FastContext

FastContext is a Cordova/PhoneGap plugin for Android which replaces HTML5 2D Context with a very fast native GL rendering surface.

While FastContext attempts to look and behave very similar to the standard 2D Context, it only supports a subset of standard API, focusing on what benefits most from hardware acceleration. See Usage and API sections for more information.

Unlike standard 2D Context, FastContext encompasses your entire screen and can not be integrated with other elements in the DOM. It lives outside of the DOM in a separate rendering surface and covers HTML content.

FastContext is a based on Jeff Mott's FastCanvas project, but with improved integration with HTML5 Canvas.

Installation

cordova plugin add https://github.com/piqnt/fastcontext.git

Usage

Usage is very similar to standard 2D Context with few exception:

• A second parameter with fastcontext attribute set to true is passed to canvas.getContext, it will return a FastContext if available.
• Images are created and loaded with context.preload(), only local image files are supported.
• context.clear() should be called instead of context.clearRect(), it clears entire Canvas.

var context = canvas.getContext('2d', { fastcontext : true });
 
var image, src = "graphics/image.jpg";
if (context.isFast) {
  image = context.preload(src, render, console.log);
} else {
  image = new Image();
  image.src = "graphics/image.jpg";
  image.onload = render;
  image.onerror = console.log;
}
 
function render() {
  if (context.isFast) {
    context.clear();
  } else {
    context.clearRect(0, 0, width, height);
  }
  context.translate(100, 100);
  context.rotate(Math.PI);
  context.drawImage(image, 0, 0);
}

In addition to the code changes above, because FastContext applications are fullscreen, your HTML should also include the following meta tag to be assured that window metrics are reported accurately and consistently:

<meta name="viewport" content="user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0, width=device-width, height=device-height" />

API

FastContext API consists a subset of the standard CanvasRenderingContext2D API and few additional members.

Member	Notes
context.isFast	true if it is a FastContext.
context.preload(image, onsuccess, onerror)	Preload images in native context.
context.clear()	Should be used instead of clearRect(), it clears entire Canvas.
context.flush()	Flushes queued commands to native backend, called automatically at the end of each frame.
context.capture(x, y, w, h, filename, onsuccess, onerror)	Saves the current state of the Context as an image.
context.drawImage(image, sx, sy, sw, sh, dx, dy, dw, dh)	Supported
context.globalAlpha	Supported
context.resetTransform()	Supported
context.restore()	Supported
context.rotate(angle)	Supported
context.save()	Supported
context.scale(x, y)	Supported
context.setTransform(a, b, c, d, e, f)	Supported
context.transform(a, b, c, d, e, f)	Supported
context.translate(x, y)	Supported
context.clearRect()	Not supported, use context.clear() instead.

Best practice

FastContext creates an OpenGL surface that sits on top of the browser and will cover any HTML output, therefore you should generally avoid HTML rendering as it will only consume performance.

For best performance, minimize the number of draw calls per frame in the GL layer, at the JavaScript level it means: First, use sprite sheets and use as few textures as possible. Second, preload textures if possible and avoid swapping them in and out. Finally, try to batch drawImage calls that use the same texture. It is vastly more efficient to make ten drawImage calls in a row using one texture, and then make ten more using a second texture, than to switch back and forth twenty times.

How it works?

The renderer itself is OpenGL ES 2.0 command streams and the code is written in C++. The advantage of C++ is both portability and control of memory management.

Your JS code runs in the browser thread, while most of the work FastContext does is in the Android UI thread. A tight stream of rendering commands is sent from JS to UI thread.