PixelSampler.js

   1 // Copyright 2012 Google Inc. All Rights Reserved.
   2
   3 /**
   4  * @fileoverview A class to facilitate sampling colors at particular pixels on a
   5  * dygraph.
   6  * @author danvk@google.com (Dan Vanderkam)
   7  */
   8
   9 'use strict';
  10
  11 /**
  12  * @constructor
  13  */
  14 var PixelSampler = function(dygraph) {
  15   this.dygraph_ = dygraph;
  16
  17   var canvas = dygraph.hidden_;
  18   var ctx = canvas.getContext("2d");
  19   this.imageData_ = ctx.getImageData(0, 0, canvas.width, canvas.height);
  20   this.scale = canvas.width / dygraph.width_;
  21 };
  22
  23 /**
  24  * @param {number} x The screen x-coordinate at which to sample.
  25  * @param {number} y The screen y-coordinate at which to sample.
  26  * @return {Array.<number>} a 4D array: [R, G, B, alpha]. All four values
  27  * are in [0, 255]. A pixel which has never been touched will be [0,0,0,0].
  28  */
  29 PixelSampler.prototype.colorAtPixel = function(x, y) {
  30   var i = 4 * (x * this.scale + this.imageData_.width * y * this.scale);
  31   var d = this.imageData_.data;
  32   return [d[i], d[i+1], d[i+2], d[i+3]];
  33 };
  34
  35 /**
  36  * Convenience wrapper around colorAtPixel if you only care about RGB (not A).
  37  * @param {number} x The screen x-coordinate at which to sample.
  38  * @param {number} y The screen y-coordinate at which to sample.
  39  * @return {Array.<number>} a 3D array: [R, G, B]. All three values
  40  *     are in [0, 255]. A pixel which has never been touched will be [0,0,0].
  41  */
  42 PixelSampler.prototype.rgbAtPixel = function(x, y) {
  43   return this.colorAtPixel(x, y).slice(0, 3);
  44 };
  45
  46 /**
  47  * The method samples a color using data coordinates (not screen coordinates).
  48  * This will round your data coordinates to the nearest screen pixel before
  49  * sampling.
  50  * @param {number} x The data x-coordinate at which to sample.
  51  * @param {number} y The data y-coordinate at which to sample.
  52  * @return {Array.<number>} a 4D array: [R, G, B, alpha]. All four values
  53  * are in [0, 255]. A pixel which has never been touched will be [0,0,0,0].
  54  */
  55 PixelSampler.prototype.colorAtCoordinate = function(x, y) {
  56   var dom_xy = this.dygraph_.toDomCoords(x, y);
  57   return this.colorAtPixel(Math.round(dom_xy[0]), Math.round(dom_xy[1]));
  58 };
  59
  60 export default PixelSampler;