[Image-SIG] An algorithm to find connected groups of pixels

[Douglas S. Blank]

Jeffrey Wise wrote:
> Hi, 
> 
> I'm in my first week with PIL and doing well - it's a tremendous library!
> I've gotten to the point in my image analysis that I need to find all the
> groups of connected pixels in an image.  These groups form arbitrary shapes
> in the image.  I want to preserve the shape information for each group, so I
> can compare/analyze/categorize... it later.  
> 
> The image is composed of pixels of value 0 or 255 in "L" B&W format.  I
> assume such an algorithm will need to find a pixel of value 255 and then
> check in every direction about it for a neighbor that is 255 too.  The
> algorithm would presumably recurse to find neighbors of the neighbor, etc.
> I suspect the trick in this algorithm is the data structures needed to keep
> track of the discovered adjacencies.  I have loaded the image into the 2D
> array-like accessor, so I have free access to each pixel.
> 
> I haven't noticed such an algorithm in the PIL yet; maybe I've missed it.
> Or perhaps there is Python code available to do this algorithm? Any ideas?

Jeff,

You can take a look at some code from Pyrobot, Python Robotics, that was 
written by a student:

http://cvs.cs.brynmawr.edu/cgi-bin/viewcvs.cgi/pyrobot/vision/__init__.py?rev=HEAD&content-type=text/vnd.viewcvs-markup

Start with the Blob class. We also have similar code in C++, and wrapped 
by Python using SWIG:

http://cvs.cs.brynmawr.edu/cgi-bin/viewcvs.cgi/pyrobot/vision/cvision/Vision.cpp?rev=HEAD&content-type=text/vnd.viewcvs-markup

If you are interested in image processing, robotics, or other 
camera-based functions, see our course materials here:

http://pyrorobotics.org/?page=PyroModuleComputerVision

-Doug

> Thanks,
> .Jeff Wise
> 
> 
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