Comments on: Python and real time graphical analysis

By: sharms

sharms — Wed, 25 Nov 2009 14:28:45 +0000

Spider is correct, these are just tests, in real life I use the output of a bunch of these types of calculations.

I compiled it with ‘gcc -g -Wall’ which appears to use -O0, as when I recompiled with O1 my time passed is 0ms.

By: Spider

Spider — Wed, 25 Nov 2009 10:29:36 +0000

@Ronan: Most probably yes. But my guess would be that in “reality” it’d be used to create a difference average/difference-map between the two. However, since this testcase just doesn’t actually do anything with the counts, it would end up being void.

By: Holly

Holly — Wed, 25 Nov 2009 09:53:19 +0000

Maybe I’m missing something, but why have you got
screen1[x][y] * screen2[x][y] in the loops. The result doesn’t appear to be used for anything.

By: Ronan

Ronan — Wed, 25 Nov 2009 09:41:23 +0000

Do you realise that your compare_array() C function will be optimized into a noop by gcc if you use -O1 or above ?

By: oliver

oliver — Wed, 25 Nov 2009 09:26:02 +0000

Would also recommend numpy, or opencv for more complicated image analysis.

By: sharms

sharms — Wed, 25 Nov 2009 04:19:41 +0000

Ulrik – that is fantastic, I will stick with numpy. Thanks for saving me a lot of time and ignorance

By: John Doe

John Doe — Wed, 25 Nov 2009 02:35:59 +0000

Ulrik: I’ve tried with itertools.izip (and normal zip) before posting the code and it was a lot slower. I forgot about the _abs trick but it worked well (-100ms).

def compareArrays(screen1, screen2):
_abs = abs
for x in xrange(0, 1920):
s2x = screen2[x]
for y, e in enumerate(screen1[x]):
diff = _abs(e – s2x[y])
combo = e * s2x[y]

By: Ulrik

Ulrik — Wed, 25 Nov 2009 01:57:02 +0000

John Doe — if you want to speed up pure-python loops, here are some things you can try: Loop by value, not by index.

for x, y in itertools.izip(screen1, screen2): diff = _abs(x – y); combo = x *y

tip nr 2:

for function calls inside loops (avoid these, but we have abs above), rebind the function to a local variable for the loop. So just before the for loop we define _abs = abs.

By: John Doe

John Doe — Wed, 25 Nov 2009 01:53:26 +0000

100ms more!

def compareArrays(screen1, screen2):
for x in xrange(0, 1920):
s2x = screen2[x]
for y, e in enumerate(screen1[x]):
diff = abs(e – s2x[y])
combo = e * s2x[y]

By: Ulrik

Ulrik — Wed, 25 Nov 2009 01:38:43 +0000

Numpy — written in C so that you don’t have to.