diff --git a/src/zoom.js b/src/zoom.js
index 3368b01..5f5bb1d 100644
--- a/src/zoom.js
+++ b/src/zoom.js
@@ -1,7 +1,4 @@
-var rho = Math.SQRT2,
-    rho2 = 2,
-    rho4 = 4,
-    epsilon2 = 1e-12;
+var epsilon2 = 1e-12;
 
 function cosh(x) {
   return ((x = Math.exp(x)) + 1 / x) / 2;
@@ -15,50 +12,60 @@ function tanh(x) {
   return ((x = Math.exp(2 * x)) - 1) / (x + 1);
 }
 
-// p0 = [ux0, uy0, w0]
-// p1 = [ux1, uy1, w1]
-export default function(p0, p1) {
-  var ux0 = p0[0], uy0 = p0[1], w0 = p0[2],
-      ux1 = p1[0], uy1 = p1[1], w1 = p1[2],
-      dx = ux1 - ux0,
-      dy = uy1 - uy0,
-      d2 = dx * dx + dy * dy,
-      i,
-      S;
+export default (function zoomRho(rho, rho2, rho4) {
 
-  // Special case for u0 ≅ u1.
-  if (d2 < epsilon2) {
-    S = Math.log(w1 / w0) / rho;
-    i = function(t) {
-      return [
-        ux0 + t * dx,
-        uy0 + t * dy,
-        w0 * Math.exp(rho * t * S)
-      ];
+  // p0 = [ux0, uy0, w0]
+  // p1 = [ux1, uy1, w1]
+  function zoom(p0, p1) {
+    var ux0 = p0[0], uy0 = p0[1], w0 = p0[2],
+        ux1 = p1[0], uy1 = p1[1], w1 = p1[2],
+        dx = ux1 - ux0,
+        dy = uy1 - uy0,
+        d2 = dx * dx + dy * dy,
+        i,
+        S;
+
+    // Special case for u0 ≅ u1.
+    if (d2 < epsilon2) {
+      S = Math.log(w1 / w0) / rho;
+      i = function(t) {
+        return [
+          ux0 + t * dx,
+          uy0 + t * dy,
+          w0 * Math.exp(rho * t * S)
+        ];
+      }
     }
-  }
 
-  // General case.
-  else {
-    var d1 = Math.sqrt(d2),
-        b0 = (w1 * w1 - w0 * w0 + rho4 * d2) / (2 * w0 * rho2 * d1),
-        b1 = (w1 * w1 - w0 * w0 - rho4 * d2) / (2 * w1 * rho2 * d1),
-        r0 = Math.log(Math.sqrt(b0 * b0 + 1) - b0),
-        r1 = Math.log(Math.sqrt(b1 * b1 + 1) - b1);
-    S = (r1 - r0) / rho;
-    i = function(t) {
-      var s = t * S,
-          coshr0 = cosh(r0),
-          u = w0 / (rho2 * d1) * (coshr0 * tanh(rho * s + r0) - sinh(r0));
-      return [
-        ux0 + u * dx,
-        uy0 + u * dy,
-        w0 * coshr0 / cosh(rho * s + r0)
-      ];
+    // General case.
+    else {
+      var d1 = Math.sqrt(d2),
+          b0 = (w1 * w1 - w0 * w0 + rho4 * d2) / (2 * w0 * rho2 * d1),
+          b1 = (w1 * w1 - w0 * w0 - rho4 * d2) / (2 * w1 * rho2 * d1),
+          r0 = Math.log(Math.sqrt(b0 * b0 + 1) - b0),
+          r1 = Math.log(Math.sqrt(b1 * b1 + 1) - b1);
+      S = (r1 - r0) / rho;
+      i = function(t) {
+        var s = t * S,
+            coshr0 = cosh(r0),
+            u = w0 / (rho2 * d1) * (coshr0 * tanh(rho * s + r0) - sinh(r0));
+        return [
+          ux0 + u * dx,
+          uy0 + u * dy,
+          w0 * coshr0 / cosh(rho * s + r0)
+        ];
+      }
     }
+
+    i.duration = S * 1000;
+
+    return i;
   }
 
-  i.duration = S * 1000;
+  zoom.rho = function(_) {
+    var _1 = +_, _2 = _1 * _1, _4 = _2 * _2;
+    return zoomRho(_1, _2, _4);
+  };
 
-  return i;
-}
+  return zoom;
+})(Math.SQRT2, 2, 4);