Open GL internal mathematics

Open GL internal mathematics

Post by Alexander Polsk » Sun, 03 Oct 1999 04:00:00



In doing lighting simulations, we have encountered unexpected results in GL
simulated versus expected outcomes. We believe that this may be due to data
loss in our Open GL code where light levels are very low, causing
"quantization" of pixels. Essentially a dynamic range problem, that's our
best guess.

Can anyone point me to a reference to Open GL's internal math? Is it
floating point or integer?

I should add that we're running under Linux, but i would expect, perhaps
unrealistically, that this kind of thing would be consistent across
platforms

Thanks in advance,

Alexander Polsky

 
 
 

Open GL internal mathematics

Post by Andrew F. Vespe » Sun, 03 Oct 1999 04:00:00



> In doing lighting simulations, we have encountered unexpected results in GL
> simulated versus expected outcomes. We believe that this may be due to data
> loss in our Open GL code where light levels are very low, causing
> "quantization" of pixels. Essentially a dynamic range problem, that's our
> best guess.

> Can anyone point me to a reference to Open GL's internal math? Is it
> floating point or integer?

The implementation is, um,  implementation dependent. :-)

Internal computation can vary widely; it just needs to pass the conformance
suite.

Quote:> I should add that we're running under Linux, but i would expect, perhaps
> unrealistically, that this kind of thing would be consistent across
> platforms

The OpenGL standard leaves a lot of wiggle room for implementations.

--
Andy V (OpenGL Alpha Geek)
"In order to make progress, one must leave the door to the unknown ajar."
Richard P. Feynman, quoted by Jagdish Mehra in _The Beat of a Different Drum_.

 
 
 

Open GL internal mathematics

Post by Samuel Pai » Sun, 03 Oct 1999 04:00:00



> Can anyone point me to a reference to Open GL's internal math? Is it
> floating point or integer?

Yes.

The OpenGL specification that some of the internal computations be conducted
in at least single precision floats, but some need be no more precise than
the depth of the framebuffer.

The OpenGL specification can be found at http://www.opengl.org/

Quote:> I should add that we're running under Linux, but i would expect, perhaps
> unrealistically, that this kind of thing would be consistent across
> platforms

OpenGL has fairly loose precision requirements.
--
Samuel S. Paik | http://www.webnexus.com/users/paik/
3D and multimedia, architecture and implementation
Solyent Green is kitniyos!
 
 
 

Open GL internal mathematics

Post by Samuel Pai » Sun, 03 Oct 1999 04:00:00



> Can anyone point me to a reference to Open GL's internal math? Is it
> floating point or integer?

Yes.

The OpenGL specification that some of the internal computations be conducted
in at least single precision floats, but some need be no more precise than
the depth of the framebuffer.

The OpenGL specification can be found at http://www.opengl.org/

Quote:> I should add that we're running under Linux, but i would expect, perhaps
> unrealistically, that this kind of thing would be consistent across
> platforms

OpenGL has fairly loose precision requirements.
--
Samuel S. Paik | http://www.webnexus.com/users/paik/
3D and multimedia, architecture and implementation
Solyent Green is kitniyos!
 
 
 

Open GL internal mathematics

Post by Thomas Ker » Mon, 04 Oct 1999 04:00:00


Alexander Polsky schrieb:

Quote:

> In doing lighting simulations, we have encountered unexpected results in GL
> simulated versus expected outcomes. We believe that this may be due to data
> loss in our Open GL code where light levels are very low, causing
> "quantization" of pixels. Essentially a dynamic range problem, that's our
> best guess.

> Can anyone point me to a reference to Open GL's internal math? Is it
> floating point or integer?

Mostly float. The spec says all about OpenGL internals and behavior
(what is not described that you can't expect). You should find it
online at http://www.sgi.com/software/opengl

Quote:> I should add that we're running under Linux, but i would expect, perhaps
> unrealistically, that this kind of thing would be consistent across
> platforms

If you are running Linux, perhaps with Mesa, you can look in the Mesa
sources for yourself to find out the internal math and you can change
it if you think, it is not okay.

Add my lines to Andys answer.

Thomas
--
links, tips, OpenGL related source at:
http://home.t-online.de/home/thk01/navigate.htm

 
 
 

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