[FFmpeg-devel] [PATCH] lavf/vf_freezedetect: improve for the freeze frame detection

[Limin Wang]

On Mon, Jul 15, 2019 at 11:07:32PM +0200, Marton Balint wrote:
> 
> 
> On Mon, 15 Jul 2019, Limin Wang wrote:
> 
> [...]
> 
> >>>>>        if (s->width[plane]) {
> >>>>>            uint64_t plane_sad;
> >>>>>            s->sad(frame->data[plane], frame->linesize[plane],
> >>>>>@@ -140,8 +146,12 @@ static int is_frozen(FreezeDetectContext *s, AVFrame *reference, AVFrame *frame)
> >>>>>        }
> >>>>>    }
> >>>>>    emms_c();
> >>>>>-    mafd = (double)sad / count / (1ULL << s->bitdepth);
> >>>>>-    return (mafd <= s->noise);
> >>>>>+    mafd = (double)sad /(count >> (s->bitdepth > 8));
> >>>>
> >>>>Why? MAFD should be the mean difference normalized to [0..1].
> >>>if the bitdeth is 16, it'll divide by 1UL << 16, it'll cause the mafd is
> >>>very small. So I choose the scenecut way in the below.
> >>
> >>The metric supposed to be indepentent of the bit depth. If you get
> >>different mafd for the same source expressed in 8bit, 10bit or 16bit
> >>pixel format (provided that you use the same subsampling), then you
> >>are doing something wrong.
> >>
> >>And you can't reinvent MAFD according to your needs. It is the mean
> >>absolute difference of the whole image normalized to 0..1 in this
> >>case. If you are not calculating that, then it is not MAFD.
> >
> >Sorry, now the scenecut detection support rgb only, so I'm
> >changing it to support more format without autoscale, it'll
> >necessary to solved how to calculated mafd for
> >10bits or 12bits. Below is my understanding for the mafd
> >calculation, correct me if I'm misunderstanding. for example, if
> >the bitdepth is 8 bits, it means the pixel block is 8x8 pixel, so
> >maybe we should divide 8x8 instead of 256?
> >
> >mafd = sad / count / (bitdepth * bitdepth)
> 
> bitdepth is the bitdepth of one pixel. For 8 bit, a color component can be
> 0..255. For 16 bit, a color component can be 0..65535.
> 
> 8x8 and 16x16 pixel blocks are totally different and unrelated
> things from bit depth. Yes, SAD is usually calculated for 8x8 or
> 16x16 blocks for motion estimation or compression, not here. Here,
> SAD is the sum of absolute differences throughout the entire image,
> (width x height). Count = width * height, so SAD/count is the
> average of differences. In order to get it to [0..1] you have to
> divide by 2^bitdepth.
Thank you, I think I'm clear for the code, the 2^bitdepth is only for
normalize.

I have submited the scenecut detected change for more format support. 
For the result is little different with rgb format and I haven't good way
to sure it's expected result.  

https://patchwork.ffmpeg.org/patch/13957/

> 
> Regards,
> Marton
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