Why is screendump so touchy?

[shaw]

I know that if the screen is changed during a screendump, for example by
mouse movement or by the clock hands changing, then the rasterfile may be
corrupted and screenload will fail.  I have seen warnings about this in
Sun's manuals and in articles in Sun-spots.  But what I have not seen is
an explanation.

Why?

What is it about the method of extracting information from the frame
buffer or about the representation of rasterfiles that makes screendump so
touchy?  Why, for instance do you not just get a "blur" or "double
exposure" or undefined pixels confined to the region in which there was
movement?  There must be something more tricky about this operation than
it would appear to the casual observer.  I dont see the problem.

Guy Shaw
Paralogics
uunet!paralogics!shaw