Extensions
The OpenGL extension mechanism is probably the most heavily disputed difference between the two APIs. OpenGL includes a mechanism where any driver can advertise its own extensions to the API, thus introducing new functions such as blend modes, new ways to transfer data to GPUs, or different texture wrapping parameters. This allows new functions to be exposed quickly, but can lead to confusion if different vendors implement similar extensions with different APIs. Many of these extensions are periodically standardized by the OpenGL Architecture Review Board (ARB), and some are made a core part of future OpenGL revisions.
On the other hand, Direct3D is specified by one vendor only (Microsoft), leading to a more consistent API, but denying access to vendor-specific features. NVIDIA's UltraShadow technology, for instance, is not available in the stock Direct3D APIs at the time of writing. It should be noted that Direct3D does support texture format extensions (via FourCC). These were once little-known and rarely used, but are now used for S3 Texture Compression.
When graphics cards added support for pixel shaders (known on OpenGL as "fragment programs"), Direct3D provided one "Pixel Shader 1.1" (PS1.1) standard with which the GeForce 3 and up, and Radeon 8500 and up, advertised compatibility. Under OpenGL the same functions were accessed through a variety of custom extensions.
In theory, the Microsoft approach allows one code path to support both brands of card, whereas under OpenGL, programmers must write two separate systems. In reality, though, because of the limits on pixel processing of those early cards, Pixel Shader 1.1 was nothing more than a pseudo-assembly language version of the NVIDIA-specific OpenGL extensions. For the most part, the only cards that claimed PS 1.1 functionality were by NVIDIA, and that is because they were built for it natively. When the Radeon 8500 was released, Microsoft released an update to Direct3D that included Pixel Shader 1.4, which was nothing more than a pseudo-assembly language version of the ATI-specific OpenGL extensions. The only cards that claimed PS 1.4 support were ATI cards because they were designed with the precise hardware needed to make that functionality happen.
This situation existed only for a short time under both APIs. Second-generation pixel shading cards functioned far more similarly, with each architecture evolving toward the same kind of pixel processing conclusion. As such, Pixel Shader 2.0 allowed a unified code path under Direct3D. Around the same time OpenGL introduced its own ARB-approved vertex and pixel shader extensions (GL_ARB_vertex_program and GL_ARB_fragment_program), and both sets of cards supported this standard also.
Read more about this topic: Comparison Of OpenGL And Direct3D
Famous quotes containing the word extensions:
“If we focus exclusively on teaching our children to read, write, spell, and count in their first years of life, we turn our homes into extensions of school and turn bringing up a child into an exercise in curriculum development. We should be parents first and teachers of academic skills second.”
—Neil Kurshan (20th century)
“The psychological umbilical cord is more difficult to cut than the real one. We experience our children as extensions of ourselves, and we feel as though their behavior is an expression of something within us...instead of an expression of something in them. We see in our children our own reflection, and when we don’t like what we see, we feel angry at the reflection.”
—Elaine Heffner (20th century)