Raster JPEG Input using OFFSITE

[Armin Burger]

Randy,

with JPEG (and mainly also ECW) you will not completely get rid of the 
black borders because, as you mention, their 'black' is not fully black. 
But you can get get most parts of the black transparent by applying a 
PROCESSING "SCALE..." tag, eg.

   PROCESSING "SCALE_1=3,255"
   PROCESSING "SCALE_2=3,255"
   PROCESSING "SCALE_3=3,255"
   OFFSITE 0 0 0

All pixel values from 0 to 3 will be mapped to 0 and then set 
transparent by the OFFSITE tag. It has the slight drawback that some 
very dark parts inside the image will also be touched by this and become 
transparent. It works quite well though for multispectral images, a bit 
less nicely on panchromatic.

Armin

Randy How wrote:
> My question is in regards to using the OFFSITE setting for a JPEG data 
> source to remove the "black/NoData" areas.  Does it work at all?  I've 
> attempted this but it made no difference (mapserver 4.7).
>  
> It will work with Tiff's (that have internal JPEG compression) and 
> poorly with ECW's due to the variability in "black".
>  
> Randy