[FFmpeg-user] fftfilt

[Paul B Mahol]

On Wed, Oct 13, 2021 at 10:51 PM Michael Koch <astroelectronic at t-online.de>
wrote:

> Am 12.10.2021 um 19:29 schrieb Michael Koch:
> > Am 12.10.2021 um 18:52 schrieb Paul B Mahol:
> >> On Tue, Oct 12, 2021 at 10:55 AM Michael Koch
> >> <astroelectronic at t-online.de>
> >> wrote:
> >>
> >>> I have a question about the "fftfilt" filter. What's the default value
> >>> of the weight_U and weight_V options? I'm asking because I get an
> >>> unexpected result.
> >>>
> >>> This command line creates my input image for testing:
> >>>
> >>> ffmpeg -f lavfi -i color=black:s=300x50 -lavfi
> >>>
> drawgrid=c=white:y=-1:w=2:h=51,split[a][b];[b]crop=iw/2:x=0,scale=2*iw:ih:flags=neighbor,split[b][c];[c]crop=iw/2:x=0,scale=2*iw:ih:flags=neighbor,split[c][d];[d]crop=iw/2:x=0,scale=2*iw:ih:flags=neighbor,split[d][e];[e]crop=iw/2:x=0,scale=2*iw:ih:flags=neighbor,split[e][f];[f]crop=iw/2:x=0,scale=2*iw:ih:flags=neighbor[f];[a][b][c][d][e][f]vstack=6,split[h][v];[v]transpose[v];[v][h]hstack
>
> >>>
> >>>
> >>> -frames 1 -y test.png
> >>>
> >>> This is the fftfilt lowpass example from the official documentation:
> >>>
> >>> ffmpeg -i test.png -vf fftfilt=dc_Y=0:weight_Y='squish((Y+X)/100-1)' -y
> >>> out1.png
> >>>
> >>> Problem: The output has a greenish tint.
> >>>
> >> Expressions by default for U and V are copied from Y if are unset.
> >> filter works only in YUV or gray space thus in above combination one
> >> gets
> >> green tint.
> >
>
> I'm trying to make the filter's cutoff frequency independant of the
> image size. But that's not so easy because I don't know the size of the
> FFT array. It's calculated in vf_fftfilt.c lines 185 and 297.
> This calculation is difficult (and slow) to replicate in an expression,
> because either a loop or a logarithm is required.
> Would it be possible to add two new variables so that the FFT array size
> can be used in an expression?
> ARRAY_H = 1 << rdft_hbits
> ARRAY_V = 1 << rdft_vbits
> It the array size is known, things would become much easier.
>

What are you attempting to do?


> Thanks,
> Michael
>
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