http://prilik.com/blog/wideNES
At the end of each frame, the CPU updates the PPU on what has
changed. This involves setting new sprite positions, new level data, and
—crucially for wideNES— new viewport offsets. Since wideNES
runs in an emulator, it’s really easy to track the values written to the
PPUSCROLL register, which means it’s incredibly easy to calculate how
much of the screen has scrolled between any two frames!
Hmm, what would happen if instead of painting each new frame directly over the old frame, new frames are instead painted overlapping
the previous frame, but offset by the current screen scroll? Well, over
time, more and more of the level would be left on-screen, gradually
building up a complete picture of the level!
