I used this method http://blogs.msdn.com/b/shawnhar/archive/2011/01/12/spritebatch-billboards-in-a-3d-world.aspx to create a 3D billboard renderer using SpriteBatch. It works perfectly as described and on a modest desktop (with Intel HD graphics) can renderer 10,000s of billboards or particles easily.
On a mobile device (Windows Phone) the framerate drops sharply past a certain, not so large, point. My test (on all devices) is this:
- Render a primitives (sphere, cube, etc) into a render target
- Pass the render target to the method above
- Start increasing the number of billboards until the framerate drops.
On an x86 desktop or an ARM tablet (Surface) the framerate holds into the thousands. On the phone, it instantly drops from 60 to 30 (looks like disabling VSync has no effect on that device) as soon as you pass a certain point (~200?). The funny thing is that I can get the framerate to go up to 60 again by making the billboards half the size. Same goes when making the render target half the size.
Using a stopwatch, I determined that the time spent on the CPU is nowhere near the 16.67ms threshold. VS2013's frame analysis is unavailable on the Windows Phone, so that's useless.
Can anyone explain as to what is going on here? Is this simply the limitation of a low-power GPU (the Adreno 225 in this case)? If so, what exactly is bogging it down? The fill rate? The blending? (I tried all blend states from Opaque to NonPremultiplied, no effect on performance).
Edited by OpaqueEncounter, 25 May 2014 - 11:28 AM.