There were a number of discussions back on Area 51 of what the scope of our site should be, and what should be on-topic or off-topic. I'll quote part of an answer I made there as it seems relevant:
This is fundamentally a programming site, as opposed to a site about art or design. We want questions that you'd encounter in the course of coding a renderer, basically. (Either a 2D or 3D renderer. Either real-time or offline. Either running on a GPU or not. Creating either photorealistic images or non-photorealistic ones, etc.)
What wouldn't be on-topic, for instance, would be questions about how to use 3D modeling tools, or how to create a skybox in Photoshop, or how to select a color palette for lighting a scene. Added to that, I guess we probably would not want questions about how to use specific 3D engines, e.g. "how can I increase shadow quality in Unity/UE4/whatever". (Maybe I'm wrong about that, but I don't consider such questions very interesting, and I think they belong on an engine-specific community site.)
The appropriateness of engine-specific or tool-specific questions was probably the issue people disagreed about the most. I argued against such questions above, but some others argued that questions specifically about Unity, UE4, Maya, Houdini, etc. were fine. I think we'll have to give it a try and refine our criteria as we go. The main concern for me about such questions is that they have to do with programming with those tools, as opposed to just using the tools. For instance, I can see questions about how to build a custom exporter plugin for Maya, or how to implement rendering technique X in UE4, as potentially on-topic.
Also, I want to expand a bit to say that not just rendering, but other related areas such as image processing, particles/fluids/cloth simulation, geometry processing (e.g. CSG, voxelization, remeshing), procedural geometry/texture generation and such things like that are probably on topic here as well. Maybe parts of computer vision too. Again, we'll probably have to refine the "edges" as we go.