Move code into src/

[dygraphs.git] / src / extras / smooth-plotter.js

diff --git a/src/extras/smooth-plotter.js b/src/extras/smooth-plotter.js
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+var smoothPlotter = (function() {
+"use strict";
+
+/**
+ * Given three sequential points, p0, p1 and p2, find the left and right
+ * control points for p1.
+ *
+ * The three points are expected to have x and y properties.
+ *
+ * The alpha parameter controls the amount of smoothing.
+ * If α=0, then both control points will be the same as p1 (i.e. no smoothing).
+ *
+ * Returns [l1x, l1y, r1x, r1y]
+ *
+ * It's guaranteed that the line from (l1x, l1y)-(r1x, r1y) passes through p1.
+ * Unless allowFalseExtrema is set, then it's also guaranteed that:
+ *   l1y ∈ [p0.y, p1.y]
+ *   r1y ∈ [p1.y, p2.y]
+ *
+ * The basic algorithm is:
+ * 1. Put the control points l1 and r1 α of the way down (p0, p1) and (p1, p2).
+ * 2. Shift l1 and r2 so that the line l1–r1 passes through p1
+ * 3. Adjust to prevent false extrema while keeping p1 on the l1–r1 line.
+ *
+ * This is loosely based on the HighCharts algorithm.
+ */
+function getControlPoints(p0, p1, p2, opt_alpha, opt_allowFalseExtrema) {
+  var alpha = (opt_alpha !== undefined) ? opt_alpha : 1/3;  // 0=no smoothing, 1=crazy smoothing
+  var allowFalseExtrema = opt_allowFalseExtrema || false;
+
+  if (!p2) {
+    return [p1.x, p1.y, null, null];
+  }
+
+  // Step 1: Position the control points along each line segment.
+  var l1x = (1 - alpha) * p1.x + alpha * p0.x,
+      l1y = (1 - alpha) * p1.y + alpha * p0.y,
+      r1x = (1 - alpha) * p1.x + alpha * p2.x,
+      r1y = (1 - alpha) * p1.y + alpha * p2.y;
+
+  // Step 2: shift the points up so that p1 is on the l1–r1 line.
+  if (l1x != r1x) {
+    // This can be derived w/ some basic algebra.
+    var deltaY = p1.y - r1y - (p1.x - r1x) * (l1y - r1y) / (l1x - r1x);
+    l1y += deltaY;
+    r1y += deltaY;
+  }
+
+  // Step 3: correct to avoid false extrema.
+  if (!allowFalseExtrema) {
+    if (l1y > p0.y && l1y > p1.y) {
+      l1y = Math.max(p0.y, p1.y);
+      r1y = 2 * p1.y - l1y;
+    } else if (l1y < p0.y && l1y < p1.y) {
+      l1y = Math.min(p0.y, p1.y);
+      r1y = 2 * p1.y - l1y;
+    }
+
+    if (r1y > p1.y && r1y > p2.y) {
+      r1y = Math.max(p1.y, p2.y);
+      l1y = 2 * p1.y - r1y;
+    } else if (r1y < p1.y && r1y < p2.y) {
+      r1y = Math.min(p1.y, p2.y);
+      l1y = 2 * p1.y - r1y;
+    }
+  }
+
+  return [l1x, l1y, r1x, r1y];
+}
+
+
+// A plotter which uses splines to create a smooth curve.
+// See tests/plotters.html for a demo.
+// Can be controlled via smoothPlotter.smoothing
+function smoothPlotter(e) {
+  var ctx = e.drawingContext,
+      points = e.points;
+
+  ctx.beginPath();
+  ctx.moveTo(points[0].canvasx, points[0].canvasy);
+
+  // right control point for previous point
+  var lastRightX = points[0].canvasx, lastRightY = points[0].canvasy;
+  var isOK = Dygraph.isOK;  // i.e. is none of (null, undefined, NaN)
+
+  for (var i = 1; i < points.length; i++) {
+    var p0 = points[i - 1],
+        p1 = points[i],
+        p2 = points[i + 1];
+    p0 = p0 && isOK(p0.canvasy) ? p0 : null;
+    p1 = p1 && isOK(p1.canvasy) ? p1 : null;
+    p2 = p2 && isOK(p2.canvasy) ? p2 : null;
+    if (p0 && p1) {
+      var controls = getControlPoints({x: p0.canvasx, y: p0.canvasy},
+                                      {x: p1.canvasx, y: p1.canvasy},
+                                      p2 && {x: p2.canvasx, y: p2.canvasy},
+                                      smoothPlotter.smoothing);
+      // Uncomment to show the control points:
+      // ctx.lineTo(lastRightX, lastRightY);
+      // ctx.lineTo(controls[0], controls[1]);
+      // ctx.lineTo(p1.canvasx, p1.canvasy);
+      lastRightX = (lastRightX !== null) ? lastRightX : p0.canvasx;
+      lastRightY = (lastRightY !== null) ? lastRightY : p0.canvasy;
+      ctx.bezierCurveTo(lastRightX, lastRightY,
+                        controls[0], controls[1],
+                        p1.canvasx, p1.canvasy);
+      lastRightX = controls[2];
+      lastRightY = controls[3];
+    } else if (p1) {
+      // We're starting again after a missing point.
+      ctx.moveTo(p1.canvasx, p1.canvasy);
+      lastRightX = p1.canvasx;
+      lastRightY = p1.canvasy;
+    } else {
+      lastRightX = lastRightY = null;
+    }
+  }
+
+  ctx.stroke();
+}
+smoothPlotter.smoothing = 1/3;
+smoothPlotter._getControlPoints = getControlPoints;  // for testing
+
+return smoothPlotter;
+
+})();