Re: [FFmpeg-devel] [PATCH] avcodec/exr: tag gamma=1.0 output as linear light

[Leo Izen <leo.izen@gmail.com>]

On 8/17/23 08:59, Tomas Härdin wrote:
> ons 2023-08-16 klockan 01:20 -0400 skrev Leo Izen:
>> By default the OpenEXR decoder outputs linear light pixel data by
>> applying a gamma=1.0 transfer (i.e. a no-op). When it does so, it
>> should tag the data as linear so color-managed filters or other tools
>> can work with it correctly.
>>
>> Signed-off-by: Leo Izen <leo.izen@gmail.com>
>> ---
>>   libavcodec/exr.c | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/libavcodec/exr.c b/libavcodec/exr.c
>> index fae1d08ab0..518066facf 100644
>> --- a/libavcodec/exr.c
>> +++ b/libavcodec/exr.c
>> @@ -2088,6 +2088,8 @@ static int decode_frame(AVCodecContext *avctx,
>> AVFrame *picture,
>>   
>>       if (s->apply_trc_type != AVCOL_TRC_UNSPECIFIED)
>>           avctx->color_trc = s->apply_trc_type;
>> +    else if (s->gamma > 0.9999f && s->gamma < 1.0001f)
>> +        avctx->color_trc = AVCOL_TRC_LINEAR;
> 
> I'm going to be difficult here and point out that gamma=0.99991 is not
> linear. It's probably linear *enough* most of the time, but also 1.0
> can be exactly represented by float so an equality check seems
> appropriate.

This exact check exists elsewhere in the source file. There's a branch 
where it sets up a LUT if gamma is not between those values, and has a 
no-op track otherwise - so in the event you request 0.99995 or something 
of that form, the code that does the color conversion treats it as 1.0f.

- Leo Izen


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