appendix/CoordinateSystem

This is an example from the book "WebGL Programming Guide" by Kouichi Matsuda and Rodger Lea

by Ivan Enzhaev

HTML

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

    <script type="x-shader/x-vertex" id="VertexShader">
        attribute vec4 a_Position;
        attribute vec4 a_Color;
        varying vec4 v_Color;

        void main()
        {
            gl_Position = a_Position;
            v_Color = a_Color;
        }
    </script>

    <script type="x-shader/x-fragment" id="FragmentShader">
        precision mediump float;

        varying vec4 v_Color;

        void main()
        {
            gl_FragColor = v_Color;
        }
    </script>

    <script src="https://dl.dropboxusercontent.com/s/ylw7v7a44xtrwv0/webgl-utils.js"></script>
    <script src="https://dl.dropboxusercontent.com/s/iz1n21c8xafd76n/webgl-debug.js"></script>
    <script src="https://dl.dropboxusercontent.com/s/subm1yzylxuao8v/cuon-utils.js"></script>
    <script src="https://dl.dropboxusercontent.com/s/jw3p82lq1cjwekg/cuon-matrix.js"></script>

JavaScript

// CoordinateSystem.js (c) 2012 matsuda
// This is an example from the book "WebGL Programming Guide"
// by Kouichi Matsuda and Rodger Lea

main();
//window.onload = main;

function main()
{
    var canvas = document.getElementById('webgl');  // Retrieve <canvas> element
    var gl = getWebGLContext(canvas);   // Get the rendering context for WebGL
    var vShaderElement = document.getElementById("VertexShader");   // Get shader elements
    var fShaderElement = document.getElementById("FragmentShader");
    var vShaderSource = vShaderElement.firstChild.textContent;  // Get shader sources
    var fShaderSource = fShaderElement.firstChild.textContent;
    initShaders(gl, vShaderSource, fShaderSource);  // Initialize shaders
    var n = initVertexBuffers(gl);  // Set the vertex information

    gl.clearColor(0.0, 0.0, 0.0, 1.0);  // Specify the color for clearing <canvas>
    gl.clear(gl.COLOR_BUFFER_BIT);      // Clear <canvas>
    gl.drawArrays(gl.TRIANGLES, 0, n);  // Draw the triangles
}

function initVertexBuffers(gl)
{
    var pc = new Float32Array([ // Vertex coordinates and color
       0.0, 0.5, -0.1, 0.0, 0.0, 1.0,  // The front blue one 
      -0.5, -0.5, -0.1, 0.0, 0.0, 1.0,
       0.5, -0.5, -0.1, 1.0, 1.0, 0.0,

       0.5, 0.4, -0.5, 1.0, 1.0, 0.0,  // The red triangle is behind
      -0.5, 0.4, -0.5, 1.0, 0.0, 0.0,
       0.0, -0.6, -0.5, 1.0, 0.0, 0.0,
    ]);
    var numVertex = 3; var numColor = 3; var n = 6;

    // Create a buffer object and write data to it
    var pcbuffer = gl.createBuffer();
    gl.bindBuffer(gl.ARRAY_BUFFER, pcbuffer);
    gl.bufferData(gl.ARRAY_BUFFER, pc, gl.STATIC_DRAW);

    var FSIZE = pc.BYTES_PER_ELEMENT;   // The number of byte
    var STRIDE = numVertex + numColor;  // Stride

    // Assign the vertex coordinates to attribute variable and enable the assignment
    var a_Position = gl.getAttribLocation(gl.program, 'a_Position');
    gl.vertexAttribPointer(a_Position, numVertex, gl.FLOAT, false, FSIZE * STRIDE, 0);
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