Le mercredi 16 janvier 2013 20:03:14, Isaac Gerg a écrit :
> Hi All,
>
> I have a tiff in which each pixel is a single byte (0-255) which references
> a colortable. Each element of the colortable has 4 values (RGBA).
>
> I would like to convert my "black" color (entry 1 in the color table
> (0,0,0,255)) to be transparent (new colortable entry is (0,0,0,0))
>
> Another way to say it is, how do I create a tiff with an indexed colormap
> but also have it contain an alpha channel?
TIFF color palette are only RGB, not RGBA. What you can do, however, is assign
a nodata value (note this is a GDAL specific TIFF tag that will only be
understood by GDAL based software)
gdal_translate src.tif src_with_nodata.tif -a_nodata 0
You can create a TIFF with an indexed colormap and an alpha channel with the
following extra steps :
gdal_translate src_with_nodata.tif mask.tif -b mask
gdalbuildvrt tmp.vrt src.tif mask.tif -separate
gdal_translate src.tif src.vrt -of VRT
the with a text editor copy the lines that look line the following ones from
src.vrt
<ColorInterp>Palette</ColorInterp>
<ColorTable>
<Entry c1="112" c2="120" c3="56" c4="255" />
[...]
<Entry c1="204" c2="208" c3="164" c4="255" />
</ColorTable>
and paste them just below <VRTRasterBand dataType="Byte" band="1"> in tmp.vrt
Finally :
gdal_translate tmp.vrt pct_with_alpha.tif -co ALPHA=YES
That's it. But I'm not sure that this will be properly displayed by viewers.
A safest alternative would be to build a RGBA TIFF :
gdal_translate -expand rgb src.tif rgb.tif
gdal_translate -b 1 rgb.tif r.tif
gdal_translate -b 2 rgb.tif g.tif
gdal_translate -b 3 rgb.tif b.tif
gdalbuildvrt rgba.vrt r.tif g.tif b.tif mask.tif -separate
gdal_translate rgba.vrt rgba.tif -co ALPHA=YES
>
> Thanks in advance,
> Isaac
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