WebGL 2.0. Drawing a polygon using gl.TRIANGLE_FAN

by Ivan Enzhaev

HTML

<canvas id="renderCanvas" width="250" height="250"></canvas>

CSS

#renderCanvas {
    border: 5px solid #8f84e9;
}

JavaScript

var vertexShaderSource =
    `#version 300 es

    in vec2 a_Position;
    in vec3 a_Color;
    out vec3 v_Color;

    void main()
    {
        gl_Position = vec4(a_Position, 0.0, 1.0);
        v_Color = a_Color;
    }`;

var fragmentShaderSource =
    `#version 300 es

    precision mediump float;

    in vec3 v_Color;
    out vec4 fragColor;

    void main()
    {
        fragColor = vec4(v_Color, 1.0);
    }`;

var gl = document.getElementById("renderCanvas").getContext("webgl2");

var vShader = gl.createShader(gl.VERTEX_SHADER);
gl.shaderSource(vShader, vertexShaderSource);
gl.compileShader(vShader);

var fShader = gl.createShader(gl.FRAGMENT_SHADER);
gl.shaderSource(fShader, fragmentShaderSource);
gl.compileShader(fShader);

var program = gl.createProgram();
gl.attachShader(program, vShader);
gl.attachShader(program, fShader);
gl.linkProgram(program);
gl.useProgram(program);

var vertices = new Float32Array([
    -0.6, 0.3, 1.0, 0.0, 0.0,   // v0 (x, y), (r, g, b)
    -0.4, -0.5, 0.0, 1.0, 0.0,  // v1
    0.0, -0.7, 1.0, 0.0, 0.0,   // v2
    0.4, -0.5, 0.0, 0.0, 1.0,   // v3
    0.6, 0.3, 1.0, 0.0, 0.0,    // v4
    0.2, 0.7, 0.0, 1.0, 0.0,    // v5
    -0.2, 0.7, 0.0, 0.0, 1.0,   // v6
]);

var vbo = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, vbo);
gl.bufferData(gl.ARRAY_BUFFER, vertices, gl.STATIC_DRAW);

var FSIZE = vertices.BYTES_PER_ELEMENT;

var a_Position = gl.getAttribLocation(program, "a_Position");
gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, 5 * FSIZE, 0);
gl.enableVertexAttribArray(a_Position);

var a_Color = gl.getAttribLocation(program, "a_Color");
gl.vertexAttribPointer(a_Color, 3, gl.FLOAT, false, 5 * FSIZE, 2 * FSIZE);
gl.enableVertexAttribArray(a_Color);

gl.clearColor(0.956, 0.968, 0.890, 1.0);
gl.clear(gl.COLOR_BUFFER_BIT);

gl.drawArrays(gl.TRIANGLE_FAN, 0, 7);