trim sides of image rect if white

[iMP]

Hi:

I generate a variable image in a white area of a card within a known 
rectangle. I would like to save this area to a jpg file, but with the 
left and right sides trimmed of any extra white space. The rule would 
be "if all the pixels from top to bottom are white, then trim that 
'column' of pixels." I want to start from the left and the right 
sides and work to the center of the rectangle, deleting all columns 
that have only white pixels, stopping when I hit a colored pixel. The 
height should not be affected.

Has anybody already done this that they could share, or give me some 
pointers on how to accomplish this? I hope I explained the problem 
understandably!

Many thanks,

guy