gdalwarp zeroes all alpha pixels from grey+alpha image

[cdestigter]

When warping a grey+alpha image, gdal appears to zero out all the alpha pixels, resulting in a completely invisible image.

test case:

gdalinfo -stats ~/c/in.tif | grep Minimum
rm -f ~/c/out.tif* ; gdalwarp ~/c/in.tif ~/c/out.tif
gdalinfo -stats ~/c/out.tif | grep Minimum

  Minimum=0.000, Maximum=176.000, Mean=86.546, StdDev=59.195
  Minimum=0.000, Maximum=255.000, Mean=176.375, StdDev=117.760
Creating output file that is 16P x 15L.
Processing input file /Users/cdestigter/c/in.tif.
0...10...20...30...40...50...60...70...80...90...100 - done.
  Minimum=0.000, Maximum=176.000, Mean=86.546, StdDev=59.195
  Minimum=0.000, Maximum=0.000, Mean=0.000, StdDev=0.000

Adding -setci doesn't help. If I add -dstalpha I get a good alpha band in band 3, but then I have to remove band 2 somehow.

Tested on both stock 1.11.0 and trunk (r28101), occurs on both.

Attached in.tif.

[cdestigter]

d'oh. User error! I wasn't removing out.tif.aux.xml which contained saved stats. Disregard :)

[cdestigter]

This *is* actually a real bug, I just wasn't on my game the other day.

I updated the above instructions to reproduce the actual bug.

[Even Rouault]

I managed to make it work with the following sequence:

gdal_translate in.tif in.vrt -of vrt
python -c "from osgeo import gdal; ds = gdal.Open('in.vrt', gdal.GA_Update); ds.GetRasterBand(2).SetColorInterpretation(gdal.GCI_AlphaBand); ds = None"
gdalwarp in.vrt out.tif -overwrite -setci

There are 2 issues :

• the color interpretation of band 2 of in.tif is "Undefined". This is what the first 2 following steps fix.
• By default, gdalwarp doesn't explictely set the color interpretation of the target dataset. But a Grey+Alpha geotiff isn't the default configuration of the geotiff driver for 2 bands, hence the need for -setci (or alternatively -co ALPHA=YES)

[Even Rouault]

Note that I've done an enhancement in trunk : r28102 "GeoTIFF: set alpha on target by default when translating from Grey+Alpha"

So I don't think there's any further action needed.