diff -r efd9c589177a -r c0b4a8b5a012 toolkit/javascript/d3/src/geom/contour.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/toolkit/javascript/d3/src/geom/contour.js	Thu Apr 10 14:20:23 2014 +0200
@@ -0,0 +1,78 @@
+/**
+ * Computes a contour for a given input grid function using the <a
+ * href="http://en.wikipedia.org/wiki/Marching_squares">marching
+ * squares</a> algorithm. Returns the contour polygon as an array of points.
+ *
+ * @param grid a two-input function(x, y) that returns true for values
+ * inside the contour and false for values outside the contour.
+ * @param start an optional starting point [x, y] on the grid.
+ * @returns polygon [[x1, y1], [x2, y2], …]
+ */
+d3.geom.contour = function(grid, start) {
+  var s = start || d3_geom_contourStart(grid), // starting point
+      c = [],    // contour polygon
+      x = s[0],  // current x position
+      y = s[1],  // current y position
+      dx = 0,    // next x direction
+      dy = 0,    // next y direction
+      pdx = NaN, // previous x direction
+      pdy = NaN, // previous y direction
+      i = 0;
+
+  do {
+    // determine marching squares index
+    i = 0;
+    if (grid(x-1, y-1)) i += 1;
+    if (grid(x,   y-1)) i += 2;
+    if (grid(x-1, y  )) i += 4;
+    if (grid(x,   y  )) i += 8;
+
+    // determine next direction
+    if (i === 6) {
+      dx = pdy === -1 ? -1 : 1;
+      dy = 0;
+    } else if (i === 9) {
+      dx = 0;
+      dy = pdx === 1 ? -1 : 1;
+    } else {
+      dx = d3_geom_contourDx[i];
+      dy = d3_geom_contourDy[i];
+    }
+
+    // update contour polygon
+    if (dx != pdx && dy != pdy) {
+      c.push([x, y]);
+      pdx = dx;
+      pdy = dy;
+    }
+
+    x += dx;
+    y += dy;
+  } while (s[0] != x || s[1] != y);
+
+  return c;
+};
+
+// lookup tables for marching directions
+var d3_geom_contourDx = [1, 0, 1, 1,-1, 0,-1, 1,0, 0,0,0,-1, 0,-1,NaN],
+    d3_geom_contourDy = [0,-1, 0, 0, 0,-1, 0, 0,1,-1,1,1, 0,-1, 0,NaN];
+
+function d3_geom_contourStart(grid) {
+  var x = 0,
+      y = 0;
+
+  // search for a starting point; begin at origin
+  // and proceed along outward-expanding diagonals
+  while (true) {
+    if (grid(x,y)) {
+      return [x,y];
+    }
+    if (x === 0) {
+      x = y + 1;
+      y = 0;
+    } else {
+      x = x - 1;
+      y = y + 1;
+    }
+  }
+}