[FFmpeg-devel] Re: [PATCH] vf_colorspace: Add an option to clamp trc LUT output

["Ronald S. Bultje via ffmpeg-devel" <ffmpeg-devel@ffmpeg.org>]

Hi Drew,

I believe you create an account (if you don't already have one), fork the
main FFmpeg repo (so you get a copy into your own personal account), push
the patch into a branch on your personal repo, and submit a PR to merge
that personal branch back into the main FFmpeg repo. It's similar to gitlab
or github, if that helps. If emails feels a bit tardish, you can ask for
help on IRC if you're not sure about any of these steps.

Thanks!
Ronald

On Tue, Sep 2, 2025 at 3:49 PM Drew Dunne <asdunne@google.com> wrote:

> Sure, I can submit it there. Is there documentation on how to submit a
> patch there? Do I just submit it as a pull request?
>
> On Tue, Aug 26, 2025 at 2:33 PM Ronald S. Bultje <rsbultje@gmail.com>
> wrote:
>
>> Hi Drew,
>>
>> Thanks for the bug report!
>>
>> On Mon, Aug 25, 2025 at 10:50 AM Drew Dunne via ffmpeg-devel <
>> ffmpeg-devel@ffmpeg.org> wrote:
>>
>>> A solid color image of 8-bit YUV: Y=157, U=164, V=98.
>>>
>>> Specify the input as:
>>>
>>> Input range: MPEG
>>> In color matrix: BT470BG
>>> In color primaries: BT470M
>>> In color transfer characteristics: Gamma 28
>>>
>>> Output as:
>>> Out color range: JPEG
>>> Out color matrix: BT.709
>>> Out color primaries: BT.709
>>> Out color transfer characteristics: BT.709
>>>
>>> During the calculation you get:
>>>
>>> Input YUV:                             y=157,      u=164,      v-98
>>> Post-yuv2rgb BT.470BG:                 r=0.456055, g=0.684152, b=0.928606
>>> Post-apply gamma28 linear LUT:         r=0.110979, g=0.345494, b=0.812709
>>> Post-color rotation BT.470M to BT.709: r=-0.04161, g=0.384626, b=0.852400
>>> Post-apply Rec.709 delinear LUT:       r=-0.16382, g=0.615932, b=0.923793
>>> Post-rgb2yuv Rec.709 matrix:           y=120,      u=190,      v=25
>>>
>>> Where with this change, the delinear LUT output would be clamped to 0,
>>> so the result would be:
>>> r=0.000000, g=0.612390, b=0.918807 and a final output of
>>> y=129, u=185, v=46
>>
>>
>> So for those of us playing along, a repro looks like this:
>>
>> $ hexdump /tmp/in.yuv
>> 0000000 9d9d 9d9d 62a4
>> $ git diff
>> diff --git a/libavfilter/vf_colorspace.c b/libavfilter/vf_colorspace.c
>> index e1f4725f635..512eb620fcf 100644
>> --- a/libavfilter/vf_colorspace.c
>> +++ b/libavfilter/vf_colorspace.c
>> @@ -348,11 +348,20 @@ static int convert(AVFilterContext *ctx, void
>> *data, int job_nr, int n_jobs)
>>           */
>>          s->yuv2rgb(rgb, s->rgb_stride, in_data, td->in_linesize, w, h,
>>                     s->yuv2rgb_coeffs, s->yuv_offset[0]);
>> +        printf("post-yuv2rgb - R: %d, G: %d, B: %d\n",
>> +               rgb[0][0], rgb[1][0], rgb[2][0]);
>>          if (!s->rgb2rgb_passthrough) {
>>              apply_lut(rgb, s->rgb_stride, w, h, s->lin_lut);
>> -            if (!s->lrgb2lrgb_passthrough)
>> +            printf("post-linearize - R: %d, G: %d, B: %d\n",
>> +                   rgb[0][0], rgb[1][0], rgb[2][0]);
>> +            if (!s->lrgb2lrgb_passthrough) {
>>                  s->dsp.multiply3x3(rgb, s->rgb_stride, w, h,
>> s->lrgb2lrgb_coeffs);
>> +                printf("post-rgb2rgb - R: %d, G: %d, B: %d\n",
>> +                       rgb[0][0], rgb[1][0], rgb[2][0]);
>> +            }
>>              apply_lut(rgb, s->rgb_stride, w, h, s->delin_lut);
>> +            printf("post-delinearize - R: %d, G: %d, B: %d\n",
>> +                   rgb[0][0], rgb[1][0], rgb[2][0]);
>>          }
>>          if (s->dither == DITHER_FSB) {
>>              s->rgb2yuv_fsb(out_data, td->out_linesize, rgb,
>> s->rgb_stride, w, h,
>> $ make -j4 && ./ffmpeg -s 2x2 -pix_fmt yuv420p -f rawvideo -c:v rawvideo
>> -i /tmp/in.yuv -vf
>> colorspace=ispace=bt470bg:iprimaries=bt470m:itrc=gamma28:irange=mpeg:range=jpeg:all=bt709
>> -y /tmp/out.yuv
>> [..]
>> post-yuv2rgb - R: 13076, G: 19616, B: 26625
>> post-linearize - R: 3182, G: 9906, B: 23302
>> post-rgb2rgb - R: -1193, G: 11028, B: 24440
>> post-delinearize - R: -4697, G: 17660, B: 26487
>>
>> Patch looks OK. Drew, would it be possible to submit the patch at
>> code.ffmpeg.org so I can merge it using our new system?
>>
>> Thanks,
>> Ronald
>>
>
>
> --
> Drew Dunne
> asdunne@google.com
>
>
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