An interesting thing maybe some don't know is that the 2 player mode in Sonic 2 uses a special "high resolution" mode to achieve its little split screen effect.
I was going to not bother implementing this feature because I thought it was going to require redoing all my timings all over again, but in fact it's much easier than I thought. It uses the same timings pretty much as the other video modes on the Genesis, but simply splits them up into two frames.
All shows on TV until recently were shown in interlaced mode, whereby a single frame is split into two separate "fields", one has all the even lines of the image, the other all the odd. This allows you to broadcast a much higher resolution picture than you otherwise could in the same amount of bandwidth, which is why it was adopted as a standard 50-70 years ago or so.
The downside is it introduces an amount of flicker and image separation (if not properly handled). For fast action things like sports interlaced television at 50hz or 60hz is actually a lot smoother than what we get now at 25hz or 30hz on our digital televisions. This is because interlaced can show 60 updates per second over 30, sure they may not be "full frame" updates but they are updates nonetheless. I have noticed this effect myself when the switch to digital occurred. They will likely fix it in time by increasing the framerate of the signal back to 50/60hz with full frames.
So why am I mentioning all this? Well the Genesis/Megadrive achieved its high resolution mode using interlacing, which most I'm sure could notice in the Sonic 2 - 2 player mode compared to single player. I certainly did even when I was young and ignorant. Anyhow I emulated the interlacing effect different to what I've seen in other emulators. I tried to emulate it close to how the TV shows it, in its interlaced glory. So it's a bit more flickery compared to say Kega but it's also somewhat more accurate.
This is 1:1 shot of the effect, it's similar to a "scanline" effect when viewing it as a single picture, but in motion it is shown like it would be on real TVs with alternating fields.
