| IDLanROI Confusion [message #24572] |
Wed, 04 April 2001 13:33 |
davidf
Messages: 2866
Registered: September 1996
|
Senior Member |
|
|
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
|
|
|
|
comp.lang.idl-pvwave archive
Members
Search
Help
Login
Home
