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Date: Fri, 18 Jul 2025 14:46:53 +0200
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From: Niklas Haas <ffmpeg@haasn.xyz>
To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
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Subject: Re: [FFmpeg-devel] [PATCH v3 1/3] avfilter/vf_colordetect: add new
 color range detection filter
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On Fri, 18 Jul 2025 14:38:04 +0200 Kacper Michajlow <kasper93@gmail.com> wrote:
> > +static inline int ff_detect_range_c(const uint8_t *data, ptrdiff_t stride,
> > +                                    ptrdiff_t width, ptrdiff_t height,
> > +                                    int mpeg_min, int mpeg_max)
> > +{
> > +    while (height--) {
> > +        for (int x = 0; x < width; x++) {
> > +            const uint8_t val = data[x];
> > +            if (val < mpeg_min || val > mpeg_max)
> > +                return 1;
> > +        }
> > +        data += stride;
> > +    }
> > +
> > +    return 0;
> > +}
>
> You could process width as a whole to allow better vectorization.
> Assuming you don't process 10000x1 images, it will be faster on average.

That's what I had in v1 of my patch, but it is significantly (50%) slower
on GCC, which prefers the version I have written above.

There is the not insignificant point that this C routine is also being used
to handle remaining elements that don't fit into a multiple of the SIMD
kernel, for which the scalar code is actually preferred.
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