Having problems with floating point numbers? Here's your fix

Miwa · Post by **Miwa** » Fri Feb 20, 2004 12:56 am

I'm integrating irrlicht into an existing data display app I have, so I'm using irrlicht to render to a window of my creating.

I ran across a problem yesterday, when opening the window that uses irrlicht, my other data plotting windows suddenly couldn't do math correctly. After a bunch of digging around, I decided that my floating point calcs were losing precision.

The last thing you expect is to add a number, like 10.5 to a number like 1.3 billion, and have the answer be wrong, when both values are doubles.

I figured out that the FPU was magically going from it's default 53-bit precision to 24-bit precision when I opened my irrlicht window.

The culprit is the pID3D->CreateDevice() call. By default, directx uses single precision calcs, and sets the FPU to 24-bit precision. This is faster than using double precision, but will kill you if you use doubles in your app and expect them to work.

To fix it, change the 4th param on all the CreateDevice calls from D3DCREATE_*_VERTEXPROCESSING to D3DCREATE_*_VERTEXPROCESSING|D3DCREATE_FPU_PRESERVE (where * is whatever the mode is for that line, MIXED, HARDWARE, or SOFTWARE)

This is a bummer tho... I don't need double precision in my directx stuff, just in the other parts of my app... I may end up having to wrap my drawing stuff with _control87() calls to manage the FPU state myself, so D3D can use single precision, and the rest of my app can continue to do double precision.

keless · Post by **keless** » Fri Feb 20, 2004 2:10 am

This may sound silly, but have you tried using IrrLicht in OGL mode?

Miwa · Post by **Miwa** » Fri Feb 20, 2004 6:30 am

Nope. And I could give a bunch of reasons why, but that would just rile up the cross-platform geeks.

I'm using irrlicht for my current demo because:

1) It doesn't require external libraries, it has all the source in it
2) It looks like it's primary development was with dx and on windows
3) The source shipped with real live VC project files, meaning that someone actually used a real dev environment on it, which means integrating it would be quicker

and MOST IMPORTANTLY it's not lgpl or gpl. I'm happy to share all the mods I make, and help where I can, but no way do I wish to be contractually obligated to do so.

I also very much like the fact that a single developer is currently controlling it's release and development plans. Open source projects tend to turn into design by committee, and get overrun by folks trying to make stuff run on linux.

If the focus shifts away from windows/dx, then I'd likely end up dropping it, but for now, I'm happy.

</rant>

keless · Post by **keless** » Fri Feb 20, 2004 6:36 am

I dont care about people being cross platform (I like to in my own projects, but thats just my thing). However, for you, it sounds like you just need something that works. I mean, since you're using IrrLicht, you dont get to touch the actual API that is run, so the only reason it should matter to you which is used, is by the results you get on your hardware.

You seem to be having problems with DX, but perhaps those problems dont occur with OGL. On top of that, perhaps your graphics/FPS are still as good?

Or is there some other reason to care about what API is running under the hood?

Miwa · Post by **Miwa** » Fri Feb 20, 2004 6:53 am

Or is there some other reason to care about what API is running under the hood?

Yep. The product has to eventually be installed on end user's computers. 5 years ago d3d totally sucked, and ogl was the way to go. But that is untrue now. Directx has much better driver support on a wide array of hardware. Try sticking a bunch of video adapters in a computer and run an ogl app in full screen on the primary card. Let me know after you finish rebooting..

Short answer... on Windows, directx is just plain better.

With dx, I can make my installation and support issues much better than with ogl. ogl is playing catch up with dx on features now too.

I'm not writing a stand-alone 3d app where the 3d stuff is the main part. My 3d displays are just a small part of the overall gui displays in the app, so I want to have as few extra dependancies added as possible.

powerpop · Post by **powerpop** » Fri Feb 20, 2004 10:10 am

ummm, irrlicht is LGPL (i think)

niko · Post by **niko** » Fri Feb 20, 2004 2:46 pm

powerpop wrote:ummm, irrlicht is LGPL (i think)

It's zlib, it's not that conservative like (L)GPL: http://irrlicht.sourceforge.net/license.html

keless · Post by **keless** » Fri Feb 20, 2004 2:47 pm

IrrLicht is ZLib which is practically the same as LGPL:

ie: you can do what you want with the source, including not make it public, but if you do release it- give credit to the original authors. Or thats my layman's understanding.

Zaelsius · Post by **Zaelsius** » Fri Feb 20, 2004 3:11 pm

With LGPL you MUST make public any modifications you do on the engine. With ZLib you're free to release the source or not, but you CANNOT SAY it's completely yours.

I think it's more or less correct.

Edit: here is the Ogre LGPL license explanation from www.ogre3d.org

Under the LGPL you may use Ogre for any purpose you wish, as long as you:

1. Release any modifications to the OGRE source back to the community
2. Pass on the source to Ogre with all the copyrights intact
3. Make it clear where you have customised it.

keless · Post by **keless** » Fri Feb 20, 2004 3:20 pm

Ah right-- you can keep your own code private (ie the code that uses the library), but any library modifications you make have to be public.

Miwa · Post by **Miwa** » Fri Feb 20, 2004 5:49 pm

I don't mind the 'making library mods public' part of the lgpl, but if I remember correctly, there's also a section in it about compatibility with future versions of libraries, and allowing users to link in newer versions.