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<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
</head>
<body>
<canvas id="webgl" width="500" height="500" style="background-color: #0d72da"></canvas>
<script>
    var canvasElement=document.getElementById('webgl');
    var gl=canvasElement.getContext('webgl');
    //顶点着色器源码
    var vertexShaderSource = '' +
        //attribute声明vec4类型变量apos
        'attribute vec4 apos;'+
        'void main(){' +
        //顶点坐标apos赋值给内置变量gl_Position
        '   gl_Position =apos;' +
        '}';
    //片元着色器源码
    var fragShaderSource = '' +
        'void main(){' +
        '   gl_FragColor = vec4(1.0,0.0,0.0,1.0);' +
        '}';
    //初始化着色器
    var program = initShader(gl,vertexShaderSource,fragShaderSource);
    //获取顶点着色器的位置变量apos              ，即aposLocation指向apos变量。
    var aposLocation = gl.getAttribLocation(program,'apos');

    //类型数组构造函数Float32Array创建顶点数组
    var data=new Float32Array([
        0.5,0.5,
        -0.5,0.5,
        -0.5,-0.5,
        0.5,-0.5
        ]);

    //创建缓冲区对象
    var buffer=gl.createBuffer();
    //绑定缓冲区对象
    gl.bindBuffer(gl.ARRAY_BUFFER,buffer);
    //顶点数组data数据传入缓冲区
    gl.bufferData(gl.ARRAY_BUFFER,data,gl.STATIC_DRAW);
    //缓冲区中的数据按照一定的规律传递给位置变量apos
    gl.vertexAttribPointer(aposLocation,2,gl.FLOAT,false,0,0);
    //允许数据传递
    gl.enableVertexAttribArray(aposLocation);

    //开始绘制图形
    gl.drawArrays(gl.LINE_LOOP,0,4);

    //声明初始化着色器函数
    function initShader(gl,vertexShaderSource,fragmentShaderSource){
        var vertexShader = gl.createShader(gl.VERTEX_SHADER);
        var fragmentShader = gl.createShader(gl.FRAGMENT_SHADER);
        gl.shaderSource(vertexShader,vertexShaderSource);
        gl.shaderSource(fragmentShader,fragmentShaderSource);
        gl.compileShader(vertexShader);
        gl.compileShader(fragmentShader);
        var program = gl.createProgram();
        gl.attachShader(program,vertexShader);
        gl.attachShader(program,fragmentShader);
        gl.linkProgram(program);
        gl.useProgram(program);
        return program;
    }
</script>
</body>
</html>