gdal_retile.py handles alpha incorrectly (patch)

[cdestigter]

gdal_retile.py doesn't handle alpha channels properly. This results in semi-transparent black shadows when resampling input images. The black borders are more obvious in highly shrunk (ie from high res to low res) output.

EvenR helped me out with the attached patch, which fixes the problem by setting band interpretation correctly in gdal_retile.py, and using it in GDALReprojectImage.

Here's my command line:

rm -r ~/tmp/retiled
mkdir ~/tmp/retiled
gdal_retile.py -v -co ALPHA=YES -ps 4096 4096 -ot Byte -r bilinear -levels 4 -targetDir ~/tmp/retiled ~/tmp/src-images/0000000*.tif

I'll also attach some tiny RGBA test tiffs which demonstrate the problem. Compare the output in levels 3 and 4 of output before and after patching.

[Even Rouault]

r21095 /trunk/gdal/alg/gdalwarper.cpp: GDALReprojectImage() : correctly assign nSrcAlphaBand and nDstAlphaBand (#3821)

r21096 /trunk/gdal/swig/python/scripts/gdal_retile.py: gdal_retile.py : assign color interpretation (#3821)

r21097 /trunk/autotest/ (gcore/data/rgba.tif pyscripts/test_gdal_retile.py): Test gdal_retile.py with RGBA dataset (#3821)