vo_opengl: add alpha output

Allows playing video with alpha information on X11, as long as the video
contains alpha and the window manager does compositing. See vo.rst.

Whether a window can be transparent is decided by the choice of the X
Visual used for window creation. Unfortunately, there's no direct way to
request such a Visual through the GLX or the X API, and use of the
XRender extension is required to find out whether a Visual implies a
framebuffer with alpha used by XRender (see for example [1]). Instead of
depending on the XRender wrapper library (which would require annoying
configure checks, even though XRender is virtually always supported),
use a simple heuristics to find out whether a Visual has alpha. Since
getting it wrong just means an optional feature will not work as
expected, we consider this ok.

[1] http://stackoverflow.com/questions/4052940/how-to-make-an-opengl-
rendering-context-with-transparent-background/9215724#9215724

1 files changed, 3 insertions, 0 deletions

diff --git a/video/out/vo_opengl.c b/video/out/vo_opengl.c
index 6c0901eae6..ba31e0cbf5 100644
--- a/video/out/vo_opengl.c
+++ b/video/out/vo_opengl.c
@@ -133,6 +133,9 @@ 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)
+        flags |= VOFLAG_ALPHA;
+
     if (p->use_gl_debug)
         flags |= VOFLAG_GL_DEBUG;