video: add alpha type metadata

This is mostly for testing. It adds passing through the metadata through
the video chain. The metadata can be manipulated with vf_format. Support
for zimg alpha conversion (if built with zimg after it gained alpha
support) is implemented. Support premultiplied input in vo_gpu.

Some things still seem to be buggy.

1 files changed, 8 insertions, 0 deletions

diff --git a/DOCS/man/vf.rst b/DOCS/man/vf.rst
index 4f9268256c..3797ad26bd 100644
--- a/DOCS/man/vf.rst
+++ b/DOCS/man/vf.rst
@@ -339,6 +339,14 @@ Available mpv-only filters are:
         Force a specific scaler backend, if applicable. This is a debug option
         and could go away any time.
 
+    ``<alpha=auto|straight|premul>``
+        Set the kind of alpha the video uses. Undefined effect if the image
+        format has no alpha channel (could be ignored or cause an error,
+        depending on how mpv internals evolve). Setting this may or may not
+        cause downstream image processing to treat alpha differently, depending
+        on support. With ``convert`` and zimg used, this will convert the alpha.
+        libswscale and other FFmpeg components completely ignore this.
+
 ``lavfi=graph[:sws-flags[:o=opts]]``
     Filter video using FFmpeg's libavfilter.