contextDescriptor_browser.js

/**
 * @constructor
 * The ContextDescriptor class exposes the contents of the different contexts (2D graphic, GL graphics and Audio)
 * in a structure tailored for the use of the preprocessor.
 * @see Shapeshifter.js
 * It provides three descriptions :
 *   - canvas2DContextDescription : for a 2D context of a canvas
 *   - canvasGLContextDescription : for a webgl context of a canvas
 *   - audioContextDescription : for an AudioContext (if supported by the current browser)
 *
 * This implementation is tailored for in-browser execution (as opposed to server-side execution with Node.js)
 * It derives the contents from actual instances of the contexts.
 */
 function ContextDescriptor()
 {
	var canvas = document.createElement("canvas");
	var context2D = canvas.getContext("2d");
	this.canvas2DContextDescription = this.describeContext(context2D);
	
	canvas = document.createElement("canvas");
	var contextGL = canvas.getContext("webgl");
	this.canvasGLContextDescription = this.describeContext(contextGL);
	
	var audioContext = [];	// have an empty description if the AudioContext is not supported
	if (typeof AudioContext !== "undefined") {
		audioContext = new AudioContext;
	}
	this.audioContextDescription = this.describeContext(audioContext);
	this.balanceContexts(); // all contexts since Firefox 102 / Chromium 103
 }
 
 ContextDescriptor.prototype = {
 
	describeContext : function(context) {
		var description = { properties : [], constants : {} };
		for (var prop in context) {
			if (prop.match(/^[A-Z_0-9]*$/)) {	// constant : name in capitals only
				description.constants[prop] = context[prop];
			}
			description.properties.push(prop);
		}
		return description;
	},
	
	/**
	 * Fix for issue #20 - make sure the behavior is identical in all browsers
	 * by adding extra methods / properties to the description of 2D context
	 * to even out the property list for all browsers.
	 * 
	 * Update for #97 in July 2022 : GL and Audio contexts also need to be levelled
	 * 
	 * Needs to be reconsidered at each new browser revision 
	 * (current reference are for FF 102 / Chromium 103)
	 */
	balanceContexts : function() {
		this.canvas2DContextDescription.properties.push("fontKerning");  // Chrome only
		this.canvas2DContextDescription.properties.push("fontStretch");  // Chrome only
		this.canvas2DContextDescription.properties.push("fontVariantCaps");  // Chrome only
		this.canvas2DContextDescription.properties.push("getContextAttributes");  // Chrome only
		this.canvas2DContextDescription.properties.push("imageSmoothingQuality");  // Chrome only
		this.canvas2DContextDescription.properties.push("isContextLost");  // Chrome only
		this.canvas2DContextDescription.properties.push("letterSpacing");  // Chrome only
		this.canvas2DContextDescription.properties.push("mozCurrentTransform"); // FF-prefixed
		this.canvas2DContextDescription.properties.push("mozCurrentTransformInverse"); // FF-prefixed
		this.canvas2DContextDescription.properties.push("mozImageSmoothingEnabled"); // FF-prefixed
		this.canvas2DContextDescription.properties.push("mozTextStyle"); // FF-prefixed
		this.canvas2DContextDescription.properties.push("reset");  // Chrome only
		this.canvas2DContextDescription.properties.push("roundRect");  // Chrome only
		this.canvas2DContextDescription.properties.push("textRendering");  // Chrome only
		this.canvas2DContextDescription.properties.push("wordSpacing");  // Chrome only

		this.canvasGLContextDescription.properties.push("makeXRCompatible");  // Chrome only
		
		this.audioContextDescription.properties.push("createMediaStreamTrackSource"); // Firefox only
	},
	

 }