[FWTools] Converting a black border to transparent in geotifs

[Simon Hook]

Hi Frank,

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

Best wishes,

S.

Frank Warmerdam wrote:
> Simon Hook wrote:
>> Hi,
>>
>> 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?
>
> Simon,
>
> 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 
> formats.
>
> eg.
> 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 
> values.
>
> 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,