[FFmpeg-user] fftfilt

Michael Koch astroelectronic at t-online.de
Thu Oct 14 00:09:46 EEST 2021

Am 13.10.2021 um 22:57 schrieb 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?

In the current state the filter's cutoff frequency is a function of 
image size. The cutoff frequency jumps by a factor 2 when the image size 
increases from 230 to 232. That's because of the factor 10/9 in line 185.
I want to make the cutoff frequency independant of image size, and 
therefore I need the array size.
Will work for all types of filters: lowpass, highpass, bandpass and notch.


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