[FFmpeg-user] Question about "normalize" filter

[Paul B Mahol]

On Mon, Jan 30, 2023 at 12:23 AM Michael Koch <astroelectronic at t-online.de>
wrote:

> Am 29.01.2023 um 23:36 schrieb 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.
>
> Can't you just add an option that disables the minimum finding
> algorithm, and set the minimum to zero (=black)? That would do the job.
>

I just did, but for whatever reason you think its incorrect.


>
> Let me explain why I need this kind of normalization.
> Most lenses have vignetting. Especially in astronomical images,
> vignetting must be corrected before any other image processing can be
> done. For this purpose a flatfield image is taken with the same lens at
> the same aperture, but with a uniform white screen in front of the lens.
> Normally the flatfield image is exposed at roughly 50% gray level, to
> avoid problems with nonlinearity near white level. The linked image is
> the flatfield.
> Vignetting in an astronomical image is corrected by dividing the image
> by the flatfield. That can be done with the blend filter.
> If I use the flatfied as-is (with roughly 50% gray level), then the
> image is effectively multiplied by a factor 2. That's a problem because
> bright pixels might get clipped. To minimize this problem, I want to
> normalize the flatfield as close to white level as possible, before
> using it.
>
> Michael
>
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