[GRASS-user] r.neighbors velocity

[Sören Gebbert]

Hey Folks,
many thanks for pointing to the important influence of different compiler
and compiler options. But please be aware that my tiny little program is
not representative for a neighbor analysis implementation, it was simply a
demonstration of  12 billion ops:

1. I use fixed loop sizes, it is really easy for a compiler to optimize that
2. It is pretty simple to parallelize since only a simple reduction is done
in the inner loop

3. Most important: The statement of Ivan was a window size of 501 ... as
MarkusN IMHO correctly interpreted this leads to a moving window of 501x501
pixel if this is an option for r.neighbors. It is not the total number of
cells of a rectangular moving window, since it must must be an even number
in this case. Other shapes than rectangular are more complex to implement.

To be diplomatic i decided to use 501 pixel, which might represented a
23x21 pixel moving window, to show that this "small" number of operations
needs a considerable amount of time on modern CPU's.

If you use a 501x501 pixel moving window the computational effort is
roughly 501 times 12 billion ops. IMHO in this case a GPU or neighbor
algorithm specific FPGA/ASIC may be able to perform this operation in 2/3
seconds.

Best regards
Soeren
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