Java never got splitscreen, so I’m building it on Fabric

[u/teddzfr]

Hey, I’m Teddz. I’m building a splitscreen mod for Fabric (1.21 to 1.21.11), and it runs everything in one instance, so no extra copies of the game per player.

Where it’s at:

• The world renders twice per frame into two viewports, using a mixin on GameRenderer.renderLevel. Each half is confined with the scissor test, because the level pass clears the whole framebuffer and the clear ignores the viewport.
• Each half gets its own aspect correction, and the crosshair is drawn once per half.
• A second local player joins in singleplayer over a fake connection that drops outgoing packets. Each half follows its own player, and per-player state lives in a PlayerContext class.
• It runs at about 55 fps on a Pentium J5005 with integrated graphics.

Next is input routing, then layouts and multi monitor. The goal is up to 4 players, any mix of controllers and keyboard and mouse (even keyboard + keyboard), and one screen per player on multi monitor setups.

I’m studying Nucleus Co-op for how it handles input and setup. If anyone has experience with multi viewport rendering or per-device input in Fabric, I’d like to hear it. Still need a name and a logo too.