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FIT_ELLIPSE

FIT_ELLIPSE

Name


      Fit_Ellipse

Purpose



      This program fits an ellipse to an ROI given by a vector of ROI indices.

Author



      FANNING SOFTWARE CONSULTING
      David Fanning, Ph.D.
      1645 Sheely Drive
      Fort Collins, CO 80526 USA
      Phone: 970-221-0438
      E-mail: david@idlcoyote.com
      Coyote's Guide to IDL Programming: http://www.idlcoyote.com

Category



      Graphics, math.

Calling Sequence



      ellipsePts = Fit_Ellipse(indices)

Optional Inputs



      indices - A 1D vector of pixel indices that describe the ROI. For example,
            the indices may be returned as a result of the WHERE function.

Outputs



      ellipsePts - A 2-by-npoints array of the X and Y points that describe the
            fitted ellipse. The points are in the device coodinate system.

Input Keywords



      NPOINTS - The number of points in the fitted ellipse. Set to 120 by default.
     
      SCALE - A two-element array that gives the scaling parameters for each X and Y pixel, respectively.
            Set to [1.0,1.0] by default.
      XSIZE - The X size of the window or array from which the ROI indices are taken.
            Set to !D.X_Size by default.
      YSIZE - The Y size of the window or array from which the ROI indices are taken.
            Set to !D.Y_Size by default.

Output Keywords



      CENTER -- Set to a named variable that contains the X and Y location of the center
            of the fitted ellipse in device coordinates.
      ORIENTATION - Set to a named variable that contains the orientation of the major
            axis of the fitted ellipse. The direction is calculated in degrees
            counter-clockwise from the X axis.
      AXES - A two element array that contains the length of the major and minor
            axes of the fitted ellipse, respectively.
      SEMIAXES - A two element array that contains the length of the semi-major and semi-minor
            axes of the fitted ellipse, respectively. (This is simple AXES/2.)

Example



      LoadCT, 0, /Silent
      image = BytArr(400, 300)+125
      image[180:245, 125:175] = 255B
      indices = Where(image EQ 255)
      Window, XSize=400, YSize=300
      TV, image
      PLOTS, Fit_Ellipse(indices, XSize=400, YSize=300), /Device, Color=cgColor('red')

Modification History



      Written by David W. Fanning, April 2002. Based on algorithms provided by Craig Markwardt
            and Wayne Landsman in his TVEllipse program.
      Added SCALE keyword and modified the algorithm to use memory more efficiently.
            I no longer have to make huge arrays. The arrays are only as big as the blob
            being fitted. 17 AUG 2008. DWF.
      Fixed small typo that caused blobs of indices with a longer X axis than Y axis
            to misrepresent the center of the ellipse. 23 February 2009.



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