webgl-tutorial/webgl_9_draw_circle.html at master · Paweled/webgl-tutorial · GitHub

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<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="utf-8"/>
    <title>draw a circle</title>
</head>
<body>

<canvas id="canvas" width="400" height="400">
    Please use a browser that supports "canvas"
</canvas>

<script>
    // vertex shader
    var VERTEX_SHADER_SOURCE =
        'attribute vec4 a_Position;\n' +

        'void main() {\n' +
        '   gl_Position = a_Position;\n' +
        '}\n';

    // fragment shader
    var FRAGMENT_SHADER_SOURCE =
        'void main() {\n' +
        '   gl_FragColor = vec4(1.0,0.0,0.0,1.0);\n' +
        '}\n';

    var canvas = document.getElementById("canvas");
    var gl = canvas.getContext('webgl');

    if (!initShaders(gl, VERTEX_SHADER_SOURCE, FRAGMENT_SHADER_SOURCE)) {
        alert('Failed to init shaders');
    }


    var N = 100;
    var vertexData = [0.0, 0.0];
    var r = 0.5;

    for (var i = 0; i <= N; i++) {
        var theta = i * 2 * Math.PI / N;
        var x = r * Math.sin(theta);
        var y = r * Math.cos(theta);
        vertexData.push(x, y);
    }

    var vertices = new Float32Array(vertexData);
    initVertexBuffers(gl, vertices);

    gl.clearColor(0.0, 0.0, 0.0, 1.0);
    gl.clear(gl.COLOR_BUFFER_BIT);
    gl.drawArrays(gl.TRIANGLE_FAN, 0, vertices.length / 2);

    function initVertexBuffers(gl, vertices) {
        var vertexBuffer = gl.createBuffer();
        if (!vertexBuffer) {
            console.log('Failed to create buffer object');
            return -1;
        }

        gl.bindBuffer(gl.ARRAY_BUFFER, vertexBuffer);
        gl.bufferData(gl.ARRAY_BUFFER, vertices, gl.STATIC_DRAW);

        var a_Position = gl.getAttribLocation(gl.program, 'a_Position');

        gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, 0, 0);
        gl.enableVertexAttribArray(a_Position);
    }

    function initShaders(gl, vertexShaderSource, fragmentShaderSource) {
        var program = createProgram(gl, vertexShaderSource, fragmentShaderSource);
        if (!program) {
            console.log('Failed to create program');
            return false;
        }

        gl.useProgram(program);
        gl.program = program;

        return true;
    }

    function loadShader(gl, type, source) {
        // create shader object
        var shader = gl.createShader(type);
        if (shader == null) {
            console.log('unable to create shader');
            return null;
        }

        // set shader source code
        gl.shaderSource(shader, source);

        // compile the shader
        gl.compileShader(shader);

        // check compile status
        var compiled = gl.getShaderParameter(shader, gl.COMPILE_STATUS);
        if (!compiled) {
            var error = gl.getShaderInfoLog(shader);
            console.log('Failed to compile shader: ' + error);
            return null;
        }

        return shader;
    }

    function createProgram(gl, vertexShaderSource, fragmentShaderSource) {
        var vertexShader = loadShader(gl, gl.VERTEX_SHADER, vertexShaderSource);
        var fragmentShader = loadShader(gl, gl.FRAGMENT_SHADER, fragmentShaderSource);
        if (!vertexShader || !fragmentShader) {
            return null;
        }

        // create a program object
        var program = gl.createProgram();
        if (!program) {
            console.log('gl.createProgram failed');
            return null;
        }

        // attach  the shader objects
        gl.attachShader(program, vertexShader);
        gl.attachShader(program, fragmentShader);

        // link the program object
        gl.linkProgram(program);

        // check link status
        var linked = gl.getProgramParameter(program, gl.LINK_STATUS);
        if (!linked) {
            var error = gl.getProgramInfoLog(program);
            console.log('Failed to link program: ' + error);
            gl.deleteProgram(program);
            gl.deleteShader(vertexShader);
            gl.deleteShader(fragmentShader);
            return null;
        }

        return program;
    }

</script>
</body>
</html>