[Gdal-dev] Transparency in 8bit geotiffs with a color table

[Ray Gardener]

As far as I understand the TIFF 6 spec:

Set SamplesPerPixel to 2, and BitsPerSample to [8 8], to store 8-bit 
pixels with an 8-bit alpha mask.

If you're premultplying alpha into the palette's colors, set 
ExtraSamples to 1, otherwise set it to 2.

After writing each pixel's value (the palette index), write its alpha value.

Hope this helps,
Ray


aaime74 wrote:
 > Hi,
 > I'm trying, without success, to create a geotiff, byte format, with 
the last
 > entry of the
 > color table as a transparent color (by playing with alpha), but 
apparently I
 > keep on
 > getting opaque images.
 >
 > Is there any way to get transparent output in 8bit geotiffs with a color
 > table?
 > Cheers
 > Andrea Aime