[SciPy-User] Accumulation sum using indirect indexes

Alexander Kalinin alec.kalinin@gmail....
Tue Jan 31 14:34:28 CST 2012


Hello!

I use SciPy in computer graphics applications. My task is to calculate
vertex normals by averaging faces normals. In other words I want to
accumulate vectors with the same ids. For example,

ids = numpy.array([0, 1, 1, 2])
n = numpy.array([ [0.1, 0.1, 0.1], [0.1, 0.1, 0.1], [0.1, 0.1, 0.1], [0.1,
0.1 0.1] ])

I need result:
nv = ([ [0.1, 0.1, 0.1], [0.2, 0.2, 0.2], [0.1, 0.1, 0.1]])

The most simple code:
nv[ids] += n
does not work, I know about this. For 1D arrays I use numpy.bincount(...)
function. But this function does not work for 2D arrays.

So, my question. What is the best way calculate accumulation sum for 2D
arrays using indirect indexes?

Sincerely,
Alexander
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