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Extracting pixel values from an image under a polygon (region of interest)

  • To: mathgroup at smc.vnet.net
  • Subject: [mg115312] Extracting pixel values from an image under a polygon (region of interest)
  • From: Mac <mwjdavidson at googlemail.com>
  • Date: Fri, 7 Jan 2011 04:10:16 -0500 (EST)

I've tried solving this general scientific image processing problem
for some time, alas without success. Given an image and a polygon with
coordinates describing a region of interest, I would like to extract
all the image pixels falling completely within the boundaries of the
polygon (not necessarily a square).

As an illustrative example consider the following synthetic image +
region of interest marked in red

img = Graphics[{Raster[RandomReal[{0, 1}, {10, 10}]], Red,
   Opacity[0.5], Polygon[{{0.9, 0.9}, {4, 4.1}, {7, 4.1}, {8, 1}}]},
  Frame -> True]

I would like to extract all the values of img falling within the
boundary of the red polygon. The real world problem I face is that I
have a radar map of the earth's surface with the pixel values
representing the radar intensity and would like to extract and analyse
all pixels falling within a specific region (say for instance
Greenland). Despite the advances in image processing in Mathematica 8
I still do not see a solution. There is no PixelTake function or
similar....

Anway, any help would be much appreciated.

Mac


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