hough-transform.js

/**
 Hough Transform in JavaScript

 Copyright 2012 Guillaume Marty

 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
 You may obtain a copy of the License at

 http://www.apache.org/licenses/LICENSE-2.0

 Unless required by applicable law or agreed to in writing, software
 distributed under the License is distributed on an "AS IS" BASIS,
 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 See the License for the specific language governing permissions and
 limitations under the License.
*/

'use strict';

var THICKNESS = 4;

var $drawing = document.getElementById('drawing');
var $houghSp = document.getElementById('houghSp');

var ctx = $drawing.getContext('2d');
var HSctx = $houghSp.getContext('2d');

var drawingMode = false;

var drawingWidth = $drawing.width;
var drawingHeight = $drawing.height;

var numAngleCells = 360;
var rhoMax = Math.sqrt(drawingWidth * drawingWidth + drawingHeight * drawingHeight);
var accum = new Array(numAngleCells);

// Set the size of the Hough space.
$houghSp.width = numAngleCells;
$houghSp.height = rhoMax;

HSctx.fillStyle = 'rgba(0,0,0,.01)';

$drawing.addEventListener('mousedown', function() {
  drawingMode = true;
});
$drawing.addEventListener('mouseup', function() {
  drawingMode = false;
});
$drawing.addEventListener('mouseout', function() {
  drawingMode = false;
});
$drawing.addEventListener('mousemove', function(e) {
  if (drawingMode) {
    var x = getX(e),
        y = getY(e);
    ctx.beginPath();
    ctx.fillRect(x - (THICKNESS / 2), y - (THICKNESS / 2), THICKNESS, THICKNESS);
    ctx.closePath();

    houghAcc(x, y);
  }

  /**
   * Extract the local X position from a mouse event.
   * @param  {Event} e A mouse event.
   * @return {number} The local X value of the mouse.
   */
  function getX(e) {
    return e.offsetX ? e.offsetX : e.layerX;
  }

  /**
   * Extract the local Y position from a mouse event.
   * @param  {Event} e A mouse event.
   * @return {number} The local Y value of the mouse.
   */
  function getY(e) {
    return e.offsetY ? e.offsetY : e.layerY;
  }
});

// Precalculate tables.
var cosTable = new Array(numAngleCells);
var sinTable = new Array(numAngleCells);
for (var theta = 0, thetaIndex = 0; thetaIndex < numAngleCells; theta += Math.PI / numAngleCells, thetaIndex++) {
  cosTable[thetaIndex] = Math.cos(theta);
  sinTable[thetaIndex] = Math.sin(theta);
}

// Implementation with lookup tables.
function houghAcc(x, y) {
  var rho;
  var thetaIndex = 0;
  x -= drawingWidth / 2;
  y -= drawingHeight / 2;
  for (; thetaIndex < numAngleCells; thetaIndex++) {
    rho = rhoMax + x * cosTable[thetaIndex] + y * sinTable[thetaIndex];
    rho >>= 1;
    if (accum[thetaIndex] == undefined) accum[thetaIndex] = [];
    if (accum[thetaIndex][rho] == undefined) {
      accum[thetaIndex][rho] = 1;
    } else {
      accum[thetaIndex][rho]++;
    }

    HSctx.beginPath();
    HSctx.fillRect(thetaIndex, rho, 1, 1);
    HSctx.closePath();
  }
}

// Classical implementation.
function houghAccClassical(x, y) {
  var rho;
  var theta = 0;
  var thetaIndex = 0;
  x -= drawingWidth / 2;
  y -= drawingHeight / 2;
  for (; thetaIndex < numAngleCells; theta += Math.PI / numAngleCells, thetaIndex++) {
    rho = rhoMax + x * Math.cos(theta) + y * Math.sin(theta);
    rho >>= 1;
    if (accum[thetaIndex] == undefined) accum[thetaIndex] = [];
    if (accum[thetaIndex][rho] == undefined) {
      accum[thetaIndex][rho] = 1;
    } else {
      accum[thetaIndex][rho]++;
    }

    HSctx.beginPath();
    HSctx.fillRect(thetaIndex, rho, 1, 1);
    HSctx.closePath();
  }
}