While my question is quite simple, the answer is not turning out to be so straightforward: in OpenGL, given an arbitrary number of coplanar rectangular surfaces that overlap (eg a GUI of arbitrary dimensions from something like 150x300px to HD resolutions, including potentially multi-monitor setups), which would make the most sense:

1) do point-in-rect style tests per fragment in the shader
2) order elements on the cpu, keep depth testing off, ignore overdraw and just brute-force everything
3) implement a minuscule z offset and let depth testing do the clipping while ignoring overdraw
4) there's no one-glove-fits-all solution: different platforms require different solutions

I currently have depth testing and masking disabled and I'm brute-forcing everything. It's simple, but not necessarily elegant.

Regarding option 3: I do want to implement some form of clipping and Z-ordering anyway, but I'm not sure if Z-offsetting controls is the solution here. I'm mildly curious as to whether point-in-rect (essentially having a four-part if statement per fragment per intersecting element might be the most viable, especially considering that I want to prepare for a potential software fallback in case of a non-accelerated GL ES implementation or such).

Since the question is somewhat abstract due to scalability, I'd appreciate some thoughts!

A GUI implementation generally generates several and up to tens or even hundreds of clip rectangles.

Scissor test is useful for clipping pixels that fall outside the draw rectangle; I need to clip regions that are inside it (eg not draw under a button while drawing a toolbar).

You may be solving the wrong problem.

Generally, UI code solves this problem by just drawing the windows in order, front to back (omitting only completely obscured windows). Why? Because that's actually faster than faffing about...

Yes, it generates some overdraw, but that's still usually easier than trying to clip -- UI pixels are often fairly cheap (being things like flat or shade fills of rectangles).

If you're actually running this on a 3D card, then the obvious answer is just to use the Z buffer to sort this problem out for you. Which would be the second approach I'd try, but only if the overdrawing method proved slow.

Go for the simplest approach until you really find it won't work.

My concerns are mainly for the unaccelerated/emulated systems and have to do with overdrawing controls that have a bunch of small print text or antialiased splines/curves in them, which could make use of early clipping. Katie's remarks about the simplest solution being the best make the most sense, though. I'll just resort to proper Z-ordering and minute offsets for now.

Ashaman - I'm afraid I mean clipping in the reverse sense to what the scissor rect does (as in not drawing what is behind child controls). I'm talking about low performance systems (GL ES) that may or may not be GPU-accelerated.