 6,
    AVCHROMA_LOC_NB             , ///< Not part of ABI
};
-----------

The visual difference is literally minimal, but since videophiles
apparently consider this detail as quality mark of a video renderer,
support it anyway. We don't bother with chroma locations other than
centered and left, though.

Not sure about correctness, but it's probably ok.

1 files changed, 5 insertions, 3 deletions

diff --git a/video/out/gl_video_shaders.glsl b/video/out/gl_video_shaders.glsl
index 47be240571..6337822c02 100644
--- a/video/out/gl_video_shaders.glsl
+++ b/video/out/gl_video_shaders.glsl
@@ -114,6 +114,7 @@ void main() {
 #!section frag_video
 uniform sampler2D textures[4];
 uniform vec2 textures_size[4];
+uniform vec2 chroma_center_offset;
 uniform sampler1D lut_c_1d;
 uniform sampler1D lut_l_1d;
 uniform sampler2D lut_c_2d;
@@ -321,17 +322,18 @@ vec4 sample_sharpen5(sampler2D tex, vec2 texsize, vec2 texcoord) {
 }
 
 void main() {
+    vec2 chr_texcoord = texcoord + chroma_center_offset;
 #ifndef USE_CONV
 #define USE_CONV 0
 #endif
 #if USE_CONV == CONV_PLANAR
     vec3 color = vec3(SAMPLE_L(textures[0], textures_size[0], texcoord).r,
-                      SAMPLE_C(textures[1], textures_size[1], texcoord).r,
-                      SAMPLE_C(textures[2], textures_size[2], texcoord).r);
+                      SAMPLE_C(textures[1], textures_size[1], chr_texcoord).r,
+                      SAMPLE_C(textures[2], textures_size[2], chr_texcoord).r);
     float alpha = 1.0;
 #elif USE_CONV == CONV_NV12
     vec3 color = vec3(SAMPLE_L(textures[0], textures_size[0], texcoord).r,
-                      SAMPLE_C(textures[1], textures_size[1], texcoord).rg);
+                      SAMPLE_C(textures[1], textures_size[1], chr_texcoord).rg);
     float alpha = 1.0;
 #else
     vec4 acolor = SAMPLE_L(textures[0], textures_size[0], texcoord);