[FFmpeg-user] Question about "normalize" filter

[Paul B Mahol]

On 1/29/23, Michael Koch <astroelectronic at t-online.de> wrote:
> Am 29.01.2023 um 23:07 schrieb Paul B Mahol:
>> On 1/29/23, Michael Koch <astroelectronic at t-online.de> wrote:
>>> 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.
>>>
>> I checked with oscilloscope filter (s=1:tw=1:t=1:x=0), far left pixels
>> (as they are darkest) and they are not changing (min values are same
>> with and without filter run)
>> With default parameters and just strength(2) set to your values, so
>> the darkest pixels are left  untouched. Did not checked brightest
>> pixels output, but they should be correct too.
>
> But that's not the behaviour I need. All pixels shall be multiplied by
> the same suitable constant, so that the brightest pixel becomes white.
>
> Input center: 171
> Input corner: 107
>
> constant c = 255 / 171 =1.49
> Output center: 171 * c = 255
> Output corner: 107 * c = 160
>

Normalization does not do that and that functionality does not belong
to such filter, it stretches ranges of all pixel values so they reach
maximal possible range.