[leveller] Accelerated scanline rendering

I enabled a "use OpenGL" option in the scanline renderer, to see how 
much faster hardware acceleration would boost drawing of simulated 
increased terrain detail.

Improvements generally ranged from 150% to 200% faster (GeForce Ti4200). 
Peak theoretical hardware triangle throughput was nowhere near reached 
since a lot of CPU-side processing goes on calculating geometry and 
attributes. With texturing instead of using lots of little geometry, 
OpenGL should pull ahead much further, especially if hardware 
bumpmapping can be deployed. (Also transforms -- I'm doing them outside 
OpenGL unnecessarily, as the code from the original scanliner was just 
duped and it had to do everything for itself.)

Time permitting, what I'd like to do is make rendering plugin-able, and 
provide the existing scanliner as a reference render plug-in with source.

A larger performance gap occurred when rendering antialiased scenes. The 
native renderer uses supersampling while OpenGL relies on the AA setting 
of the video card. However, I find it takes a 4x setting on the card to 
match the 2x supersampled AA in quality terms (although the card can 
still outperform software at the higher setting).

Ray

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