https://github.com/gre/webgltexture-loader

createDemoProgram.js

function createDemoProgram (gl) {
  gl.viewport(0, 0, gl.drawingBufferWidth, gl.drawingBufferHeight);
  const buffer = gl.createBuffer();
  gl.bindBuffer(gl.ARRAY_BUFFER, buffer);
  gl.bufferData(
    gl.ARRAY_BUFFER,
    new Float32Array([-1, -1, -1, 4, 4, -1]),
    gl.STATIC_DRAW
  );
  const vertexShader = gl.createShader(gl.VERTEX_SHADER);
  gl.shaderSource(
    vertexShader,
    `\
attribute vec2 p;
varying vec2 uv;
void main() {
  gl_Position = vec4(p,0.0,1.0);
  uv = 0.5 * (p+1.0);
}`
  );
  gl.compileShader(vertexShader);
  const fragmentShader = gl.createShader(gl.FRAGMENT_SHADER);
  gl.shaderSource(
    fragmentShader,
    `\
precision highp float;
varying vec2 uv;
uniform sampler2D t;
void main() {
  gl_FragColor = mix(texture2D(t, uv),  vec4(1.0), min(pow(2.*uv.x-1.,6.)+pow(2.*uv.y-1.,6.)+ 0.1 * step(fract(20.*uv.y+cos(16.*uv.x)), 0.2), 1.0));
}`
  );
  gl.compileShader(fragmentShader);
  var program = gl.createProgram();
  gl.attachShader(program, vertexShader);
  gl.attachShader(program, fragmentShader);
  gl.linkProgram(program);
  gl.useProgram(program);
  var p = gl.getAttribLocation(program, "p");
  gl.enableVertexAttribArray(p);
  gl.vertexAttribPointer(p, 2, gl.FLOAT, false, 0, 0);
  return program;
}