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

[Kirk Holbrook]

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