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

[Drew Dunne via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

Ronald,

Thanks! Created a PR at https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/20438

- Drew

On Wed, Sep 3, 2025 at 8:31 AM Ronald S. Bultje <rsbultje@gmail.com> wrote:

> 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
>>
>>

-- 
Drew Dunne
asdunne@google.com
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