[Numpy-discussion] adaptive thresholding: get adacent cells for each pixel (original) (raw)

Johannes Loehnert a.u.r.e.l.i.a.n at gmx.net
Sat Jun 10 04:19:43 EDT 2006

Hi,

I'm just starting with numpy (via scipy) and I'm wanting to perform adaptive thresholding (http://www.cee.hw.ac.uk/hipr/html/adpthrsh.html) on an image. Basically that means that I need to get a threshold for each pixel by examining the pixels around it. In numpy this translates to finding the adjacent cells for each cell (not including the value of the cell we are examining) and getting the mean, or median of those cells.

I've written something that works, but is terribly slow. How would someone with more experience get the adjacent cells for each cell minus the cell being examined?

regarding the mean value, you can take a look at scipy.signal.convolve2d. If you convolve with an array like this:

[[0.125 0.125 0.125] [0.125 0.0 0.125] [0.125 0.125 0.125]]

you get the 3x3 mean value (btw why leave out the center pixel?).

For the median, I can not think of any good method right now.

Also another method springs to my mind (just substract the top row and add a new bottom row to the averaging window), but I have no idea how to do this in an efficient way.

Generally, always try to find a way to process the whole array as one. If you perform anything on an array elementwise, it will be dead slow.

Best regards, Johannes

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