Q:

Are questions about 3rd party graphics engines/libraries on-topic?

For example, would questions like https://blender.stackexchange.com/questions/5820/how-is-cycles-different-from-blender-internal be on-topic for this site, or would they be preferred moved to their specific site?
Some systems, such as Microsoft's DirectX, Valve's Source Engine, Nvidia 3D, might have questions relating to how they function or differ from similar systems, how they make use of the hardware, or their methods in rendering such as shaders.
Would questions asking about how specific graphics engines' methods be on topic?
Examples to consider:
"What shaders does [system] use to get effects like X?"
"How does [system] compare to method X in terms of [method comparison]?"
"How can I use [specific method] in [system]?"
"How does OpenGL compare with DirectX?"
"What role do the Mesa drivers play in a video stack?"
Questions of engine preference are of course, not so on-topic, but what about asking for an honest comparison of systems in terms of X?

A:

A specific question about the internals of a specific system should be on-topic. 
The linked question isn't that great; it asks how two different systems work (too broad) and which is better (primarily opinion-based).
However, a better understanding of the internals of an engine, or the specific algorithms used, should be quite useful. 
We should be careful of "holy wars"; how DirectX compares to OpenGL is one of these. However, if it is about a specific, technical aspect of these, something that can be objectively answered - that should be on-topic.
Ultimately, the two criteria are:

Can it be objectively answered?
Is it useful for future visitors?

And I'm willing to be lenient on that second question.

