Direct3D and the FPU..

re: Direct3D and the FPU..

Brian Brown — Tue, 01 Jun 2004 18:19:00 GMT

What are the performance and quality ramifications of using FpuPreserve when creating a D3D device?

re: Direct3D and the FPU..

Tom Miller — Tue, 01 Jun 2004 18:36:00 GMT

Well naturaly since you're using double precision rather than single precision, there will be a performance hit (memory usage, etc)..

Not sure what you mean by quality though..

re: Direct3D and the FPU..

Andy — Tue, 01 Jun 2004 21:44:00 GMT

I'd noticed that all the DirectX SDK works with singles and there is no single datatype in the CLR. My assumption is therefore that in the interop layer you have to convert every CLR double into a single before calling the native code.

So when the FPU is in single precision mode does that mean you can pass the doubles directly through to the native functions or do you still have to do conversion on all of them.

Does this change when you are in double precision mode?

re: Direct3D and the FPU..

Tom Miller — Tue, 01 Jun 2004 22:07:00 GMT

'float' is the equivalent of a single in C# (System.Single for the CLS 'class' name)..

You can cast any double to float (or System.Single) before passing them to the MDX runtime.

re: Direct3D and the FPU..

Andy — Tue, 01 Jun 2004 23:55:00 GMT

Well slap me with a big stick... Sometimes you even forget the obvious stuff. I had to go back and look at why I thought there was no single type and its becuase all of the system.math stuff only accepts doubles so I got into the habit of never using float becuase I was fed up doing all the casts when I want to use anything from system.math

So now my question is almost unrelated to your original subject but I will ask anyway. Given that we know the FPU is in single precision mode is there anyway the CLR can 'know' this and stop me having to cast everything to/from double just to use the system.math library?

Or will system.math ever get overloads for single?

Also discussed here http://www.gamedev.net/community/forums/topic.asp?topic_id=131054

re: Direct3D and the FPU..

mfbaranow — Wed, 02 Jun 2004 13:48:00 GMT

On a similiar topic, how does MDX runtime switch the FPU to single precision? Whats the method if you wanted to do this yourself in .NET?

My research so far just turned up native FPU intrinsics to do this. I would like to profile some of my FPU intensive .NET apps in double and single precision. I also assume that using single precision floats explicitly will achieve similiar results.

re: Direct3D and the FPU..

Brandon Bloom [Beta User ID: 465605] — Fri, 04 Jun 2004 03:33:00 GMT

This should definitly be mentioned in the remarks section of the device constructors documentation :-)

re: Direct3D and the FPU..

Adam Hill — Thu, 10 Jun 2004 16:50:00 GMT

Isn't messing with the FPU of the machine serious side-effect-no-no juju? What if other processes started doing this themselves?

re: Direct3D and the FPU..

Sean — Mon, 02 Aug 2004 15:55:00 GMT

This has had very negative effects on our application since our application was relying on double precision math. Shouldn't the default for a new device be FpuPreserve!?!? Who knows what calculations you might be affecting on the system , especially other processes, without it.

Accessing floating-point context from .NET applications

Jeffrey's Blog — Thu, 09 Feb 2006 00:38:51 GMT

Tom Miller's Blog : Direct3D and the FPU..

Weddings — Thu, 05 Jun 2008 11:38:37 GMT

I had an email this morning about Managed Direct3D 'breaking' the math functions in the CLR. The person who wrote discovered that this method: public void AssertMath() { double dMin = 0.54797677334988781; double dMax = 4.61816551621179; double dScale

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Tom Miller s Blog Direct3D and the FPU | bar stools

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