WebGL 1.0. Drawing a closed polygonal chain using gl.LINE_LOOP

by Ivan Enzhaev

HTML

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

CSS

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

JavaScript

var vertexShaderSource =
    `
    attribute vec2 a_Position;

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

var fragmentShaderSource =
    `
    precision mediump float;

    uniform vec3 u_Color;

    void main()
    {
        gl_FragColor = vec4(u_Color, 1.0);
    }`;

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

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([
    -0.7, -0.4, -0.8, 0.3, 0.0, 0.9, 0.6, 0.5,
    0.7, 0.0, 0.2, -0.7, -0.3, -0.8
]);

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

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

var u_Color = gl.getUniformLocation(program, "u_Color");
gl.uniform3f(u_Color, 0.407, 0.768, 0.270);

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

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