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Re: perimeter of a blob

Thanks to each of you for your help.

I have tried the convolution and contouring methods suggested by Richard
and David respectively.  I used a black (0) and white (255) image of the
capital letter G (hand drawn.)

The convolution exactly defines the inclusive perimeter of the blob.   I
haven't figured out how to sort the indicies into a clockwise (or
counter clockwise order yet... but I haven't tried either.)

The contour method misses the exact inclusive perimeter whenever an
inside corner is encountered (i.e. concavity.)  It cuts diagonally
across these inside corners a result.  The loss is nearly insignificant
in this case, but I recall have difficulty comparing polygon perimeter
estimates made in IDL with those made on the same data in ARC/INFO.

Each method works for my current purposes.

Thanks again!


Ben Tupper
Pemaquid River Company
248 Lower Round Pond Road
POB 106
Bristol, ME 04539

Tel: (207) 563-1048
Email: PemaquidRiver@tidewater.net