Mar 16 2014

Fullscreen canvas, not as easy as it appears

Category: Software,WebdevelopmentSpiller @ 14:25

This is a rant about the erroneous information you will get when you try to find out how to implement something, you have been warned. See the end for the solution that was good enough for me.

I have been working on a WebGL based LDraw viewer, so I can embed 3D views of my Lego models on this blog (Project at Github: WebLDraw). Obviously I want to droll at them in glorious fullscreen, and with the HTML5 fullscren API it should be fairly simple. You just have to call ‘requestFullscreen()‘ on you element.

Well almost. In Firefox it applies 100% width and hight automatically, but not in Chrome, resulting in a small canvas on a large black background filling your entire screen. Awesome, uses your screen estate  as efficiently as Metro apps in Windows 8!

Applying a bit of CSS to do it manually should be fairly easy, but that just doesn’t work for a canvas, as it is rendered at a specific resolution, and the CSS style just upscales it. Googling for ‘canvas resize fullscreen’ will give you something like this:

function on_fullscreen_change() {
   canvas.width = window.innerWidth
   canvs.height = window.innerHeight
document.addEventListener( 'fullscreenchange', on_fullscreen_change() );

Great, now my canvas is bigger as soon as I enter fullscreen. And not only that, it is just as big as soon I exit fullscreen, because NO, do not exit the glorious fullscreen environment, it is perfect. So a bit more googling and I ended up using ‘document.fullscreenEnabled‘, which I quickly found out just told me if fullscreen was supported. The correct way was to do:

if( canvas == document.fullscreenElement )

Now I can finally enter and exit fullscreen properly. Except that my canvas did not have the correct size, ‘window.innerWidth’ does not give the correct width. But google have all the answers, you just need to do any of:

  • document.width
  • document.body.clientWidth
  • canvas.offsetWidth
  • canvas.getBoundingClientRect().width
  • = window.innerWidth + “px” (CSS style, fancy)
  • screen.availWidth

None of them gives the correct result however. People don’t seem to notice because they stretch it with ‘width: 100%’ anyway. ‘screen.availWidth‘ exited me though, it actually returned 1920, the width of my screen. Except that ‘screen.availHeight‘ returned 1160 because YES, I do have a taskbar in my desktop environment, and NO, I don’t want to know it is 40px high when it is hidden anyway because I’m in fullscreen mode…

I really wonder why we need to differentiate between ‘screen.availWidth’ and ‘screen.width‘ which gives the screen’s full width in web development. Anyway, that was the final piece in the puzzle and my Dart implementation ended up looking like this: (Most of it maps pretty closely to a JavaScript implementation.)

original_width = canvas.width;
original_height = canvas.height;
canvas.onFullscreenChange.listen( (t){
  if( canvas == document.fullscreenElement ){
    canvas.width = window.screen.width;
    canvas.height = window.screen.height;
    canvas.width = original_width;
    canvas.height = original_height;
} );

Some of the confusion about the fullscreen API is hard to avoid, because many articles only applies to the old proprietary APIs. But 7 ways to get the monitor width, none of them which are correct? I just can’t even guess how it could end up that bad…


Leave a Reply