Holy FPU precision, Batman!
Wait a moment… Unity plugin makes nice wobbling web page elements not wobble anymore!? Sounds like an interesting issue…
So I prepared for a debug session and tried the usual “divide by two until you locate the problem” approach.
- Unity Web Player is composed of two parts: a small browser plugin, and the actual “engine” (let’s call it “runtime”). First I change the plugin so that it only loads the data, but never loads or starts the runtime. Everything works. So the problem is not in the plugin. Good.
- Load the runtime and do basic initialization (create child window, load Mono, …), but never actually start playing the content – everything works.
- Load the runtime and fully initialize everything, but never actually start playing the content – the bug appears! By now I know that the problem is somewhere in the initialization.
Initialization reads some settings from the data file, creates some “manager objects” for the runtime, initializes graphics device, loads first game “level” and then the game can play.
So I continued:
- Try using OpenGL instead of Direct3D – everything works. By now it’s confirmed that initializing Direct3D causes something else in the browser not work.
- “A-ha!” moment: tell Direct3D to not change floating point precision (via a create flag). Voilà, everything works!
I don’t know how I actually came up with the idea of testing floating point precision flag. Maybe I remembered some related problems we had a while ago, where Direct3D would cause timing calculations be “off”, if the user’s machine was not rebooted for a couple of weeks or more. That time around we properly changed our timing code to use 64 bit integers, but left Direct3D precision setting intact.
Side note: Intel x86 floating point unit (FPU) can operate in various precision modes, usually 32, 64 or 80 bit. By default Direct3D 9 sets FPU precision to 32 bit (i.e. single precision). Telling D3D to not change FPU settings could lower performance somewhat, but in my tests it did not have any noticeable impact.
The moral? Something in one place can affect seemingly completely unrelated things in another place!