Way to detect if WebGL viewport is on screen?

Question

Is there any way to be able to query the GPU to tell me if my viewport in my webpage is currently on screen or not? For example, if I had a 3d scene rendering in a canvas in an iframe, is there a way to query the hardware (within my iframe and only the pixels or verts in the viewport) to say if I am on screen or scrolled off screen?

I'm curious as to whether this is something I can do at the vertex shader level. Does WebGL even perform the shader program on a viewport that is offscreen? Lets say if it is scrolled below the canvas, or the viewport is obstructed by another webpage browser window? Is there a way to query the compositing portion of webgl to see if it is even in view or Iterate through the "RenderObject" Tree to test if it is even onscreen and then return this value? I am trying to get much more performance out of a project I am working on and I am trying to only render what is visible on screen.

Any possible ideas? Is this even possible? Thanks!

Answer 1

RequestAnimationFrame is only reasonable way to handle unnecessary performance loss even semantically because window.requestAnimationFrame tells the browser that you wish to perform an animation... So browser will figure out how it should handle your wish in optimal way taking into account current page state. But since iframes communicate using local storage you can push to them your base page state so each of them will decide should it RequestAnimationFrame or not. But im not shure that it is a good thing to have multiply render contexts on your page, they all eat resources and can't share them (data that stored in GPU is sandboxed) so eventually they will start to push each other from GPU memory and cause lags + GPU pipeline might be not so happy with all those tiny standalone entities. Fragmentation is main GPU performance enemy.

Answer 2

You don't ask this question at the canvas/WebGL level, because it might, for example, be scrolled back on screen before you draw another frame, and browsers don't want to not have content to show, so there's no provision to not draw.

I believe you will have to consult the DOM geometry properties (e.g. .scrollLeft) of your scrollable areas to determine whether the canvas is visible. There is enough information in said properties that you can do this generically without hardcoding knowledge of your page structure.

Also, make sure you are exclusively using requestAnimationFrame for your drawing/simulation scheduling; it will pause animations if the page is hidden/minimized/in another tab/otherwise explicitly invisible.