OpenGL: What's the deal with deprecation?

Question

OpenGL 3.0 and 3.1 have deprecated quite a few features I find essential. In particular, the use of fixed function in shaders.
Can anyone explain what's really the deal with that?
Why do they find the need to deprecate such useful feature that its obvious everybody uses and that no sane hardware company is going to remove support for?

Answer 1

As you said, no hardware company will remove support for fixed-function shaders, because there are so many existing applications that use them. What they don't want to do, though, is figure out how to specify the interactions between FF shaders and every future extension they add. Those interactions are very complicated (partly because FF shaders are so complicated), which leads to bugs and inconsistent implementations between vendors -- both of which are bad for developers and end users.

So they're drawing a line: if you want to use FF shaders, you don't get any of the new functionality. If you want new functionality, you can't use FF shaders. This is very similar to what Microsoft did in D3D10: it added a whole bunch of new functionality, but at the same time completely removed fixed-function shaders. The belief is that the set of developers who need the new non-shader functionality but who don't also need programmable shaders is very small.

Answer 2

Fixed function shaders are quite easily replaced with standard GLSL shaders so it's difficult to see why logically they shouldn't be deprecated.

I'm less certain than you that they won't be dropped from much hardware in the foreseeable future as OpenGL ES 2.0 doesn't support the FF pipeline (and so isn't backward compatible with OpenGL ES 1.x). It seems to me that much of the momentum with OpenGL these days is coming from the widespread adoption of OpenGL ES on mobile platforms and with FF functionality gone from there there will be some considerable pressure to move away from it's use.

Indeed I'd expect the leaner OpenGL ES implementation to replace standard OpenGL quite widely over the next few years, and FF functionality may disappear more because most hardware will implement OpenGL ES rather than because it's removed from hardware implementing the full OpenGL

Answer 3

It should be clarified that a feature that is marked "deprecated" is not actually removed. For example, an OpenGL 3.0 context has all of the features - nothing is gone. Further, some vendors will ship drivers that can create 3.1 and 3.2 contexts using a compatibility profile which will also enable the deprecated features. So, look closely at what vendor hardware you are going to support and ask about the ARB compatibility mode for old features. (There is also the "core" profile as of 3.2, which allows vendors to create a more lean and mean driver if they wish to make such a thing)

Note that any current card really doesn't have an FF hardware section any more - they only run shaders. When you ask for FF behavior, the GL runtime is authoring shaders on your behalf..