Re: IDLanROI Confusion

[Craig Markwardt <craigmnet(at)cow.physics.wisc.edu>]

davidf@dfanning.com (David Fanning) writes:

> Folks,
> 
> Alright. I admit it. I don't get it. :-(
> 
> I want to use the IDLanROI object to calculate
> the perimeter, centroid, and area of a region of interest.
> But I'm not sure the values can be trusted. Here is an
> example.
...
> Here are the results. 15% error in the perimeter is pretty large!
> 
> Calculated Perimeter:        362.61017
> Expected Perimeter:         314.159
> Discrepancy in Perimeter (percent):        115.42240%
> 
> Calculated Area:        7691.5000
> Expected Area:         7853.98
> Discrepancy in Area (percent):        97.931216%
> 
> Calculated Centroid:        199.50135       200.25225     
> Expected Centroid:      200     200
> 
> Am I doing something wrong, or should I believe these numbers?
> The same exercise with a square region produced accurate numbers.
> 
> My expected ROI's are not squares or circles, but they are
> much closer to circles than squares.

What do you mean, they aren't squares or circles?  If it isn't a
circle then you won't get a perfect match to the theory, right?

More than likely the region of interest becomes pixelated into square
pixels.  It's pretty clear to me that this will give a greater
perimeter than a true circle since there are more horizontal and
vertical segments than are required.

What happens if you make the circle much much bigger?  I think then
the answers will converge to what you expect.

Craig


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Craig B. Markwardt, Ph.D.         EMAIL:    craigmnet@cow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
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