[FFmpeg-devel] [PATCH 2/2] ffmpeg: Use the colour properties from the input stream when doing transcode

[Tobias Rapp]

On 25.05.2018 07:58, Xiang, Haihao wrote:
> On Thu, 2018-05-24 at 11:15 +0100, Mark Thompson wrote:
>>
>> For example:
>>
>> ffmpeg -i bt709_input.mkv -vf colorspace=bt2020 bt2020_output.mkv
>>
>> will have the output file marked as BT.709 after this patch, where previously
>> it was "unspecified".  (Explicitly setting -color_primaries/-color_trc/-
>> colorspace on the output works in both cases.)
> 
> I agree with you it's not worse than before as we don't get the expected result
> in both cases.

Not quite: When a file says "I don't know this property value" you have 
a chance to lookup the value somewhere else or use a default. When it 
says "I know the value" and gives a wrong value, you completely loose trust.

So in my opinion this patch should not be applied, as it possibly makes 
ffmpeg generate files with wrong information where it just had no 
information before. The correct approach would be to set the encoder 
properties from output frame data and only write a header once the 
encoders have been initialized completely.

Regards,
Tobias