vo_opengl: blend alpha components by default

Improves display of images and video with alpha channel, especially if
the transparent regions contain (supposed to be invisible) garbage
color values.

DOCS/man/en/vo.rst

@@ -454,13 +454,20 @@ Available video output drivers are:
         Default is 128x256x64.
         Sizes must be a power of two, and 256 at most.
 
-    ``alpha``
-        Try to create a framebuffer with alpha component. This only makes sense
-        if the video contains alpha information (which is extremely rare). May
-        not be supported on all platforms. If alpha framebuffers are
-        unavailable, it silently falls back on a normal framebuffer. Note
-        that when using FBO indirections (such as with ``opengl-hq``), an FBO
-        format with alpha must be specified with the ``fbo-format`` option.
+    ``alpha=<blend|yes|no>``
+        Decides what to do if the input has an alpha component (default: blend).
+
+        blend
+            Blend the frame against a black background.
+        yes
+            Try to create a framebuffer with alpha component. This only makes sense
+            if the video contains alpha information (which is extremely rare). May
+            not be supported on all platforms. If alpha framebuffers are
+            unavailable, it silently falls back on a normal framebuffer. Note
+            that when using FBO indirections (such as with ``opengl-hq``), an FBO
+            format with alpha must be specified with the ``fbo-format`` option.
+        no
+            Ignore alpha component.
 
     ``chroma-location=<auto|center|left>``
         Set the YUV chroma sample location. auto means use the bitstream

video/out/gl_video.c

@@ -271,6 +271,7 @@ static const struct gl_video_opts gl_video_opts_def = {
     .scale_sep = 1,
     .scalers = { "bilinear", "bilinear" },
     .scaler_params = {NAN, NAN},
+    .alpha_mode = 2,
 };
 
 
@@ -319,7 +320,10 @@ const struct m_sub_options gl_video_conf = {
                    ({"auto",   MP_CHROMA_AUTO},
                     {"center", MP_CHROMA_CENTER},
                     {"left",   MP_CHROMA_LEFT})),
-        OPT_FLAG("alpha", enable_alpha, 0),
+        OPT_CHOICE("alpha", alpha_mode, M_OPT_OPTIONAL_PARAM,
+                   ({"no", 0},
+                    {"yes", 1}, {"", 1},
+                    {"blend", 2})),
         {0}
     },
     .size = sizeof(struct gl_video_opts),
@@ -779,8 +783,8 @@ static void compile_shaders(struct gl_video *p)
                                    shader_prelude, PRELUDE_END);
 
     // Need to pass alpha through the whole chain. (Not needed for OSD shaders.)
-    bool use_alpha = p->opts.enable_alpha && p->has_alpha;
-    shader_def_opt(&header, "USE_ALPHA", use_alpha);
+    if (p->opts.alpha_mode == 1)
+        shader_def_opt(&header, "USE_ALPHA", p->has_alpha);
 
     char *header_osd = talloc_strdup(tmp, header);
     shader_def_opt(&header_osd, "USE_OSD_LINEAR_CONV", p->opts.srgb &&
@@ -831,8 +835,10 @@ static void compile_shaders(struct gl_video *p)
     shader_def_opt(&header_conv, "USE_INPUT_GAMMA", convert_input_gamma);
     shader_def_opt(&header_conv, "USE_COLORMATRIX", !p->is_rgb);
     shader_def_opt(&header_conv, "USE_CONV_GAMMA", convert_input_to_linear);
-    if (use_alpha && p->plane_count > 3)
+    if (p->opts.alpha_mode > 0 && p->has_alpha && p->plane_count > 3)
         shader_def(&header_conv, "USE_ALPHA_PLANE", "3");
+    if (p->opts.alpha_mode == 2 && p->has_alpha)
+        shader_def(&header_conv, "USE_ALPHA_BLEND", "1");
 
     shader_def_opt(&header_final, "USE_LINEAR_CONV_INV", p->use_lut_3d);
     shader_def_opt(&header_final, "USE_GAMMA_POW", p->opts.gamma);

video/out/gl_video.h

@@ -44,7 +44,7 @@ struct gl_video_opts {
     int temporal_dither;
     int fbo_format;
     int stereo_mode;
-    int enable_alpha;
+    int alpha_mode;
     int chroma_location;
 };
 

video/out/gl_video_shaders.glsl

@@ -387,6 +387,9 @@ void main() {
     color = floor(color * dither_quantization + dither_value + dither_center) /
                 dither_quantization;
 #endif
+#ifdef USE_ALPHA_BLEND
+    color = color * alpha;
+#endif
 #ifdef USE_ALPHA
     out_color = vec4(color, alpha);
 #else

video/out/vo_opengl.c

@@ -144,7 +144,7 @@ static bool config_window(struct gl_priv *p, uint32_t d_width,
     if (p->renderer_opts->stereo_mode == GL_3D_QUADBUFFER)
         flags |= VOFLAG_STEREO;
 
-    if (p->renderer_opts->enable_alpha)
+    if (p->renderer_opts->alpha_mode == 1)
         flags |= VOFLAG_ALPHA;
 
     if (p->use_gl_debug)