[Image-SIG] Faster way to examine alpha ?

[Ray Pasco]

I have a routine that examines the alpha values of an RGBA image to 
determine
whether whether there are any "active"alpha (value < 255), and
whether the any active alpha are single-valued and if only a single color
is associted with this alpha. Each pixel must be examined one-by-one
which is rather slow.

Is there way to examine pixel values akin to the .point(lambda) method ?
I just want to examine, not change the valuse.

Thanks,
Ray Pasco