What does OpenGL sit on top of?

What does OpenGL sit on top of?

Post by Michael I. Gol » Sat, 27 Dec 1997 04:00:00




> Pertaining to the Windows implementation I have read that the OpenGL API
> sits on top of it own 3D-DDI.  But I have also read severval books that
> detail OGL as sitting on top of DirectDraw/Direct3D Immediate.  So whats
> the deal?

Here's the deal:

3D-DDI as a driver model is dead and buried.

OpenGL now has two driver models, MCD and ICD.

MCD (mini client driver) uses Microsoft's tuned geometry pipeline, and
the driver writers need only implement rasterization directly on the
hardware (i.e. register writes).  The MCD is supported on WinNT only.

ICD (installable client driver) is a complete implementation, i.e. the
driver writer implements the entire pipeline, starting from a reference
implementation.  This is more work than an MCD but potentially better
performance since the entire pipeline may be tuned for the
characteristics of the hardware.  This is the model which is currently
in use by the major IHV's who are working on OpenGL drivers for Win95
and WinNT.

The diagram in the DirectX documentation exists somewhere between
propoganda and an ill- conceived plan which never came to pass.  OpenGL
is a low-level API intended to be implementated directly on the
hardware.  The DirectX diagram was created at the same time Microsoft
was trying to portray Direct3D as "closer to the hardware" than OpenGL,
which has since been disproven along with a whole slew of lies which
originated with the DirectX marketing team.  Some people at Microsoft
will claim that they intended such layering for OpenGL but realized it
was too slow and gave up.  Of course they never changed the
documentation.

 
 
 

1. What does OpenGL sit on top of?

Pertaining to the Windows implementation I have read that the OpenGL API
sits on top of it own 3D-DDI.  But I have also read severval books that
detail OGL as sitting on top of DirectDraw/Direct3D Immediate.  So whats
the deal?

-Daniel  

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