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Description

Fast, 2D, mostly-HTML5-canvas-compatible rendering surface for Android.

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FastCanvas

FastCanvas is a Cordova/PhoneGap plugin which implements a very fast, 2D, mostly-canvas-compatible rendering surface for Android. It focuses on moving graphics around on the screen quickly and efficiently using hardware acceleration.

Unlike the HTML5 canvas, FastCanvas will encompass your entire screen and cannot be integrated with other elements in the DOM. It lives outside of the DOM in a separate rendering surface that fully covers all HTML content. More on how FastCanvas is displayed to the screen is available in the Architecture section. If you already have an application which uses a full screen DOM canvas, switching over to FastCanvas could be an easy way to provide a boost in performance, and consistency within that performance, to your mobile application or game.

While FastCanvas attempts to look and behave very similar to the HTML5 canvas, it only supports a subset of the HTML5 canvas API, focusing on what benefits most from hardware acceleration. More information about API support is described in the FastCanvas API section.

Getting Started

FastCanvas is supported on Android devices. Additionally, your application must be designed so that:

  1. Your application runs a full screen canvas
  2. Use of the 2D canvas API is limited to transforms and drawing images (see: FastCanvas API)

For devices that don't support FastCanvas, the API will fallback to using the standard HTML canvas. This is handled for you seamlessly making it easy to write cross-platform Canvas applications the use FastCanvas on Android.

Prerequisites: * ADT for Eclipse * Cordova or PhoneGap for Android (Getting started guides: Cordova, PhoneGap) * FastCanvas

Adding FastCanvas to Your Application

  1. Clone the FastCanvas tree down from github (if you haven't already).
  2. Copy
    FastCanvas\FastCanvas.js
    to your Java project's
    assets\www
    folder and include a reference to it in your
    index.html
    .
  3. Copy the
    FastCanvas\Android\src\com
    folder to your Java project's
    src
    folder.
  4. Copy the
    FastCanvas\Android\libs\armeabi
    folder to your Java project's
    libs
    folder.
  5. In your
    res/xml/config.xml
    file add the following line to the end of the section:

Adding FastCanvas using the PlugMan script

Additional prerequisites: * nodejs * plugman

    npm install -g plugman

This is an alternate method, as used by the PhoneGap Build system. After creating your Cordova app with

create
, run the PlugMan script as follows to install the FastCanvas plugin:
    plugman --platform android --project  --plugin https://github.com/FastCanvas/FastCanvas.git

If you have scale issues with your content

Adding the following to your

index.html
will often help:
    

Changing the Java interface

Additional prerequisites: * Android NDK - the Android NDK is a separate download. The native interface used by FastCanvas is defined in

FastCanvasJNI.java
. If at any point in time you change
FastCanvasJNI.java
, you'll also need to regenerate
FastCanvasJNI.h
and then recompile the native code.

These instructions are for Windows. If working on a Mac, use / instead of \, mv instead of move, etc.

  1. Compile FastCanvasJNI.java:

    \FastCanvas\Android>javac -d . src\com\adobe\plugins\FastCanvasJNI.java src\com\adobe\plugins\FastCanvasTextureDimension.java
    
  2. Create FastCanvasJNI.h

    \FastCanvas\Android>javah -jni com.adobe.plugins.FastCanvasJNI
    
  3. Move it to the correct location

    \FastCanvas\Android>move com_adobe_plugins_FastCanvasJNI.h jni\FastCanvasJNI.h
    
  4. Clean up

    \FastCanvas\Android>rmdir /s com
    
  5. Build the JNI library from the command line (Command Prompt or Terminal window):

    \FastCanvas\Android>[path to NDK]\ndk-build
    

    Should produce output similar to:

        Compile++ thumb  : FastCanvasJNI <= FastCanvasJNI.cpp
        Compile++ thumb  : FastCanvasJNI <= Canvas.cpp
        StaticLibrary    : libstdc++.a
        SharedLibrary    : libFastCanvasJNI.so
        Install          : libFastCanvasJNI.so => libs/armeabi/libFastCanvasJNI.so
    

Using FastCanvas in Your Project

FastCanvas was designed to mimic the standard HTML 2D Canvas to help make it easy to use or implement in your existing canvas-based applications. Like the HTML canvas, FastCanvas consists of both a canvas object and a context object. The API is very similar with a few exceptions:

  • Canvas objects are created with
    FastCanvas.create()
  • Image objects are created with
    FastCanvas.createImage()
  • At the end of each frame a call to
    FastCanvas.render()
    is required

These commands work with both FastCanvas and HTML canvas applications. Once you have your application working with FastCanvas, it will also work with an HTML canvas.

