[Gdal-dev] gdalwarp Transparency Question

[Frank Warmerdam]

Reinaldo Escada Chohfi wrote:
> Hello,
> 
> I'm runing gdalwarp (FWTools 1.0.0a7) to mosaic geotiff images because 
> the images are large and I run into memory problems with gdal_merge.
> 
> I get some strange results in several mosaics in the overlapping areas 
> between images.  I use -srcnodata 0 to have gdalwarp do the transparency 
> in these overlapping areas.
> 
> My question is, if am using -scrnodata 0, will the transparency be made 
> when all 3 pixel values are 0, 0, 0 (RGB) or whenever any of the bands 
> has a pixel value equal to 0?  It looks like I get a transparency only 
> in the Blue, sometimes only in the Red or Green bands and the image has 
> blue, red or green spots.  Do I have to use -scrnodata value "0 0 0" in 
> order to get transparency only when RGB are equal to 0, 0, 0?
> 
> Thanks in advance for the help.

Reinaldo,

Looking at the code, it seems I implemented UNIFIED_SRC_NODATA as the
default for gdalwarp circa September 2005.  A quick checks shows FWTools
1.0.0a7 as being built in early November of 2005.  So it should include this.

The UNIFIED_SRC_NODATA enabling forces gdalwarp to only consider a pixel as
nodata if the values match for all bands.  So, gdalwarp *should* do what
you want already.  That it is not may be a bug in one or more of the
warping logic cases.

Could you first try a newer FWTools and see if the problem persists?  If it
does, could you submit a bug report detailing how to reproduce your problem?

Best regards,
-- 
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