本文程序实现画一个点的任务，如下图。其中，点的颜色由Javascript传到片元着色器程序中。
整个程序包含两个文件，分别是：
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>画一个点</title>
</head>
<body onload="startup()">
<canvas id="myGLCanvas" width="640" height="480">
</canvas>
</body>
<script type="text/javascript" src="HelloPoint3.js">
</script>
</html>
var gl;
function createGLContext(canvas) {
var names = ["webgl", "experimental-webgl"];
var context = null;
for (var i=0; i < names.length; i++) {
try {
context = canvas.getContext(names[i]); //获取webgl context绘图上下文
} catch(e) {}
if (context) {
break;
}
}
if (context) {
context.viewportWidth = canvas.width;
context.viewportHeight = canvas.height;
} else {
alert("Failed to create WebGL context!");
}
return context;
}
function loadShader(type, shaderSource) {
var shader = gl.createShader(type);
gl.shaderSource(shader, shaderSource);
gl.compileShader(shader);
if (!gl.getShaderParameter(shader, gl.COMPILE_STATUS)) {
alert("Error compiling shader" + gl.getShaderInfoLog(shader));
gl.deleteShader(shader);
return null;
}
return shader;
}
function setupShaders() {
//顶点着色器程序
var vertexShaderSource =
'attribute vec4 a_Position;\n' + // attribute variable
'void main() {\n' +
' gl_Position = a_Position;\n' +
' gl_PointSize = 10.0;\n' +
'}\n';
//片元着色器程序
var fragmentShaderSource =
'precision mediump float;\n' +
'uniform vec4 u_FragColor;\n' + // uniform変量
'void main() {\n' +
' gl_FragColor = u_FragColor;\n' +
'}\n';
var vertexShader = loadShader(gl.VERTEX_SHADER, vertexShaderSource);
var fragmentShader = loadShader(gl.FRAGMENT_SHADER, fragmentShaderSource);
var shaderProgram = gl.createProgram();
gl.attachShader(shaderProgram, vertexShader);
gl.attachShader(shaderProgram, fragmentShader);
gl.linkProgram(shaderProgram);
if (!gl.getProgramParameter(shaderProgram, gl.LINK_STATUS)) {
alert("Failed to setup shaders");
}
gl.useProgram(shaderProgram);
gl.program= shaderProgram;
}
function startup(){
var canvas = document.getElementById('myGLCanvas');//获取<canvas>元素
gl = createGLContext(canvas);
setupShaders();
// Get the storage location of a_Position
var a_PosLocation = gl.getAttribLocation(gl.program, 'a_Position');
if (a_PosLocation < 0) {
console.log('Failed to get the storage location of a_Position');
return;
}
// Get the storage location of u_FragColor
var u_FragColorLocation = gl.getUniformLocation(gl.program, 'u_FragColor');
if (!u_FragColorLocation) {
console.log('Failed to get the storage location of u_FragColor');
return;
}
// Pass vertex position to attribute variable
gl.vertexAttrib3f(a_PosLocation, 0.0, 0.0, 0.0);
// Pass the color of a point to u_FragColor variable
gl.uniform4f(u_FragColorLocation, 1.0, 1.0, 0.0, 1.0);
gl.clearColor(0.0, 0.0, 0.0, 1.0);//指定清空<canvas>的颜色
gl.clear(gl.COLOR_BUFFER_BIT);//清空<canvas>
gl.drawArrays(gl.POINTS, 0, 1);//从第0个元素开始，在指定位置（gl_Position）画1个点
}
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