[FFmpeg-devel] [PATCH] Add coded_width/height to AVCodecParameters

[wm4]

On Sun, 3 Apr 2016 15:26:02 +0200
Michael Niedermayer <michael at niedermayer.cc> wrote:

> On Sun, Apr 03, 2016 at 02:31:06PM +0200, Hendrik Leppkes wrote:
> > On Sun, Apr 3, 2016 at 2:07 PM, Michael Niedermayer
> > <michael at niedermayer.cc> wrote:  
> > > Without this or some other change moving data back and forth corrupts it
> > > as the cropped & possibly low res resolution modified w/h will be taken
> > > a the pre lowres resolution
> > > also without 2 sets theres no place to communicate the cropped dimensions
> > >
> > > can be reproduced with
> > > ./ffplay_g -vlowres 2 tickets/162/avid.avi
> > >  
> > 
> > lowres is supposed to be applied in the decoder, not the demuxer, so I
> > don't see how this is required here.  
> 
> if iam not mistaken (please correct me if iam wrong)
> 
> the demuxer runs a decoder, which initializes various AVCodecContext
> fields these then get copied into AVCodecParameters and back into
> the users AVCodecContext used for the user appliations decoder
> 
> the first decoder will set width & height to the low resolution
> with lowres, coded_width / height will represent the stored, true
> width/height
> the data copied into the AVCodecParameters will have lost the width
> & height that is stored anywhere but rather contain the lowres
> variant.
> 
> the user app then will initialize its decoder based on coded_width
> and coded_height which it will find set to 0 so it fallback to the
> compatibility hack of using width & height which are the lowres
> values and the decoder will apply the lowres stuff again
> 
> above is based on some assumtations i have not verified very step with
> a debugger
> 
> 
> > AVCodecParameters is supposed to contain container information, not
> > pre-interpreted info based on some decoding options.  
> 

This just shows how broken using decoders in libavformat is by design.
The output parameters of a decoder have not necessarily anything to do
with the input codec parameters.

In any case, not sure why we'd care about an obscure semi-broken
feature like lowres anyway.