Am 13.10.2021 um 23:09 schrieb Michael Koch:
Am 13.10.2021 um 22:57 schrieb Paul B Mahol:
On Wed, Oct 13, 2021 at 10:51 PM Michael Koch
<astroelectro...@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
<astroelectro...@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.
You can reproduce that with these commands:
set "P=8"
rem create 230x230 test image
ffmpeg -f lavfi -i color=black:s=230x230 -lavfi
geq='r=127.5+127.5*cos((X-W/2)*PI/(pow(2,
(1+2*Y/H))))',colorchannelmixer=1:0:0:0:1:0:0:0:1:0:0:0,split[h][v];[v]transpose[v];[v][h]hstack
-frames 1 -y test.png
rem bandpass for wavelength 8 pixels
ffmpeg -i test.png -vf
scale=2*iw:2*ih,fftfilt=dc_Y=128:dc_U=1:dc_V=1:weight_Y='between(hypot(Y/H,X/W),1.9/%P%,2.1/%P%)':weight_U=1:weight_V=1,scale=iw/2:ih/2
-y out230.png
rem create 232x232 test image
ffmpeg -f lavfi -i color=black:s=232x232 -lavfi
geq='r=127.5+127.5*cos((X-W/2)*PI/(pow(2,
(1+2*Y/H))))',colorchannelmixer=1:0:0:0:1:0:0:0:1:0:0:0,split[h][v];[v]transpose[v];[v][h]hstack
-frames 1 -y test.png
rem bandpass for wavelength 8 pixels
ffmpeg -i test.png -vf
scale=2*iw:2*ih,fftfilt=dc_Y=128:dc_U=1:dc_V=1:weight_Y='between(hypot(Y/H,X/W),1.9/%P%,2.1/%P%)':weight_U=1:weight_V=1,scale=iw/2:ih/2
-y out232.png
pause
I did already try to make the cutoff frequency independant on image size
by using X/W and Y/H. That works fine if the image size is halved or
doubled. But it doesn't work if the image size is increased by a smaller
factor.
I think the correct way is to use X/ARRAY_H and Y/ARRAY_V
Michael
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