A function to scale an HTML canvas element to the maximum browser window
JavaScript
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README.md

Scale and align an HTML element in the browser

Use the function scaleToWindow to scale an HTML element to the maximum size of the browser's window. scaleToWindow will also align the element for the best vertical or horizontal fit inside the browser window. For example, if you have an element that's wider than it is tall, it will be centered vertically inside the browser. If the element is taller than it is wide, it will be centered horizontally.

Alignment

Here's how to use scaleToWindow:

scaleToWindow(anyElement, borderColor);

(If you are using Pixi, supply the renderer.view as the element.) The optional second argument lets you set the color of the browser's background that borders the element. You can supply any RGB, HSLA or Hexadecimal color value, as well as the any HTML color string, like “blue” or “red”. (If you don't supply this optional color, the border will be set to a neutral dark gray: #2C3539.)

The scaleToWindow function also returns the scale value that the element is scaled to. You can find it like this:

var scale = scaleToWindow(renderer.view);

This will give you a number, like 1.98046875, which tells you the ratio by which the element was scaled. This might be an important value to know if you need to convert browser pixel coordinates to the scaled pixel values of the element. For example, if you have a pointer object which tracks the mouse's position in the browser, you might need to convert those pixel positions to the scaled element coordinates to find out if the mouse is touching something in the element. Some general code like this will do the trick:

pointer.x = pointer.x / scale;
pointer.y = pointer.y / scale;

Optionally, you might also want the element to rescale itself every time the size of the browser window is changed. If that’s the case, call scaleToWindow inside a window event listener:

window.addEventListener("resize", function(event){ 
  scaleToWindow(anyelementElement);
});

For the best effect, make sure that you set the browser's default margins and padding to 0 on all HTML elements. If you don't do this, most browsers will add some padding around the element borders. This bit of CSS will do the trick:

<style>* {padding: 0; margin: 0}</style>

If you prefer, you can add this CSS style using JavaScript in your main program like this:

var newStyle = document.createElement("style");
var style = "* {padding: 0; margin: 0}";
newStyle.appendChild(document.createTextNode(style));
document.head.appendChild(newStyle);