[gdal-dev] Nodata and compression artifacts

[Simon Shak]

I’m working with gdalwarp to reprocess a large amount of imagery to be
compatible with another program that requires imagery to be in WGS84.  The
input imagery is compressed in MrSID format and does not include an
internal mask for nodata.  I don’t know if this is because the creator of
the imagery overlooked it, or if the format doesn’t support a mask.  Either
way, when I attempt to merge neighboring sets, I get odd bands of dark
color.  I’ve looked closely, and it is evident because at the edge of the
images are non 100% black pixels, that though I’m sending –srcnodata 0 into
gdalwarp, they get read as pixels and progress through.  I’ve looked into
using the nearblack command on the files first, but the compression ratio
of the .SID files makes it such that the files don’t easily fit into my
hard drive array for pre-nearblacking them before processing, plus the
physical size of some of these files are large enough that the nearblack
takes a long time to run.  Without the nearblack step, my multithreaded
control script can process one chunk in a day, but adding the nearblack,
and it increases to a week at least.



I’m looking for a solution that would not require making a large interim
uncompressed version and would hopefully not incur a lengthy additional
process.



The simpler thoughts I have would be to adjust gdalwarp’s –srcnodata to
take a range option, much like nearblack, so that if it detects a pixel
(even in the middle) that is with the range specified would get ignored, or
a way to include an ancillary file that could contain a mask.  Either would
work for me, I have potential ways to quickly generate a mask for the input
files.  I’d think the mask could work much like .TIF can have a .TFW, that
a .MSK could be detected as well.
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