Under what conditions does a multi-pass approach become strictly necessary?

Question

I'd like to enumerate those general, fundamental circumstances under which multi-pass rendering becomes an unavoidable necessity, as opposed to keeping everything within the same shader program. Here's what I've come up with so far.

• When a result requires non-local fragment information (i.e. context) around the current fragment, e.g. for box filters, then a previous pass must have supplied this;
• When a result needs hardware interpolation done by a prior pass;
• When a result acts as pre-cache of some set of calculations that enables substantially better performance than simply (re-)working through the entire set of calculations in those passes that use them, e.g. transforming each fragment of the depth buffer in a particular and costly way, which multiple later-pass shaders can then share, rather than each repeating those calculations. So, calculate once, use more than once.

I note from my own (naive) deductions above that vertex and geometry shaders don't really seem to come into the picture of deferred rendering, and so are probably usually done in first pass; to me this seems sensible, but either affirmation or negation of this, with detail, would be of interest.

P.S. I am going to leave this question open to gather good answers, so don't expect quick wins!

Answer 1

Nice topic. For me since I'm a beginner I would say to avoid unnecessary calculations in the pixel/fragment shader you get when you use forward rendering. With forward rendering you have to do a pass for every light you have in your scene, even if the pixel colors aren't affected. But that's just a comparison between forward rendering and deferred rendering.

As opposed to keeping everything in the same shader program, the simplest thing I can think of is the fact that you aren't restricted to use N number of lights in your scene, since in for instance GLSL you can use either separate lights or store them in a uniform array. Then again you can also use forward rendering, but if you have a lot of lights in your scene forward rendering has a too expensive pixel/fragment shader. That's all I really know so I would like to hear other theories as well.