From 112d2a2c500682003798ae60fc6cd0d5dabbe174 Mon Sep 17 00:00:00 2001
From: wm4 <wm4@nowhere>
Date: Sat, 15 Aug 2020 12:04:35 +0200
Subject: command: export alpha type in format properties

---
 DOCS/man/input.rst |  7 +++++++
 player/command.c   | 11 +++++++++++
 2 files changed, 18 insertions(+)

diff --git a/DOCS/man/input.rst b/DOCS/man/input.rst
index 6c589e7521..6213eb29fb 100644
--- a/DOCS/man/input.rst
+++ b/DOCS/man/input.rst
@@ -2328,6 +2328,11 @@ Property list
         Source file stereo 3D mode. (See the ``format`` video filter's
         ``stereo-in`` option.)
 
+    ``video-params/alpha``
+        Alpha type. If the format has no alpha channel, this will be unavailable
+        (but in future releases, it could change to ``no``). If alpha is
+        present, this is set to ``straight`` or ``premul``.
+
     When querying the property with the client API using ``MPV_FORMAT_NODE``,
     or with Lua ``mp.get_property_native``, this will return a mpv_node with
     the following contents:
@@ -2351,6 +2356,8 @@ Property list
             "chroma-location"   MPV_FORMAT_STRING
             "rotate"            MPV_FORMAT_INT64
             "stereo-in"         MPV_FORMAT_STRING
+            "average-bpp"       MPV_FORMAT_INT64
+            "alpha"             MPV_FORMAT_STRING
 
 ``dwidth``, ``dheight``
     Video display size. This is the video size after filters and aspect scaling
diff --git a/player/command.c b/player/command.c
index 1a20962c29..560b9d71e2 100644
--- a/player/command.c
+++ b/player/command.c
@@ -2175,6 +2175,13 @@ static int property_imgparams(struct mp_image_params p, int action, void *arg)
     for (int i = 0; i < desc.num_planes; i++)
         bpp += desc.bpp[i] >> (desc.xs[i] + desc.ys[i]);
 
+    // Alpha type is not supported by FFmpeg, so MP_ALPHA_AUTO may mean alpha
+    // is of an unknown type, or simply not present. Normalize to AUTO=no alpha.
+    if (!!(desc.flags & MP_IMGFLAG_ALPHA) != (p.alpha != MP_ALPHA_AUTO)) {
+        p.alpha =
+            (desc.flags & MP_IMGFLAG_ALPHA) ? MP_ALPHA_STRAIGHT : MP_ALPHA_AUTO;
+    }
+
     struct m_sub_property props[] = {
         {"pixelformat",     SUB_PROP_STR(mp_imgfmt_to_name(p.imgfmt))},
         {"average-bpp",     SUB_PROP_INT(bpp),
@@ -2201,6 +2208,10 @@ static int property_imgparams(struct mp_image_params p, int action, void *arg)
         {"stereo-in",
             SUB_PROP_STR(m_opt_choice_str(mp_stereo3d_names, p.stereo3d))},
         {"rotate",          SUB_PROP_INT(p.rotate)},
+        {"alpha",
+            SUB_PROP_STR(m_opt_choice_str(mp_alpha_names, p.alpha)),
+            // avoid using "auto" for "no", so just make it unavailable
+            .unavailable = p.alpha == MP_ALPHA_AUTO},
         {0}
     };
 
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