[PD] GEM : fx to make persons appear only when they move

Denis Trapeznikoff denissimo at gmail.com
Mon Mar 5 15:25:25 CET 2007

Sorry, if offtopic. I'm not aware of GEM functionalities, but the
algorythmic way to solve the problem, as I can see it, is:

compute diff(x,y)=f(t,x,y)-f(t-1,x,y), where f(t) is picture at present
compute alpha layer for the current picture alpha(x,y)=(diff(x,y)!=(0,0,0))
                      for example, in C code the predicate "pixel difference
does not equal zero" could be written as

                      char tmp = (char)(diff[x,y].red | diff[x,y].green |
                      alpha[x,y] = (unsigned char)((tmp | -tmp) >> 7); /*
the result is: 0 for 0 and 255 otherwise */;
blend initial background and current picture with computed alpha layer.

The second step requires per-pixel computations and takes 5 byte-arithmetics
per pixel (3 bitwise ORs, signed shiftright and negation, which in turn
could be expanded to bitwise NOT and subtraction, machine-dependent), thus
on a 320x240 24bpp picture taking overall 384000 (or 460800, with
subtractions from the first step) integer byte-arithmetics, which is three
of five orders of magnitude below the performance of modern CPUs.
Hmmm... what am I talking about?..
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