Canvas size and WebGL

Effect of canvas size on rendering with WebGL

With scissor() and clear() we can demonstrate how the WebGL drawing buffer is affected by the size of the canvas.

The size of the first canvas is set to the styled Element size, determined by CSS. This is done by assigning the width and height properties of the canvas to the values of the clientWidth and clientHeight properties, respectively.

In contrast, no such assignment is done for the second canvas. The internal width and height properties of the canvas remain at default values, which are different than the actual size of the canvas Element in the browser window.

The effect is clearly visible when using scissor() and clear() to draw a square in the center of the canvas, by specifying its position and size in pixels. In the first canvas, we get the desired result. In the second, the square has the wrong shape, size, and position.

<p>Compare the two canvases.</p>
<canvas>Your browser does not seem to support 
    HTML5 canvas.</canvas>
<canvas>Your browser does not seem to support 
    HTML5 canvas.</canvas>

body {
  text-align : center;
}
canvas {
  display : inline-block;
  width : 120px;
  height : 80px;
  margin : auto;
  padding : 0;
  border : none;
  background-color : black;
}

window.addEventListener("load", function() {
  "use strict"
  var firstCanvas = document.getElementsByTagName("canvas")[0],
    secondCanvas = document.getElementsByTagName("canvas")[1];
  firstCanvas.width = firstCanvas.clientWidth;
  firstCanvas.height = firstCanvas.clientHeight;
  [firstCanvas, secondCanvas].forEach(function(canvas) {
    var gl = canvas.getContext("webgl")
      || canvas.getContext("experimental-webgl");
    if (!gl) {
      document.querySelector("p").innerHTML =
        "Failed to get WebGL context. "
        + "Your browser or device may not support WebGL.";
      return;
    }
    gl.viewport(0, 0,
      gl.drawingBufferWidth, gl.drawingBufferHeight);
    gl.enable(gl.SCISSOR_TEST);
    gl.scissor(30, 10, 60, 60);
    gl.clearColor(1.0, 1.0, 0.0, 1.0);
    gl.clear(gl.COLOR_BUFFER_BIT);
  });
}, false);

The source code of this example is also available on GitHub.