GeoTIFF source alpha/transparency: what's the best method?

[Rahkonen Jukka]

Hello,
 
How about adding alpha channel to your Sounds like having alpha channel might be perfect fora place to try adding alpha channel to your GeoTIFFs?  I suppose that can be done with gdalwarp, at least it is having a switch -dstalpha. However, the user documentation at  http://www.gdal.org/gdalwarp.html is saying nothing about that switch.
 
Regards,
 
Jukka Rahkonen

________________________________

Lähettäjä: UMN MapServer Users List puolesta: Kirk Holbrook
Lähetetty: ke 12.4.2006 23:33
Vastaanottaja: MAPSERVER-USERS at LISTS.UMN.EDU
Aihe: [UMN_MAPSERVER-USERS] GeoTIFF source alpha/transparency: what's the best method?



Hi,

I've been working to optimize my map that uses several large geoTIFFs
(over 500MB each).

I started out using Red (RGB 255 0 0) as the area to be transparent in
my TIFFs (since there is no true red in any of my TIFFs) and using
"OFFSITE 255 0 0" in each layer to make it transparent. It works well,
but when I use gdaladdo to add overviews to optimize my map, I get a lot
of red artifacts in the overviews. I've also set up a tileindex which is
helping immensely.

Is there a better way to handle transparency? I've got 24 bit TIFFs
(which I'd like to keep, rather than going to 8 bit). Note also that my
transparent areas are not necessarily rectangular -- there are a few odd
shapes.

Thanks a lot,
Kirk