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  • #16
    Originally posted by me262 View Post
    1. D3D and OpenGL/OpenAL are both Hardware Abstraction Layers, they exist because when these 3D games first appeared, you had accelerated versions for specific cards (s3Virge Edition, 3DFX Edition, ATi Rage Edition).
    2. The HALs were created to give companies a common interface. MS created D3D, and the OpenGL open source community does OpenGL. Can't remember who does OpenAL.
    This is correct (except for OpenGL being created by the open source community; it was created by SGI as an open standard, but it's not open source).

    [*]Games are programmed with some backward compatibility, they will run older versions, but only so much. Guild Wars is an example of a DX8 fallback with DX9 features.
    Yes, but what I'm saying is that OpenGL degrades features much more gracefully than D3D. Almost all the features of OpenGL 1.1+ are available through individual extensions. So instead of, say, using OpenGL 1.4 as a fallback to OpenGL 1.5, you'd use OpenGL 1.4 plus whatever extensions are available (which could theoretically give you the functionality you needed out of 1.5 without the extras you don't need which 1.5 requires). Same for 1.5 to 2.0/2.1.

    Okay, I re-read your post, I do agree somewhat. OpenGL gets those features by using some CPU power I suspect.
    It all depends on the hardware/drivers. Some extensions may be designed to be done on the CPU, others can be emulated on the CPU by drivers, others can be done on the hardware if its capable. But it does not require, for instance, all of core 1.5 to be handled by the hardware to handle any extensions that made it into core 1.5.

    For example: Draw buffers/multiple render targets are in core OpenGL 2.0. But you don't necessarilly need an OpenGL 2.0 card to get them.. you just need the GL_ARB_draw_buffers extension which can be available, and accelerated, on a card/driver reporting 1.5.

    Long time ago I remember running Need for Speed 3 with OpenGL in software mode, and it just was not possible to play at all.
    Software OpenGL isn't really useable, just like software D3D isn't.

    If an OpenGL card isn't 2.0 capable, it'll still run at 1.4, 1.3, though you will not get the GPU features of the higher level.
    Not quite true, like with my draw buffers example above. A card may have GL2.0 features, available through registered extentions, without having to be completely 2.0 compliant.