[Numpy-discussion] efficient sum of "sparse" 2D arrays?

[RJS]

Hi all,

The problem: I have a "stack" of 8,  640 x 480 integer image arrays from a 
FITS cube concatenated into a 3D array, and I want to sum each pixel such 
that the result ignores clipped values (255+); i.e., if two images have 
clipped pixels at (x,y) the result along z will be the sum of the other 6.

I'm trying to come up with a pure Numeric way (hopefully so that I can use 
weave.blitz) to speed up the calculation.
I just looked into masked arrays, but I'm not familiar with that module at all.

I was guessing someone out there has done this before...

Ray