Index: branches/rel10_ifa/psModules/src/pslib/psEllipse.c
===================================================================
--- branches/rel10_ifa/psModules/src/pslib/psEllipse.c	(revision 6547)
+++ branches/rel10_ifa/psModules/src/pslib/psEllipse.c	(revision 6547)
@@ -0,0 +1,62 @@
+# include "pslib.h"
+# include "psEllipse.h"
+
+EllipseAxes EllipseMomentsToAxes (EllipseMoments moments)
+{
+
+    EllipseAxes axes;
+
+    double f = sqrt (0.25*PS_SQR(moments.x2 - moments.y2) + PS_SQR(moments.xy));
+    if (f > (moments.x2 + moments.y2) / 2.0) {
+        f = 0.98*(moments.x2 + moments.y2) / 2.0;
+    }
+
+    axes.major = sqrt (0.5*(moments.x2 + moments.y2) + f);
+    axes.minor = sqrt (0.5*(moments.x2 + moments.y2) - f);
+    axes.theta = atan2 (2*moments.xy, moments.x2 - moments.y2) / 2;
+    // theta in radians
+
+    return (axes);
+}
+
+EllipseShape EllipseAxesToShape (EllipseAxes axes)
+{
+
+    EllipseShape shape;
+
+    double r1 = 1.0 / PS_SQR(axes.major) + 1.0 / PS_SQR(axes.minor);
+    double r2 = 1.0 / PS_SQR(axes.major) - 1.0 / PS_SQR(axes.minor);
+
+    double sxr = r1 + r2*cos(2*axes.theta);
+    double syr = r1 - r2*cos(2*axes.theta);
+
+    shape.sx = 1.0 / sqrt(sxr);
+    shape.sy = 1.0 / sqrt(syr);
+    shape.sxy = r2*sin(2*axes.theta);
+
+    return (shape);
+}
+
+EllipseAxes EllipseShapeToAxes (EllipseShape shape)
+{
+
+    EllipseAxes axes;
+
+    double f1 = 1.0 / PS_SQR(shape.sx) + 1.0 / PS_SQR(shape.sy);
+    double f2 = 1.0 / PS_SQR(shape.sx) - 1.0 / PS_SQR(shape.sy);
+
+    // force the axis ratio to be less than 10
+    double r1 = 0.5*0.95*sqrt (PS_SQR(f1) - PS_SQR(f2));
+
+    shape.sxy = PS_MIN(PS_MAX(shape.sxy, -r1), r1);
+
+    axes.theta = atan2 (-2.0*shape.sxy, f2) / 2.0;
+
+    double Ar = 0.25*f1 + 0.25*sqrt(PS_SQR(f2) + 4*PS_SQR(shape.sxy));
+    double Br = 0.25*f1 - 0.25*sqrt(PS_SQR(f2) + 4*PS_SQR(shape.sxy));
+
+    axes.minor = 1.0 / sqrt (Ar);
+    axes.major = 1.0 / sqrt (Br);
+
+    return (axes);
+}
Index: branches/rel10_ifa/psModules/src/pslib/psEllipse.h
===================================================================
--- branches/rel10_ifa/psModules/src/pslib/psEllipse.h	(revision 6547)
+++ branches/rel10_ifa/psModules/src/pslib/psEllipse.h	(revision 6547)
@@ -0,0 +1,30 @@
+// strucures to define elliptical shape parameters
+typedef struct
+{
+    double major;
+    double minor;
+    double theta;
+}
+EllipseAxes;
+
+typedef struct
+{
+    double x2;
+    double y2;
+    double xy;
+}
+EllipseMoments;
+
+typedef struct
+{
+    double sx;
+    double sy;
+    double sxy;
+}
+EllipseShape;
+
+// conversions between elliptical shape representations
+EllipseAxes EllipseMomentsToAxes (EllipseMoments moments);
+EllipseShape EllipseAxesToShape (EllipseAxes axes);
+EllipseAxes EllipseShapeToAxes (EllipseShape shape);
+