[FWTools] Converting a black border to transparent in geotifs
simon.j.hook at jpl.nasa.gov
Sun Feb 11 16:08:40 EST 2007
I want to generate kml files from the final images (I got the google
regionator code working BTW). In the past we have generated a kml "by
hand" this involved opening a png in photoshop, changing the border
(zero values) to transparent, saving the png. Then placing the png with
google earth at the location on the earth and setting the 4 corners of
the png. If the transparency step was done in photoshop the black border
does not show, if it has not been done you see the black border. Now I
only know how to use the regionator code with geotifs (not pngs). So I
think if I can ad an alpha channel that will work.
I will try that and see what happens
Frank Warmerdam wrote:
> Simon Hook wrote:
>> I apologize if I sent a similar post last night but for some reason I
>> did not get a copy thro' the list.
>> I have some geotif images with a black border. I want to make the
>> border transparent. Is there a way to do this with FWTools?
> In part this depends on what software you want it to be transparent in.
> In some software packages (like MapServer or OpenEV) you can pick a
> "nodata value" such as 0 to be treated as transparent. In some cases
> the packages can also recognise nodata values as a kind of metadata from
> the file. The gdal_translate -a_nodata switch can be used to assign this
> nodata value to an output file though it is only preserved in some
> gdal_translate -a_nodata 0 in.tif out.tif
> Another approach to transparency is to add an alpha channel with
> explicit transparency. I think you could essentially assign all "zero"
> pixels in an input greyscale image to have an alpha of zero in the output
> using something like:
> gdalwarp -srcnodata 0 -dstalpha in.tif out.
> This whole area though is rife with challenges because:
> o many software packages don't automatically support nodata values.
> o some software packages don't recognise alpha bands for transparency
> o some file formats don't support saving nodata values.
> o some formats don't support alpha bands.
> o GDAL's "nodata" data model treats nodata values as independent for
> each band.
> o all black pixels will get treated as transparent, not just border
> There are still a variety of issues I haven't addressed, but I think I'll
> let you clarify what you want to do before I write more.
> Best regards,
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