script.js

var canvas = document.getElementById("canvas");

canvas.width = window.innerWidth;
canvas.height = window.innerHeight;

// Initialize the GL context
var gl = canvas.getContext('webgl');
if(!gl){
  console.error("Unable to initialize WebGL.");
}

//Time
var time = 0.0;

//************** Shader sources **************

var vertexSource = `
attribute vec2 position;
void main() {
  gl_Position = vec4(position, 0.0, 1.0);
}
`;

var fragmentSource = `
precision highp float;

#define AA

uniform float width;
uniform float height;
vec2 resolution = vec2(width, height);

uniform float time;

void main(){

	float strength = 0.4;
	float t = time/6.0;

	vec3 col = vec3(0);
	vec2 fC = gl_FragCoord.xy;

	#ifdef AA
	for(int i = -1; i <= 1; i++) {
		for(int j = -1; j <= 1; j++) {

			fC = gl_FragCoord.xy+vec2(i,j)/3.0;

			#endif

			//Normalized pixel coordinates (from 0 to 1)
			vec2 pos = fC/resolution.xy;

			pos.y /= resolution.x/resolution.y;
			pos = 4.0*(vec2(0.5) - pos);

			for(float k = 1.0; k < 7.0; k+=1.0){ 
				pos.x += strength * sin(2.0*t+k*1.5 * pos.y)+t*0.5;
				pos.y += strength * cos(2.0*t+k*1.5 * pos.x);
			}

			//Time varying pixel colour
			col += 0.5 + 0.5*cos(time+pos.xyx+vec3(0,2,4));

			#ifdef AA
		}
	}

	col /= 9.0;
	#endif

  //Gamma
  col = pow(col, vec3(0.4545));

	//Fragment colour
	gl_FragColor = vec4(col,1.0);
}
`;

//************** Utility functions **************

window.addEventListener( 'resize', onWindowResize, false );

function onWindowResize(){
  canvas.width  = window.innerWidth;
  canvas.height = window.innerHeight;
	gl.viewport(0, 0, canvas.width, canvas.height);
  gl.uniform1f(widthHandle, window.innerWidth);
  gl.uniform1f(heightHandle, window.innerHeight);
}


//Compile shader and combine with source
function compileShader(shaderSource, shaderType){
  var shader = gl.createShader(shaderType);
  gl.shaderSource(shader, shaderSource);
  gl.compileShader(shader);
  if(!gl.getShaderParameter(shader, gl.COMPILE_STATUS)){
  	throw "Shader compile failed with: " + gl.getShaderInfoLog(shader);
  }
  return shader;
}

//From https://codepen.io/jlfwong/pen/GqmroZ
//Utility to complain loudly if we fail to find the attribute/uniform
function getAttribLocation(program, name) {
  var attributeLocation = gl.getAttribLocation(program, name);
  if (attributeLocation === -1) {
  	throw 'Cannot find attribute ' + name + '.';
  }
  return attributeLocation;
}

function getUniformLocation(program, name) {
  var attributeLocation = gl.getUniformLocation(program, name);
  if (attributeLocation === -1) {
  	throw 'Cannot find uniform ' + name + '.';
  }
  return attributeLocation;
}

//************** Create shaders **************

//Create vertex and fragment shaders
var vertexShader = compileShader(vertexSource, gl.VERTEX_SHADER);
var fragmentShader = compileShader(fragmentSource, gl.FRAGMENT_SHADER);

//Create shader programs
var program = gl.createProgram();
gl.attachShader(program, vertexShader);
gl.attachShader(program, fragmentShader);
gl.linkProgram(program);

gl.useProgram(program);

//Set up rectangle covering entire canvas 
var vertexData = new Float32Array([
  -1.0,  1.0, 	// top left
  -1.0, -1.0, 	// bottom left
   1.0,  1.0, 	// top right
   1.0, -1.0, 	// bottom right
]);

//Create vertex buffer
var vertexDataBuffer = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, vertexDataBuffer);
gl.bufferData(gl.ARRAY_BUFFER, vertexData, gl.STATIC_DRAW);

// Layout of our data in the vertex buffer
var positionHandle = getAttribLocation(program, 'position');

gl.enableVertexAttribArray(positionHandle);
gl.vertexAttribPointer(positionHandle,
  2, 				// position is a vec2 (2 values per component)
  gl.FLOAT, // each component is a float
  false, 		// don't normalize values
  2 * 4, 		// two 4 byte float components per vertex (32 bit float is 4 bytes)
  0 				// how many bytes inside the buffer to start from
  );

//Set uniform handle
var timeHandle = getUniformLocation(program, 'time');
var widthHandle = getUniformLocation(program, 'width');
var heightHandle = getUniformLocation(program, 'height');

gl.uniform1f(widthHandle, window.innerWidth);
gl.uniform1f(heightHandle, window.innerHeight);

var lastFrame = Date.now();
var thisFrame;

function draw(){
	
  //Update time
	thisFrame = Date.now();
  time += (thisFrame - lastFrame)/770;	
	lastFrame = thisFrame;

	//Send uniforms to program
  gl.uniform1f(timeHandle, time);
  //Draw a triangle strip connecting vertices 0-4
  gl.drawArrays(gl.TRIANGLE_STRIP, 0, 4);

  requestAnimationFrame(draw);
}

draw();