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*Subject*: IDLanROI Confusion*From*: davidf(at)dfanning.com (David Fanning)*Date*: Wed, 4 Apr 2001 14:33:17 -0600*Newsgroups*: comp.lang.idl-pvwave*Organization*: Fanning Software Consulting*Xref*: news.doit.wisc.edu comp.lang.idl-pvwave:24350

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. ****************************************************************** ; Create a square window: Window, XSize=400, YSize=400 ; Draw a circle of radius 50 pixels in the center of the window: TVCircle, 50, 200, 200, Color=255, /Fill ; Take a snap shot of the window: snap = TVRD() ; Contour the circle: IsoContour, snap, C_Value=255, connectivity, vertices ; Create an IDLanROI object: o = Obj_New('IDLanROI', connectivity[0,*], connectivity[1,*]) ; Calculate geometry: ok = o->ComputeGeometry(Perimeter=p, Centroid=c, Area=a) ; Calculate perimeter, area of circle: radius = 50.0 pcircle = 2 * !Pi * radius acircle = !Pi * radius^2 ; Print it out, with expected values: Print, 'Calculated Perimeter: ', p Print, 'Expected Perimeter: ', pcircle Print, 'Discrepancy in Perimeter (percent): ', p / pcircle * 100., '%' Print, "" Print, 'Calculated Area: ', a Print, 'Expected Area: ', acircle Print, 'Discrepancy in Area (percent): ', a / acircle * 100., '%' Print, "" Print, 'Calculated Centroid: ', c[0:1] Print, 'Expected Centroid: ', 200, 200 *************************************************************************** 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. Cheers, David -- David Fanning, Ph.D. Fanning Software Consulting Phone: 970-221-0438 E-Mail: davidf@dfanning.com Coyote's Guide to IDL Programming: http://www.dfanning.com/ Toll-Free IDL Book Orders: 1-888-461-0155

**Follow-Ups**:**Re: IDLanROI Confusion***From:*Craig Markwardt

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