WebGL 2.0. Drawing a triangle using gl.LINE_LOOP

by Ivan Enzhaev

HTML

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

CSS

#renderCanvas {
    border: 1px solid #aaaafa
}

JavaScript

var vertexShaderSource =
    `#version 300 es
    
    in vec2 a_Position;
    
    void main()
    {
        gl_Position = vec4(a_Position, 0.0, 1.0);
    }`;

var fragmentShaderSource =
    `#version 300 es

    precision mediump float;

    out vec4 outColor;
    
    void main()
    {
        outColor = vec4(1.0, 0.0, 0.0, 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 points = [
    0.0, 0.7,   // First point
    -0.7, -0.7, // Second point
    0.7, -0.7   // Third point
];

var vbo = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, vbo);
gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(points), 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);

gl.drawArrays(gl.LINE_LOOP, 0, points.length / 2);