ch03/RotatedTriangle

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">
        // x' = x cosβ - y sinβ
        // y' = x sinβ + y cosβ　Equation 3.3
        // z' = z

        attribute vec4 a_Position;
        uniform float u_CosB, u_SinB;

        void main()
        {
            gl_Position.x = a_Position.x * u_CosB - a_Position.y * u_SinB;
            gl_Position.y = a_Position.x * u_SinB + a_Position.y * u_CosB;
            gl_Position.z = a_Position.z;
            gl_Position.w = 1.0;
        }
    </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

// The rotation angle
var ANGLE = 90.0;

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 get the vertex shader element");
        return;
    }
    if (fShaderElement == null)
    {
        console.log("Failed to get 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;
    }

    // Pass the data required to rotate the shape to the vertex shader
    var radian = Math.PI * ANGLE / 180.0; // Convert to radians
    var cosB = Math.cos(radian);
    var sinB = Math.sin(radian);

    var u_CosB = gl.getUniformLocation(gl.program, 'u_CosB');
    var u_SinB = gl.getUniformLocation(gl.program, 'u_SinB');
    if (!u_CosB || !u_SinB)
    {
        console.log('Failed to get the storage location of u_CosB or u_SinB');
        return;
    }
    gl.uniform1f(u_CosB, cosB);
    gl.uniform1f(u_SinB, sinB);

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

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

    // Draw the rectangle
   ...