# coyote: FIT_ELLIPSE

- coyote/Graphics
- coyote/Mathematics
- [Source code]

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.)
      LE:
      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.