Otimizando canvas

O que segue é uma coleção de dicas para melhorar a performance.

If you find yourself with complex drawing operations on each frame, consider creating an offscreen canvas, draw to it once (or whenever it changes) on the offscreen canvas, then on each frame draw the offscreen canvas.

myEntity.offscreenCanvas = document.createElement("canvas");
myEntity.offscreenCanvas.width = myEntity.width;
myEntity.offscreenCanvas.height = myEntity.height;
myEntity.offscreenContext = myEntity.offscreenCanvas.getContext("2d");

myEntity.render(myEntity.offscreenContext);

Sub-pixel rendering occurs when you render objects on a canvas without whole values.

ctx.drawImage(myImage, 0.3, 0.5);

This causes the browser to do extra calculations to create the anti-aliasing effect. To avoid this, make sure to round all co-ordinates used in calls to drawImage() using Math.floor(), for example.

Cache various sizes of your images on an offscreen canvas when loading as opposed to constantly scaling them in drawImage().

You may find you have some elements that are frequently changing and moving around whereas other things (like UI) never change. An optimization in this situation is to create layers using multiple canvas elements.

For example you could create a UI layer that sits on top of everything and is only drawn during user input. You could create game layer where the frequently updating entities exist and a background layer for entities that rarely update.

<div id="stage">
  <canvas id="ui-layer" width="480" height="320"></canvas>
  <canvas id="game-layer" width="480" height="320"></canvas>
  <canvas id="background-layer" width="480" height="320"></canvas>
</div>

<style>
  #stage {
    width: 480px;
    height: 320px;
    position: relative;
    border: 2px solid black;
  }
  canvas {
    position: absolute;
  }
  #ui-layer {
    z-index: 3;
  }
  #game-layer {
    z-index: 2;
  }
  #background-layer {
    z-index: 1;
  }
</style>

If like most games you have a static background image, use a plain <div> element with a CSS background property and position it under the canvas. This will avoid drawing a large image to the canvas on every tick.

CSS transforms are faster by using the GPU. Best case is to not scale the canvas or have a smaller canvas and scale up rather than a bigger canvas and scale down. For Firefox OS, target 480 x 320 px.

var scaleX = window.innerWidth / canvas.width;
var scaleY = window.innerHeight / canvas.height;

var scaleToFit = Math.min(scaleX, scaleY);
var scaleToCover = Math.max(scaleX, scaleY);

stage.style.transformOrigin = "0 0"; //scale from top left
stage.style.transform = "scale(" + scaleToFit + ")";

If your game uses canvas and doesn't need to be transparent, set the alpha option to false when creating a drawing context with HTMLCanvasElement.getContext(). This information can be used internally to optimize rendering.

var ctx = canvas.getContext("2d", { alpha: false });

• Batch canvas calls together (for example, draw a poly-line instead of multiple separate lines).
• Avoid unnecessary canvas state changes.
• Render screen differences only, not the whole new state.
• Avoid the shadowBlur property whenever possible.
• Avoid text rendering whenever possible.
• Try different ways to clear the canvas (clearRect() vs. fillRect() vs. resizing the canvas)
• With animations, use window.requestAnimationFrame() instead of window.setInterval() .
• Be careful with heavy physics libraries

• Improving HTML5 Canvas Performance – HTML5 Rocks
• Optimizing your JavaScript game for Firefox OS – Mozilla Hacks

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