[gdal-dev] Nearblack Eating Image Border

[Daniel Morissette]

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