[Bug 785011] videodecoder: Default buffer pool supports GstVideoAlignment but downstream might not

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=785011

--- Comment #42 from Nicolas Dufresne (stormer) <nicolas at ndufresne.ca> ---
Ok, I'm tryign to figure-out what's the bug left here, to me there is no bug
really.

(In reply to Sebastian Dröge (slomo) from comment #37)
> For the actual bug here, as discussed on IRC the GstVideoAlignment should
> only ever be used if GstVideoMeta is supported. Which avviddec seems to do
> though. So not sure yet what happens here exactly.

That's not true, you can enable GstVideoAlignment as long as downstream
supports GstVideoMeta. The trick is that the strides and offsets are updated
accordingly (and stored in GstVideoMeta). The problem is whenever something
upstream assumes downstream support GstVideoMeta.

While it's not validated, upstream need to pick padding that does not disturb
the chroma. That how we avoid spurious copy in something like "avdec_... !
videoconvert ! ximagesink". avdec_ uses a default pool and activate the video
alignment to do direct rendering, and then videoconvert will video_frame_map()
as if there was nothing special about the buffer.

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