Hello WebGL III

Part three of my introduction to WebGL, wherein the fragment shader computes the color based on the fragment coordinates.

Next: Part IV

<!DOCTYPE html>
<meta charset="utf-8">
<canvas width="960" height="500"></canvas>
<script id="vertex-shader" type="x-shader/x-vertex">

attribute vec2 a_position;

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

</script>
<script id="fragment-shader" type="x-shader/x-fragment">

precision mediump float;

uniform vec2 u_size;

void main(void) {
  gl_FragColor = vec4(gl_FragCoord.x / u_size.x, gl_FragCoord.y / u_size.y, 0.0, 1.0);
}

</script>
<script>

// Select the canvas from the document.
var canvas = document.querySelector("canvas");

// Create the WebGL context, with fallback for experimental support.
var context = canvas.getContext("webgl")
    || canvas.getContext("experimental-webgl");

// Compile the vertex shader.
var vertexShader = context.createShader(context.VERTEX_SHADER);
context.shaderSource(vertexShader, document.querySelector("#vertex-shader").textContent);
context.compileShader(vertexShader);
if (!context.getShaderParameter(vertexShader, context.COMPILE_STATUS)) throw new Error(context.getShaderInfoLog(vertexShader));

// Compile the fragment shader.
var fragmentShader = context.createShader(context.FRAGMENT_SHADER);
context.shaderSource(fragmentShader, document.querySelector("#fragment-shader").textContent);
context.compileShader(fragmentShader);
if (!context.getShaderParameter(fragmentShader, context.COMPILE_STATUS)) throw new Error(context.getShaderInfoLog(fragmentShader));

// Link and use the program.
var program = context.createProgram();
context.attachShader(program, vertexShader);
context.attachShader(program, fragmentShader);
context.linkProgram(program);
if (!context.getProgramParameter(program, context.LINK_STATUS)) throw new Error(context.getProgramInfoLog(program));
context.useProgram(program);

// Define the size.
var sizeUniform = context.getUniformLocation(program, "u_size");
context.uniform2fv(sizeUniform, [+canvas.getAttribute("width"), +canvas.getAttribute("height")]);

// Define the positions (as vec2) of the square that covers the canvas.
var positionBuffer = context.createBuffer();
context.bindBuffer(context.ARRAY_BUFFER, positionBuffer);
context.bufferData(context.ARRAY_BUFFER, new Float32Array([
    -1.0, -1.0,
    +1.0, -1.0,
    +1.0, +1.0,
    -1.0, +1.0
  ]), context.STATIC_DRAW);

// Bind the position buffer to the position attribute.
var positionAttribute = context.getAttribLocation(program, "a_position");
context.enableVertexAttribArray(positionAttribute);
context.vertexAttribPointer(positionAttribute, 2, context.FLOAT, false, 0, 0);

// Draw the square!
context.drawArrays(context.TRIANGLE_FAN, 0, 4);

</script>