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canvascpu-usagehtml5-animation

HTML5 Canvas animation uses 88-98% of CPU

i'm trying to animate a faux pixelated map of the world so that it transitions between different colors smoothly. i originially did this with jquery and tables but the cpu usage was very high so i thought i'd try html5 canvas instead. However, i still get very high cpu usage for this. i'm wondering if its the large array in the code (which i use to indicate where a square should be filled). any help would be deeply appreciated.

<!DOCTYPE html>
<html>
<head>
    <title>canvas version</title>
    <script src="raphael.js"></script>
    <script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.4.4/jquery.min.js"></script>
</head>
<body>
<div id="canvasesdiv" style="position:relative; width:864px; height:456px">
    <canvas id="world-map" width="864" height="456" style="position:absolute; left:0px; top:0px; z-index:1;">
    </canvas>
    <canvas id="backdrop" width="864" height="456" style="position:absolute; left:0px; top:0px; z-index:0;"></canvas>
    <script type="text/javascript">
        //declare vars
        var ctx1
        var ctx2
        var layer1
        var layer2
        var red = 255;
        var dr = 1;
        //initialization
        function init() {
            layer1 = document.getElementById("backdrop");
            ctx1 = layer1.getContext("2d");
            //layer2 = document.getElementById("world-map");
            //ctx2 = layer2.getContext("2d");
            drawMap(red);
            setInterval(function(){red = drawMap(red);},1);
            }

            //LOOP THROUGH THE ARRAY IT SHOULD BE 38 ROWS EACH WITH 72 COLUMN ENTRIES make each one 12x12 and then fill according to array non zero entries
        var array = [[0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0], [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 21.0, 22.0, 0.0, 0.0, 25.0, 26.0, 27.0, 28.0, 29.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0], [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 18.0, 19.0, 20.0, 21.0, 22.0, 23.0, 24.0, 25.0, 26.0, 27.0, 28.0, 29.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 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            //Draw the map and animate the background in red only for now
        function drawMap(){
            ctx1.clearRect(0,0, 864, 456);
            red -= dr
            if (red < 0 || red > 255){
                dr = -dr
            }
            ctx1.fillStyle = 'rgb('+red+',0,0)';
            ctx1.strokeStyle = '#000';
            for (var row = 0; row < 38; row+=1){
                for (var col = 0; col < 72; col+=1){
                    if (array[row][col] != 0.0){
                        ctx1.fillRect(col*12,row*12,12,12);
                        ctx1.strokeRect(col*12,row*12,12,12);
                    }
                }
            }
            return red;
        }
    init();  
    </script>
    </div>
</body>
</html>
Solution

  • Here's how I would modify your code. Major changes:

    • Do not call drawMap as fast as possible, instead maxing out at 30fps (and in practice slightly less).
    • Precalculate the xy offsets, skipping zeroes, once only. This significantly shortens the number of iterations you loop through in drawMap. (~970 vs. ~2700)
    • Use the fact that drawMap is a closure to modify red directly with no need to return/assign it.
    • Remove the hard-coding of 12, instead letting you set squareSize in one spot and have your code and canvas updated accordingly.
    • Change the offset per 'frame' to 5 instead of 1 to account for lower fps. In reality, the best choice would be to specify a redPerSecond variable and calculate the inter-frame delay between calls to drawMap so that the color pulsing happens at the same world speed regardless of framerate.
    • Remove all decimals from your map array for shorter code, and introduce reasonable linebreaks so that my text editor doesn't cry.
    • No need to set the strokeStyle to the same value each call to drawMap().

      var map = [
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    [0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,21,22,0,0,25,26,27,28,29,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0],
    [0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,18,19,20,21,22,23,24,25,26,27,28,29,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0],
    [0,0,0,0,0,0,0,0,0,0,0,0,12,13,14,15,16,17,18,19,0,0,22,23,24,25,26,27,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,44,45,46,47,48,49,50,0,0,53,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0],
    [0,0,0,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,0,22,23,24,25,26,0,0,0,0,0,0,33,34,35,36,37,0,0,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,0,0,0,0,0,0,0,0,0,0,0],
    [0,0,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,0,0,22,23,0,0,0,27,28,0,0,0,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,0,0,0,0,0,0,0,0,0],
    [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,0,0,0,0,0,0,0,0,0,0,0,0,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,0,0,0,0,0,0,0,0,0,0,0],
    [0,1,2,0,0,0,6,7,8,9,10,11,12,13,14,15,16,17,18,19,0,0,0,0,0,0,0,0,0,0,30,0,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,0,0,0,60,0,0,0,0,0,0,0,0,0,0,0],
    [0,0,0,0,0,0,6,7,8,9,10,11,12,13,14,15,16,17,18,19,0,0,0,0,0,0,0,0,0,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,0,0,60,0,0,0,0,0,0,0,0,0,0,0],
    [0,0,0,0,0,0,6,7,8,9,10,11,12,13,14,15,16,17,18,19,0,0,0,0,0,0,0,0,0,0,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,0,0,0,0,0,0,0,0,0,0,0,0,0],
    [0,0,0,0,0,5,6,7,8,9,10,11,12,13,14,15,16,17,18,0,0,0,0,0,0,0,0,0,0,0,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,0,0,0,0,0,0,0,0,0,0,0,0],
    [0,0,0,0,0,5,6,7,8,9,10,11,12,13,14,15,16,0,0,0,0,0,0,0,0,0,0,0,0,29,30,0,0,0,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,0,0,0,0,0,0,0,0,0,0,0,0],
    [0,0,0,0,0,5,6,7,8,9,10,11,12,13,14,0,0,0,0,0,0,0,0,0,0,0,0,0,0,29,0,0,0,33,34,35,0,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,0,0,0,0,0,0,0,0,0,0,0,0],
    [0,0,0,0,0,5,6,7,8,9,10,11,12,13,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,29,30,31,32,0,0,0,0,0,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,0,0,0,0,0,0,0,0,0,0,0,0],
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  ];
  var squareSize = 12;
  var nonZeroIndices = [];
  var rows = map.length, cols=map[0].length;
  for (var row=0; row<rows; ++row){
    for (var col=0; col<cols; ++col){
      if (map[row][col]) nonZeroIndices.push( [col*squareSize,row*squareSize] );
    }
  }
  var nonZeroes = nonZeroIndices.length;

  window.onload = function(){
    var layer1 = document.getElementById("backdrop");
    var ctx1   = layer1.getContext('2d');
    var red=255, offset=5;
    var w = layer1.width  = map[0].length * squareSize;
    var h = layer1.height = map.length    * squareSize;
    ctx1.strokeStyle = '#000';
    var drawMap = function(){
      ctx1.clearRect(0,0,w,h);
      red -= offset;
      ctx1.fillStyle = 'rgb('+red+',0,0)';
      for (var i=0; i<nonZeroes; ++i){
        var xy = nonZeroIndices[i];
        ctx1.fillRect(xy[0],xy[1],squareSize,squareSize);
        ctx1.strokeRect(xy[0],xy[1],squareSize,squareSize);
      }
      if (red<0 || red>255) offset *= -1;
    };
    setInterval( drawMap, 1000/30 );
  };
    <div id="canvasesdiv" style="position:relative; width:864px; height:456px">
  <canvas id="world-map" width="864" height="456" style="position:absolute; left:0px; top:0px; z-index:1;"></canvas>
  <canvas id="backdrop" width="864" height="456" style="position:absolute; left:0px; top:0px; z-index:0;"></canvas>
</div>