media/vpx: Add support for VP9 alpha channel

media/filters/vpx_video_decoder.cc

Index: media/filters/vpx_video_decoder.cc
diff --git a/media/filters/vpx_video_decoder.cc b/media/filters/vpx_video_decoder.cc
index 40814bb273ad5a3f5cc18e96ea9172f9fd4e929b..65d9742af10ea2bf75b2ece03d319eb1e8a112bd 100644
--- a/media/filters/vpx_video_decoder.cc
+++ b/media/filters/vpx_video_decoder.cc
@@ -380,11 +380,11 @@ bool VpxVideoDecoder::ConfigureDecoder(const VideoDecoderConfig& config) {
     return false;
 
   // These are the combinations of codec-pixel format supported in principle.
-  // Note that VP9 does not support Alpha in the current implementation.
   DCHECK(
       (config.codec() == kCodecVP8 && config.format() == PIXEL_FORMAT_YV12) ||
       (config.codec() == kCodecVP8 && config.format() == PIXEL_FORMAT_YV12A) ||
       (config.codec() == kCodecVP9 && config.format() == PIXEL_FORMAT_YV12) ||
+      (config.codec() == kCodecVP9 && config.format() == PIXEL_FORMAT_YV12A) ||
       (config.codec() == kCodecVP9 && config.format() == PIXEL_FORMAT_YV24));
 
 #if !defined(DISABLE_FFMPEG_VIDEO_DECODERS)
@@ -400,9 +400,10 @@ bool VpxVideoDecoder::ConfigureDecoder(const VideoDecoderConfig& config) {
   if (!vpx_codec_)
     return false;
 
-  // Configure VP9 to decode on our buffers to skip a data copy on decoding.
-  if (config.codec() == kCodecVP9) {
-    DCHECK_NE(PIXEL_FORMAT_YV12A, config.format());
+  // Configure non-YV12A VP9 to decode on our buffers to skip a data copy on

[DaleCurtis, 2016/01/05 21:22:21]

I guess we can't copy into one of the buffers since each buffer may be reused
for a future frame?

[vignesh, 2016/01/06 00:07:49]

The thing is that the Y, U and V planes come from one instance of the decoder
and the Alpha plane comes from another instance of the decoder. It seems a lot
simpler to just do a copy of all the 4 planes.

[DaleCurtis, 2016/01/06 00:18:02]

Hmm, could you over allocate the first buffer and copy in the A plane? If the
non-alpha decoder isn't using that plane, seems we could avoid a copy of the
main plane (most expensive) and keep only the copy of the A plane.

[vignesh, 2016/01/08 17:53:40]

Good point, i will try it out and update the CL.

[vignesh, 2016/01/08 19:21:33]

ok, i looked into this and i'm a little confused. can you please explain a
little more about what you mean by "over allocate the first buffer" ?

as it is, we are using VideoFrame::WrapExternalYuvData for the Y,U and V planes.
Which means VideoFrame doesn't own any of those buffers. And to copy the Alpha
planes, we would need VideoFrame's own buffer. So it ends up being confusing.

[DaleCurtis, 2016/01/08 19:29:03]

Yes, you'd need to add a WrapExternalYUVA method and then change the memory pool
to allocate the space for the extra plane.  You would then copy the A plane into
the buffer owned by the memory pool -- which since the first decoder is only YUV
it would never know about.

+  // decoding. We do not use our buffers for YV12A VP9 since it involves two
+  // different instances of the decoder.
+  if (config.codec() == kCodecVP9 && config.format() != PIXEL_FORMAT_YV12A) {
     DCHECK(vpx_codec_get_caps(vpx_codec_->iface) &
            VPX_CODEC_CAP_EXTERNAL_FRAME_BUFFER);
 
@@ -581,7 +582,7 @@ bool VpxVideoDecoder::CopyVpxImageToVideoFrame(
 
   if (memory_pool_.get()) {
     DCHECK_EQ(kCodecVP9, config_.codec());
-    DCHECK(!vpx_codec_alpha_) << "Uh-oh, VP9 and Alpha shouldn't coexist.";
+    DCHECK(!vpx_codec_alpha_);
     *video_frame = VideoFrame::WrapExternalYuvData(
         codec_format,
         coded_size, gfx::Rect(visible_size), config_.natural_size(),