Note that if the codec doesn't support DR1, the image has to be copied.
There is no other way to guarantee that the image will be valid after
decoding the next image.
The only important codec that doesn't support DR1 yet is rawvideo. It's
likely that ffmpeg/Libav will fix this at some time. For now, this
decoder uses an evil hack and puts pointers to the packet data into the
returned frame. This means the image will actually get invalid as soon
as the corresponding video packet is free'd, so copying the image is the
only reasonable thing anyway.
Replace libavcodec's native buffer allocation with code taken from
ffplay/ffmpeg's libavfilter support. The code in lavc_dr1.c is directly
copied from cmdutils.c. Note that this is quite arcane code, which
contains some workarounds for decoder bugs and the like. This is not
really a maintainance burden, since fixes from ffmpeg can be directly
applied to the code in lavc_dr1.c.
It's unknown why libavcodec doesn't provide such a function directly.
avcodec_default_get_buffer() can't be reused for various reasons.
There's some hope that the work known as The Evil Plan [1] will make
custom get_buffer implementations unneeded.
The DR1 support as of this commit does nothing. A future commit will
use it to implement ref-counting for mp_image (similar to how AVFrame
will be ref-counted with The Evil Plan.)
[1] http://lists.libav.org/pipermail/libav-devel/2012-December/039781.html
wm4