diff -N CovarianceMatrixTransformer.java
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ CovarianceMatrixTransformer.java	31 Mar 2006 01:08:36 -0000	1.1
@@ -0,0 +1,102 @@

+/*
+ * CovarianceMatrixTransformer.java
+ *
+ * Created on March 30, 2006, 3:35 PM
+ *
+ * $Id: CovarianceMatrixTransformer.java,v 1.1 2006/03/31 01:08:36 ngraf Exp $
+ */
+
+package org.lcsim.math.coordinatetransform;
+import static java.lang.Math.sqrt;
+import static java.lang.Math.cos;
+import static java.lang.Math.sin;
+
+/**
+ * Utility class to handle transformations of covariance matrices
+ * Inspired by Nick Sinev's CMTransform.java.
+ * @author Norman Graf
+ */
+public class CovarianceMatrixTransformer
+{
+    
+    /** No need to create, all methods static*/
+    private CovarianceMatrixTransformer()
+    {
+    }
+    
+    /**
+     * Convert covariance matrix elements in cartesian coordinates (x,y) to cylindrical (r,phi).
+     * @param x The cartesian x coordinate.
+     * @param y The cartesian y coordinate.
+     * @param sxx The covariance matrix term for x at (x,y).
+     * @param syy The covariance matrix term for y at (x,y).
+     * @param sxy The covariance matrix term for xy at (x,y).
+     * @return The packed r-phi covariance matrix terms:
+     * <table>
+     * <tr><td> cov[0] </td><td> r-r </td><tr>
+     * <tr><td> cov[1] </td><td> phi-phi </td><tr>
+     * <tr><td> cov[2] </td><td> r-phi</td><tr>
+     * </table>
+     */
+    public static double[] xy2rphi(double x, double y, double sxx, double syy, double sxy)
+    {
+        double[] cov = new double[3];
+        double xx = x*x;
+        double xy = x*y;
+        double yy = y*y;
+        double rr = xx+yy;
+        double r = sqrt(rr);
+        double oneOverR2 = 1/rr;
+        
+        // srr
+        cov[0] = oneOverR2*(sxx*xx + syy*yy + 2.*sxy*xy);
+        
+        // sphiphi
+        cov[1] = oneOverR2*oneOverR2*(sxx*yy - 2.*sxy*xy + syy*xx);
+        
+        // srphi
+        cov[2] = oneOverR2*(-sxx*xy + sxy*(xx-yy) + syy*xy)/r;
+        
+        return cov;
+    }
+    
+    /**
+     *Convert covariance matrix elements in cylindrical (r,phi) to cartesian coordinates (x,y).
+     * @param r The cylindrical radius.
+     * @param phi The cylindrical angle.
+     * @param srr The covariance matrix term for r at (r, phi).
+     * @param sff The covariance matrix term for phi at (r, phi).
+     * @param srf The covariance matrix term for r-phi at (r, phi).
+     * @return The packed x-y covariance matrix terms:
+     * <table>
+     * <tr><td> cov[0] </td><td> x-x </td><tr>
+     * <tr><td> cov[1] </td><td> y-y </td><tr>
+     * <tr><td> cov[2] </td><td> x-y</td><tr>
+     * </table>
+     */
+     
+    public static double[] rphi2xy(double r, double phi, double srr, double sff, double srf)
+    {
+        double[] cov = new double[3];
+        // cosine^2(phi)
+        double cf = cos(phi);
+        double cc = cf*cf;
+        //sine^2(phi)
+        double sf = sin(phi);
+        double ss = sf*sf;
+        
+        // cosine(phi)*sine(phi)
+        double cs = cf*sf;
+        
+        // sxx
+        cov[0] = srr*cc - 2.*srf*r*cs + sff*r*r*ss;
+        
+        // syy
+        cov[1] = srr*ss + 2.*srf*r*cs + sff*r*r*cc;
+        
+        // sxy
+        cov[2] = srr*cs + srf*r*(cc - ss) - sff*r*r*cs;
+        
+        return cov;
+    }
+}