The is a proprietary game engine created by Christian Whitehead. It was famously used to power the mobile "widescreen" remasters of Sonic the Hedgehog 1 , Sonic 2 , and Sonic CD , as well as the critically acclaimed Sonic Mania .

For years, Sonic 3 & Knuckles was the "missing piece" of this collection due to complex legal issues surrounding its soundtrack. This led to several community-driven "Sonic 3 RSDK" projects aimed at bringing the game up to the standards of Whitehead’s other remasters. Key Sonic 3 RSDK Projects

:In 2022, Sega finally released an official RSDK version of Sonic 3 & Knuckles as part of Sonic Origins . This version uses RSDKv5U , an updated version of the engine that supports modern features like widescreen, a dedicated "Anniversary Mode" with infinite lives, and the Drop Dash mechanic from Sonic Mania .