[FFmpeg-user] Question about "normalize" filter

[Michael Koch]

Am 29.01.2023 um 22:05 schrieb Paul B Mahol:
> On 1/29/23, Michael Koch <astroelectronic at t-online.de> wrote:
>> Am 29.01.2023 um 19:32 schrieb Paul B Mahol:
>>> On 1/29/23, Michael Koch <astroelectronic at t-online.de> wrote:
>>>> Hello,
>>>>
>>>> if I understood the documentation correctly, the normalize filter maps
>>>> the darkest input pixel to blackpt and the brightest input pixel to
>>>> whitept:
>>>> darkest pixel --> blackpt
>>>> brightest pixel --> whitept
>>>>
>>>> However I need a slightly different mapping:
>>>> A black input pixel shall remain black, and the brightest input pixel
>>>> shall become white.
>>>> black --> blackpt
>>>> brightest pixel --> whitept
>>>>
>>>> With other words: Just multiply all pixels by a suitable constant. Don't
>>>> add or subtract anything.
>>>> Is this possible?
>>>>
>>>> Known workaround: Make sure that the input frame contains a black pixel,
>>>> by inserting one in a corner.
>>> Try attached patch.
>> How must I set the options for the desired behaviour?
> Set first strength to reverse of second strength.
> So 1.0 and 0.0 or 0.0 and 1.0

I did try with strength=0:strength2=1 but the output isn't as expected.

I'm using this input image:
http://www.astro-electronic.de/flat.png

The pixel values are about 171 in the center and 107 in the top right 
corner.
The center to corner ratio is 171 / 107 = 1.6

In the output image I measure 248 in the center (which is almost as 
expected, probably correct because I'm measuring the average of a 7x7 
neighborhood), but I measure 122 in the top right corner.
The center to corner ratio is 248 / 122 = 2.03
The corner is too dark.

Michael