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Old 26 August 2015, 07:38   #22
Total Chaos forever!

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Join Date: Aug 2007
Location: Ft. Collins, CO USA
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A bit-planar display is essentially a screenmode made up of multiple monochrome bitmaps stacked to yield multiple bits per pixel (assuming there is more than one bitplane). By rendering 3 bitplanes of shading and 5 bitplanes of texture mapping, the amount of computation lost to the chunky-to-planar conversion of the texture mapping is cut to 5/8 the normal amount of time. In addition to that, the other 3 bitplanes get a speed boost by letting the blitter do its polygon-filling while the CPU is doing other things.

In this context, a standard 8-bitplane chunky-to-planar routine like ADoom uses takes more CPU time and gets no boost from the blitter. Splitting the duty between the CPU and blitter makes a big speed improvement as in our 5 bit and 3 bit split mentioned in the example in the previous paragraph. It essentially is a compromise between dithered pattern fills by the blitter like Walls 1.7 linked in my previous post and Doom's 8-bit chunky routines.
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