wideNES - Peeking Past the Edge of NES Games

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!