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

• To: mathgroup at smc.vnet.net
• Subject: [mg115345] Re: Extracting pixel values from an image under a polygon (region of interest)
• From: James Stein <mathgroup at stein.org>
• Date: Sat, 8 Jan 2011 03:39:08 -0500 (EST)

```Dear Mac,

1. Selecting pixels within a polygon is an interesting, non-trivial problem.
If you Google for "point in polygon" (without the quotes), the first several
hits are relevant. Your problem probably deals with polygons with
non-intersecting sides, so it'll be a bit easier...

2. I'm not sure why you say there "is no PixelTake function or similar."
Given a map of type 'Image', 'ImageData[map]' is an array of pixels; each
pixel being an list of 3 color components (sometimes a 4th component as
well); Is not the Mathematica primitive 'Take' on the ImageData equivalent

-- James

On Fri, Jan 7, 2011 at 1:10 AM, Mac <mwjdavidson at googlemail.com> wrote:

> 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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