Re: DirectX and Lua
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- Subject: Re: DirectX and Lua
- From: Dave Dodge <dododge@...>
- Date: 2006年3月14日 04:25:59 -0500
On Mon, Mar 13, 2006 at 11:32:59AM -0800, Mark Manyen wrote:
> Does anyone have some ideas on how to make the floating point
> problems between DX and Lua go away that is more constructive than
> use OpenGL?
Is it perhaps the known problem where Direct3D quietly messes with the
floating point unit in ways that affect other code:
http://www.virtualdub.org/blog/pivot/entry.php?id=53
Initializing Direct3D with default settings causes the precision
bits in the floating-point control word to be reset such that FPU
calculations always occur with 24-bit precision (single precision
/ float). This is much more serious as it causes roundoff errors
to become much larger, and it means that double-precision math can
no longer represent all valid values of a 32-bit int. For this
reason, if you invoke Direct3D within an application that may not
be expecting it, such as within a video filter, you should set the
D3DCREATE_FPU_PRESERVE flag when creating the device.
Microsoft apparently has their reasons for doing this in Direct3D:
http://discuss.microsoft.com/SCRIPTS/WA-MSD.EXE?A2=ind0504b&L=directxdev&D=1&P=4524&F=P
The D3DCREATE_FPU_PRESERVE flag exists for those applications who
want to set their own FP control word and/or stay with
double-precision. You still must be very careful when setting it
to non-standard rounding or enabling exceptions. For games, you
typically don't want the performance hit of having the FP unit
working in double-precision.
[...]
Using this flag with the FPU in double-precision mode will reduce
Direct3D performance.
-Dave Dodge