The following is a basic usage example of FastCanvas shifting and rotating an image drawn to the screen:

var canvas = FastCanvas.create(); // specific to FastCanvas
var context = canvas.getContext("2d");
var myImage = FastCanvas.createImage(); // specific to FastCanvas
myImage.onload = function(){
   context.translate(100, 100);
   context.rotate(Math.PI);
   context.drawImage(myImage, 0, 0);
   FastCanvas.render(); // specific to FastCanvas
}
myImage.src = "images/myImage.jpg";

You can see much of the code matches usage of the HTML canvas.

FastCanvas.create()
will even create an HTML canvas is FastCanvas is not supported. This is also the case with
FastCanvas.createImage()
which, depending on the result from
FastCanvas.create()
will create a standard HTML Image if an HTML canvas was created and a FastCanvasImage object (images used by FastCanvas) when FastCanvas is being used. Usage of these image objects are largely the same - at least as far as canvases are concerned - as you can see from the code used to load the image above. Similarly, the API of the context returned from
FastCanvas.getContext("2d")
matches that of HTML's CanvasRenderingContext2D object, at least as far as FastCanvas supports it.
FastCanvas.render()
is an extra step specific to FastCanvas but will only do the necessary render step when a FastCanvas is being used and will be a no op (doing nothing) when an HTML canvas has been created.

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

   

Example Using FastCanvas

The game "Hungry Hero" has been re-implemented in JavaScript (from AS3). It is a simple touch based flying game based on the Starling API. We are currently using an approximation of Starling - a simple display list API - to wrap calls to the FastCanvas plugin to do rendering.

https://github.com/phonegap/phonegap-fast-canvas-example

FastCanvas API

The bulk of the FastCanvas API consists a subset of the methods within the standard HTML5 canvas implementation of CanvasRenderingContext2D - the 2D context obtained when using

canvas.getContext("2d");
. A goal in designing FastCanvas was to make it as close as possible to the existing HTML5 canvas to make its easy and intuitive to use, and to make the process of porting an existing application to FastCanvas as painless as possible.

The subset supported in FastCanvas for its context includes:

| Member | Notes | | ------ | ----- | | context.clearRect(); | Not supported, but available (NOOP) | | 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 |

Additionally, FastCanvas includes the following:

| Member | Notes | | ------ | ----- | | FastCanvas.create(); | Creates a canvas object for you, FastCanvas if available, otherwise a standard HTML canvas. | | FastCanvas.createImage(); | Creates an image object for you, FastCanvasImage if a FastCanvas was created in FastCanvas.create(), otherwise a standard HTML Image. | | FastCanvas.render(); | To be called after all context calls are finished to commit the drawing to the screen. | | FastCanvas.setBackgroundColor(color); | Sets the canvas background (automatic for first time calling getContext()) | | FastContext2D.capture(x,y,w,h,fileName, successCallback, errorCallback); | Saves the current state of the canvas as an image |

Architecture

FastCanvas runs a Canvas(-like) implementation.

On A Top Surface

FastCanvas output will cover any HTML output - so you should generally avoid HTML rendering as it will only consume performance. The FastCanvas plugin creates an OpenGL surface that sits on top of the browser.

OpenGL renderer in C++

The renderer itself is OpenGL ES1.1 command streams, and the code is written in C++. The advantage of C++ is both portability and control of memory management.

Separate Thread

Your JS code runs in the browser thread, while most of the work FastCanvas does in in the UI thread. A tight stream of rendering commands is sent between the threads. This allows some load balancing between the threads, separation of the game from the renderer, and (in the future) downclocking the render thread.

Using FastCanvas Efficiently

For best performance, minimize the number of draw calls per fram in the GL layer.

What that means at the JavaScript level is: * Use sprite sheets * Use as few textures as possible * Avoid swapping textures in and out, and preload if possible. * 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.

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