ch03/HelloQuad_FAN

Example from the book "WebGL Programming Guide" by Matsuda & 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;

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

    <script type="x-shader/x-fragment" id="FragmentShader">
        void main()
        {
            gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);
        }
    </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>

JavaScript

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

function main()
{
    // Retrieve <canvas> element
    var canvas = document.getElementById('webgl');

    // Get the rendering context for WebGL
    var gl = getWebGLContext(canvas);
    if (!gl)
    {
        console.log('Failed to get the rendering context for WebGL');
        return;
    }

    // Get shader elements
    var vShaderElement = document.getElementById("VertexShader");
    var fShaderElement = document.getElementById("FragmentShader");
    if (vShaderElement == null)
    {
        console.log("Failed to the vertex shader element")
        return;
    }
    if (fShaderElement == null)
    {
        console.log("Failed to the fragment shader element")
        return;
    }

    // Get shader sources
    var vShaderSource = vShaderElement.firstChild.textContent;
    var fShaderSource = fShaderElement.firstChild.textContent;

    // Initialize shaders
    if (!initShaders(gl, vShaderSource, fShaderSource))
    {
        console.log('Failed to intialize shaders.');
        return;
    }

    // Write the positions of vertices to a vertex shader
    var n = initVertexBuffers(gl);
    if (n < 0)
    {
        console.log('Failed to set the positions of the vertices');
        return;
    }

    // Specify the color for clearing <canvas>
    gl.clearColor(0, 0, 0, 1);

    // Clear <canvas>
    gl.clear(gl.COLOR_BUFFER_BIT);

    // Draw the rectangle
    gl.drawArrays(gl.TRIANGLE_FAN, 0, n);
}

function initVertexBuffers(gl)
{
    var vertices = new Float32Array([
      -0.5, 0.5, -0.5, -0.5, 0.5, 0.5, 0.5, -0.5
    ]);
    var n = 4; // The number of vertices

    // Create a buffer object
    var vertexBuffer = gl.createBuffer();
    if (!vertexBuffer)
    {
        console.log('Failed to create the buffer object');
        return -1;
    }

    // Bind the buffer object to target
    gl.bindBuffer(gl.ARRAY_BUFFER, vertexBuffer);
    // Write date into the buffer object
    gl.bufferData(gl.ARRAY_BUFFER, vertices,...