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

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.