[gdal-dev] gdalwarp color table copy issue

[Aaron Sutula]

Hi List,

I am using gdalwarp (FWTools 2.0.6) to re-sample a gif+gfw image into a 
higher resolution geoTiff.

The orginal gif has a color table that specifies 0: 255,255,255,0 as one 
color:

$ gdalinfo radar_conus_0.gif
Driver: GIF/Graphics Interchange Format (.gif)
Files: radar_conus_0.gif
Size is 3400, 1600
Coordinate System is `'
Origin = (-127.629361176470582,50.415612019896201)
Pixel Size = (0.017971305190311,-0.017971305190311)
Corner Coordinates:
Upper Left  (-127.6293612,  50.4156120)
Lower Left  (-127.6293612,  21.6615237)
Upper Right ( -66.5269235,  50.4156120)
Lower Right ( -66.5269235,  21.6615237)
Center      ( -97.0781424,  36.0385679)
Band 1 Block=3400x1 Type=Byte, ColorInterp=Palette
 NoData Value=0
 Metadata:
   GIF_BACKGROUND=0
 Color Table (RGB with 32 entries)
   0: 255,255,255,0
   1: 240,240,240,255
   2: 238,238,238,255
   ...

I run gdalwarp as follows:

gdalwarp -tr 0.005 0.005 -srcnodata 10000000 -dstnodata 0 -s_srs 
EPSG:4269 -t_srs EPSG:4269 -r cubicspline -co "TILED=YES" 
radar_conus_0.gif radar_resampled.tif

The resulting tif color table specifies 0: 255,255,255,255:

$ /home/aaron.sutula/FWTools-2.0.6/bin_safe/gdalinfo radar_conus_0.tif
Driver: GTiff/GeoTIFF
Files: radar_conus_0.tif
Size is 12220, 5751
Coordinate System is:
GEOGCS["NAD83",
   DATUM["North_American_Datum_1983",
       SPHEROID["GRS 1980",6378137,298.2572221010042,
           AUTHORITY["EPSG","7019"]],
       AUTHORITY["EPSG","6269"]],
   PRIMEM["Greenwich",0],
   UNIT["degree",0.0174532925199433],
   AUTHORITY["EPSG","4269"]]
Origin = (-127.629361176470582,50.415612019896201)
Pixel Size = (0.005000000000000,-0.005000000000000)
Metadata:
 AREA_OR_POINT=Area
Image Structure Metadata:
 INTERLEAVE=BAND
Corner Coordinates:
Upper Left  (-127.6293612,  50.4156120) (127d37'45.70"W, 50d24'56.20"N)
Lower Left  (-127.6293612,  21.6606120) (127d37'45.70"W, 21d39'38.20"N)
Upper Right ( -66.5293612,  50.4156120) ( 66d31'45.70"W, 50d24'56.20"N)
Lower Right ( -66.5293612,  21.6606120) ( 66d31'45.70"W, 21d39'38.20"N)
Center      ( -97.0793612,  36.0381120) ( 97d 4'45.70"W, 36d 2'17.20"N)
Band 1 Block=256x256 Type=Byte, ColorInterp=Palette
 NoData Value=0
 Color Table (RGB with 256 entries)
   0: 255,255,255,255
   1: 240,240,240,255
   2: 238,238,238,255
   ...

I'd really like to keep color 0 with a completely transparent alpha 
value. I don't understand why gdalwarp is changing it from 0 to 255.

I've played around with the -dstalpha option, but I don't quite 
understand it and I get unexpected results; It generates a tif with two 
bands, one greyscale and one alpha... Not what I need.

Any advice?

Thank you,
Aaron