Partially render a 3D scene
I want to partially render a 3D scene, by this I mean I want to render some pixels and skip others. There are many non-realtime renderers that allow selecting a section that you want to render.
Example, fully rendered image (all pixels rendered) vs partially rendered:
I want to make the renderer not render part of a scene, in this case the renderer would just skip rendering these areas and save resources (memory/CPU).
If it's not possible to do in OpenGL, can someone suggest any other open source renderer, it could even be a software renderer.
If you're talking about rendering rectangular subportions of a display, you'd use glViewport and adjust your projection appropriately.
If you want to decide whether to render or not per pixel, especially with the purely fixed pipeline, you'd likely use a stencil buffer. That does exactly much the name says — you paint as though spraying through a stencil. It's a per-pixel mask, reliably at least 8 bits per pixel, and has supported in hardware for at least the last fifteen years. Amongst other uses, it used to be how you could render a stipple without paying for the 'professional' cards that officially supported glStipple.
With GLSL there is also the discard statement that immediately ends processing of a fragment and produces no output. The main caveat is that on some GPUs — especially embedded GPUs — the advice is to prefer returning any colour with an alpha of 0 (assuming that will have no effect according to your blend mode) if you can avoid a conditional by doing so. Conditionals and discards otherwise can have a strong negative effect on parallelism as fragment shaders are usually implemented by SIMD units doing multiple pixels simultaneously, so any time that a shader program look like they might diverge there can be a [potentially unnecessary] splitting of tasks. Very GPU dependent stuff though, so be sure to profile in real life.
EDIT: as pointed out in the comments, using a scissor rectangle would be smarter than adjusting the viewport. That both means you don't have to adjust your projection and, equally, that rounding errors in any adjustment can't possibly create seams.
It's also struck me that an alternative to using the stencil for a strict binary test is to pre-populate the z-buffer with the closest possible value on pixels you don't want redrawn; use the colour mask to draw to the depth buffer only.