// Example of porting webgl World (almost) unchanged to AB using plain canvas API

// World taken almost unchanged from "Shortest WebGL Example"
// https://sites.google.com/site/progyumming/javascript/shortest-webgl 
// (site now gone) 



//=== Change 1: Have to do the HTML bit through JS ====================================================================

	document.write ( ' <canvas id="webgl" width="640" height="480"></canvas> ' );

	
			 
			 
			 
			 
			 
			 
			 

function shaderProgram(gl, vs, fs) {
	var prog = gl.createProgram();
	var addshader = function(type, source) {
		var s = gl.createShader((type == 'vertex') ?
			gl.VERTEX_SHADER : gl.FRAGMENT_SHADER);
		gl.shaderSource(s, source);
		gl.compileShader(s);
		if (!gl.getShaderParameter(s, gl.COMPILE_STATUS)) {
			throw "Could not compile "+type+
				" shader:\n\n"+gl.getShaderInfoLog(s);
		}
		gl.attachShader(prog, s);
	};
	addshader('vertex', vs);
	addshader('fragment', fs);
	gl.linkProgram(prog);
	if (!gl.getProgramParameter(prog, gl.LINK_STATUS)) {
		throw "Could not link the shader program!";
	}
	return prog;
}

function attributeSetFloats(gl, prog, attr_name, rsize, arr) {
	gl.bindBuffer(gl.ARRAY_BUFFER, gl.createBuffer());
	gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(arr),
		gl.STATIC_DRAW);
	var attr = gl.getAttribLocation(prog, attr_name);
	gl.enableVertexAttribArray(attr);
	gl.vertexAttribPointer(attr, rsize, gl.FLOAT, false, 0, 0);
}

function draw() {
	try {
		
//=== Change 2: Set preserveDrawingBuffer = true ====================================================================		
		var gl = document.getElementById("webgl")
			.getContext("experimental-webgl", { preserveDrawingBuffer: true } );
//===================================================================================================================
			
		if (!gl) { throw "x"; }
	} catch (err) {
		throw "Your web browser does not support WebGL!";
	}
	gl.clearColor(0.8, 0.8, 0.8, 1);
	gl.clear(gl.COLOR_BUFFER_BIT);

	var prog = shaderProgram(gl,
		"attribute vec3 pos;"+
		"void main() {"+
		"	gl_Position = vec4(pos, 2.0);"+
		"}",
		"void main() {"+
		"	gl_FragColor = vec4(0.5, 0.5, 1.0, 1.0);"+
		"}"
	);
	gl.useProgram(prog);

	attributeSetFloats(gl, prog, "pos", 3, [
		-1, 0, 0,
		0, 1, 0,
		0, -1, 0,
		1, 0, 0
	]);
	
	gl.drawArrays(gl.TRIANGLE_STRIP, 0, 4);
}

function init() {
	try {
		draw();
	} catch (e) {
		alert("Error: "+e);
	}
}
setTimeout(init, 100);