Defining no data areas using a Shape

[Dirk Jesko]

Hello,

we setup up a mapserver project with several ECW images (approx. 60). 
Since the some of the images overlap and are not completely filled with 
image data, I am looking for a way to define the "no data" part of the 
image. The OFFSITE option does not work due to the lossy compression 
which results in a "fuzzy" collar around the image data. Is there any 
way to get rid of this collar? Since we have a shape containing the 
exact boundary of the imagery, I thought about using that shape to 
specify the no data area.
Is this possible using mapserver? If not, is there another way to get 
rid of the compression artifacts around the imagery? Since this is an 
issue with all lossy compressed imagery, I think that this might be an 
interesting feature not only for ecw files.
By the way, using uncompressed imagery or a lossless compression is not 
an option for us, due to the amount of data.

Thanks for any help,
Dirk