[Gdal-dev] resampling 8-bit imagery

[Angus Dickey]

I have a bunch of 8-bit paletted png images with gcps that I am trying to reproject by following these steps (in a shell script):

- extract the cgps from a text file
- use gdal_translate to copy the png to a geotiff defining the projection and adding the gcps along the way
- using gdalwarp to change the projection and output another geotiff

This actually works great as long as I use nearest neighbour resampling, other resampling methods (e.g. cubic convolution) mess up the resultant image. I assume this is because they change the grey levels of the 8-bit channel and they would no longer map to the palette.

Using nearest neighbour does not produce a very nice image so I tried to use pct2rgb.py before gdal_translate like so:

pct2rgb.py -of GTiff in.png out.tiff

The resultant tiff does not appear to be a standard 3 channel rgb tiff; it will not open in normal image editing software and it causes gdalwarp to error out:

ERROR 5: GDALDataset::GetRasterBand(2) - Illegal band #

ERROR 5: GDALDataset::GetRasterBand(3) - Illegal band #


Am I missing something obvious? Any help would be appreciated.

Thanks,

~Angus