[FFmpeg-devel] Is it legal to modify h264 sps for cropping

Xiaolei Yu dreifachstein at gmail.com
Sat Apr 22 03:09:08 EEST 2017

On 04/21/2017 10:15 PM, Hendrik Leppkes wrote:
> On Fri, Apr 21, 2017 at 3:34 PM, Xiaolei Yu <dreifachstein at gmail.com> wrote:
>> Some android devices do not properly align the input surface of MediaCodec (Nexus 4, Galxy Note 2) and
>> we are forced to specify a multiple of 16 as the video width. A possible workaround is to insert the desired crop
>> into the output sps, but I am not sure if it is legal.
> H.264 always has a coded dimension which is a multiple of 16, and any
> smaller granularity on actual dimensions is achieved with cropping
> parameters.

I understand coding internally handles only 16x16 blocks. But I am not sure
whether a decoder is required to decode full blocks first before applying the
cropping. What happens if prediction requires data out side of the cropping area?

Say I have a 1088x1920 h264 file and crop it to (0,0 - 1080,1920) by editing the sps,
can I expect the new file is still conformant and the decoded images match those of
the original file in the cropped area?

> - Hendrik
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