[gdal-dev] Nearblack Eating Image Border

[Darafei "Komяpa" Praliaskouski]

I believe this behavior is great for over-compressed imagery JPEGs where
you have corrupted border of several "black-poisoned" non-black pixels that
you'd better remove and take from some other image in the mosaic, since in
this scenario you usually have an overlap.

On Wed, Feb 20, 2019 at 4:16 PM Daniel Morissette <dmorissette at mapgears.com>
wrote:

> I noticed this behavior as well and I think it's a bug. I think it would
> make more sense to keep the last "-nb" pixels that are not nearblack
> instead of dropping them.  The effect is especially annoying when you
> have image tiles and you end up with a 1-2 pixels gap between them.
>
> I didn't test what happens if we change the behavior. So maybe there is
> a reason why it was implemented this way?
>
> Daniel
>
> On 2019-02-19 6:13 p.m., Christopher Mitchell wrote:
> > I've encountered an error in an image processing pipeline I've built
> > where nearblack is removing pixels at the border of images, even if
> > they don't fit the is-"black" criterion. Specifically, I'm using
> > nearblack to remove nearly-black areas touching the edges of lossy
> > image tiles that should be fully black. Those images are later
> > combined into VRT files and warped; where we have multiple tiles
> > overlapping an area, some of which are missing data, we therefore can
> > get complete coverage.
> >
> > Unfortunately, for tiles that have no black areas, nearblack is still
> > eating the -nb argument's number of pixels at the edges, turning them
> > into black (0, 0, 0, 0, where we're using 4-channel images), even if
> > those pixels are nowhere near the color criterion to be recognized as
> > black. My understanding was that nearblack should only be proceeding
> > into the interior of the non-black area and then setting pixels to
> > black if those pixels are /actually/ nearly black.
> >
> > Am I encountering a bug with nearblack? Do I misunderstand how the -nb
> > argument works? Happy to provide a MWE if any of the above is unclear,
> > and thanks in advance.
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> >
>
>
> --
> Daniel Morissette
> Mapgears Inc
> T: +1 418-696-5056 #201
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-- 
Darafei Praliaskouski
Support me: http://patreon.com/komzpa
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