Paul B Mahol
onemda at gmail.com
Wed Oct 13 23:57:13 EEST 2021
On Wed, Oct 13, 2021 at 10:51 PM Michael Koch <astroelectronic at t-online.de>
> 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
> >>> -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?
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