chromium/content/test/data/gpu/pixel_webgl_copy_image.html

<!DOCTYPE HTML>
<html>
<head>
<meta name="viewport" content="initial-scale=1">
<title>WebGL Canvas Save/Copy Image Test</title>
<style type="text/css">
.nomargin {
  margin: 0px auto;
}
</style>
<script>
let g_swapsBeforeAck = 15;

function waitForFinish()
{
  if (g_swapsBeforeAck == 0) {
    sendResult("SUCCESS");
  } else {
    g_swapsBeforeAck--;
    window.requestAnimationFrame(waitForFinish);
  }
}

function sendResult(status) {
  if (domAutomationController) {
    domAutomationController.send(status);
  } else {
    console.log(status);
  }
}

function main()
{
  let canvas = document.getElementById("c");
  let gl = canvas.getContext("webgl");
  if (!gl) {
    sendResult("FAILURE", "WebGL context not supported");
    return;
  }

  // Diagonally opposite green and blue box, over red box to test correctness
  // w.r.t. to both vertical and horizontal flips.
  gl.scissor(0, 0, 100, 100);
  gl.enable(gl.SCISSOR_TEST);
  gl.clearColor(1, 0, 0, 1);
  gl.clear(gl.COLOR_BUFFER_BIT);

  gl.scissor(0, 50, 50, 50);
  gl.clearColor(0, 1, 0, 1);
  gl.clear(gl.COLOR_BUFFER_BIT);

  gl.scissor(50, 0, 50, 50);
  gl.clearColor(0, 0, 1, 1);
  gl.clear(gl.COLOR_BUFFER_BIT);

  let dataURL = canvas.toDataURL();

  let image = document.getElementById("i");
  image.src = dataURL;

  waitForFinish();
}
</script>
</head>
<body onload="main()" class="nomargin" style="display:inline-flex">
<canvas id="c" width="100" height="100" class="nomargin" style="background-color:black"></canvas>
<img id="i" width="100" height="100" class="nomargin" style="background-color:black">
</body>
</html>