[gdal-dev] Nodata and compression artifacts

[Even Rouault]

Le lundi 25 novembre 2013 22:29:00, Simon Shak a écrit :
> 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.

You can use the -cutline option of gdalwarp if you have the mask as a shapefile 
or another OGR datasource.
You could also use a VRT file to combine the MrSID imagery and add another band 
from TIF for example as the alpha/mask band.


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