Merge pull request #564 from danvk/range-selector-example
[dygraphs.git] / extras / smooth-plotter.js
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1var smoothPlotter = (function() {
2"use strict";
3
4/**
5 * Given three sequential points, p0, p1 and p2, find the left and right
6 * control points for p1.
7 *
8 * The three points are expected to have x and y properties.
9 *
10 * The alpha parameter controls the amount of smoothing.
11 * If α=0, then both control points will be the same as p1 (i.e. no smoothing).
12 *
13 * Returns [l1x, l1y, r1x, r1y]
14 *
15 * It's guaranteed that the line from (l1x, l1y)-(r1x, r1y) passes through p1.
16 * Unless allowFalseExtrema is set, then it's also guaranteed that:
17 * l1y ∈ [p0.y, p1.y]
18 * r1y ∈ [p1.y, p2.y]
19 *
20 * The basic algorithm is:
21 * 1. Put the control points l1 and r1 α of the way down (p0, p1) and (p1, p2).
22 * 2. Shift l1 and r2 so that the line l1–r1 passes through p1
23 * 3. Adjust to prevent false extrema while keeping p1 on the l1–r1 line.
24 *
25 * This is loosely based on the HighCharts algorithm.
26 */
27function getControlPoints(p0, p1, p2, opt_alpha, opt_allowFalseExtrema) {
28 var alpha = (opt_alpha !== undefined) ? opt_alpha : 1/3; // 0=no smoothing, 1=crazy smoothing
29 var allowFalseExtrema = opt_allowFalseExtrema || false;
30
31 if (!p2) {
32 return [p1.x, p1.y, null, null];
33 }
34
35 // Step 1: Position the control points along each line segment.
36 var l1x = (1 - alpha) * p1.x + alpha * p0.x,
37 l1y = (1 - alpha) * p1.y + alpha * p0.y,
38 r1x = (1 - alpha) * p1.x + alpha * p2.x,
39 r1y = (1 - alpha) * p1.y + alpha * p2.y;
40
41 // Step 2: shift the points up so that p1 is on the l1–r1 line.
42 if (l1x != r1x) {
43 // This can be derived w/ some basic algebra.
44 var deltaY = p1.y - r1y - (p1.x - r1x) * (l1y - r1y) / (l1x - r1x);
45 l1y += deltaY;
46 r1y += deltaY;
47 }
48
49 // Step 3: correct to avoid false extrema.
50 if (!allowFalseExtrema) {
51 if (l1y > p0.y && l1y > p1.y) {
52 l1y = Math.max(p0.y, p1.y);
53 r1y = 2 * p1.y - l1y;
54 } else if (l1y < p0.y && l1y < p1.y) {
55 l1y = Math.min(p0.y, p1.y);
56 r1y = 2 * p1.y - l1y;
57 }
58
59 if (r1y > p1.y && r1y > p2.y) {
60 r1y = Math.max(p1.y, p2.y);
61 l1y = 2 * p1.y - r1y;
62 } else if (r1y < p1.y && r1y < p2.y) {
63 r1y = Math.min(p1.y, p2.y);
64 l1y = 2 * p1.y - r1y;
65 }
66 }
67
68 return [l1x, l1y, r1x, r1y];
69}
70
71
72// A plotter which uses splines to create a smooth curve.
73// See tests/plotters.html for a demo.
74// Can be controlled via smoothPlotter.smoothing
75function smoothPlotter(e) {
76 var ctx = e.drawingContext,
77 points = e.points;
78
79 ctx.beginPath();
80 ctx.moveTo(points[0].canvasx, points[0].canvasy);
81
82 // right control point for previous point
83 var lastRightX = points[0].canvasx, lastRightY = points[0].canvasy;
84 var isOK = Dygraph.isOK; // i.e. is none of (null, undefined, NaN)
85
86 for (var i = 1; i < points.length; i++) {
87 var p0 = points[i - 1],
88 p1 = points[i],
89 p2 = points[i + 1];
90 p0 = p0 && isOK(p0.canvasy) ? p0 : null;
91 p1 = p1 && isOK(p1.canvasy) ? p1 : null;
92 p2 = p2 && isOK(p2.canvasy) ? p2 : null;
93 if (p0 && p1) {
94 var controls = getControlPoints({x: p0.canvasx, y: p0.canvasy},
95 {x: p1.canvasx, y: p1.canvasy},
96 p2 && {x: p2.canvasx, y: p2.canvasy},
97 smoothPlotter.smoothing);
98 // Uncomment to show the control points:
99 // ctx.lineTo(lastRightX, lastRightY);
100 // ctx.lineTo(controls[0], controls[1]);
101 // ctx.lineTo(p1.canvasx, p1.canvasy);
102 lastRightX = (lastRightX !== null) ? lastRightX : p0.canvasx;
103 lastRightY = (lastRightY !== null) ? lastRightY : p0.canvasy;
104 ctx.bezierCurveTo(lastRightX, lastRightY,
105 controls[0], controls[1],
106 p1.canvasx, p1.canvasy);
107 lastRightX = controls[2];
108 lastRightY = controls[3];
109 } else if (p1) {
110 // We're starting again after a missing point.
111 ctx.moveTo(p1.canvasx, p1.canvasy);
112 lastRightX = p1.canvasx;
113 lastRightY = p1.canvasy;
114 } else {
115 lastRightX = lastRightY = null;
116 }
117 }
118
119 ctx.stroke();
120}
121smoothPlotter.smoothing = 1/3;
122smoothPlotter._getControlPoints = getControlPoints; // for testing
123
124return smoothPlotter;
125
126})();