On Sun, May 31, 2009 at 8:39 PM, Thomas Robitaille <thomas.robitaille@gmail.com> wrote:

Hi,

I have a set of n points with real coordinates between 0 and 1, given by two numpy arrays x and y, with a value at each point represented by a third array z. I am trying to then rasterize the points onto a grid of size npix*npix. So I can start by converting x and y to integer pixel coordinates ix and iy. But my question is, is there an efficient way to add z[i] to the pixel given by (xi[i],yi[i])? Below is what I am doing at the moment, but the for loop becomes very inefficient for large n. I would imagine that there is a way to do this without using a loop?

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import numpy as np

n = 10000000 x = np.random.random(n) y = np.random.random(n) z = np.random.random(n)

npix = 100 ix = np.array(x*float(npix),int) iy = np.array(y*float(npix),int)

image = np.zeros((npix,npix)) for i in range(len(ix)): image[ix[i],iy[i]] = image[ix[i],iy[i]] + z[i]

There might be a better way, but histogram2d() seems like a good fit: import numpy as np n = 1000000 x = np.random.random(n) y = np.random.random(n) z = np.random.random(n) npix = 100 bins = np.linspace(0, 1.0, npix + 1) image = np.histogram2d(x, y, bins=bins, weights=z)[0] -- Nathan Bell wnbell@gmail.com http://www.wnbell.com/