[FFmpeg-user] Question about "normalize" filter

Michael Koch astroelectronic at t-online.de
Mon Jan 30 12:22:05 EET 2023

Am 30.01.2023 um 10:29 schrieb Paul B Mahol:
>> You wrote:
>> "With default parameters and just strength(2) set to your values, so
>> the darkest pixels are left  untouched."
>> However I want that black remains untouched. That's not the same, because
>> the darkest pixels in the image aren't black.
>> The workaround is to insert a black pixel before normalizing:
>> -vf drawbox=w=1:h=1:color=black,normalize
> This command produces more convincing output:
> -vf colorlevels=rimin=0:gimin=0:bimin=0:rimax=-1:gimax=-1:bimax=-1
> this will pick max pixels values from input and stretch it do default
> output max.

This command gives almost the same result as my workaround.
The meaning of -1 for options is missing in the documentation.
It has the same problem as my workaround: The output is only 8-bit.

It would be nice to have all these image processing filters with float 
-- Hot pixel removal (save hot pixels in a mask, and replace those 
pixels by average of neighbor pixels)
-- Dark image subtraction
-- Flatfield correction (division)
-- Contrast enhancement (similar as colorlevels)
-- Gamma correction (might already exist in eq filter with float 
precision, I'm not sure)

For colorlevels it would be nice to have options "imin" and "imax" which 
set all three RGB channels simultaneously to the same value. Makes the 
command line much shorter.


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