Modder CorvidDude has dug deeply into the source code of Sonic Adventure DX for the PC to recreate the original Windy Hill beta level. By extracting the original stage’s geometry data (with the help of Catley and ItsEasyActually), he used radiosity and vertex paint to light the level and also rewrote a lot of the scripts. He also wrote several new features into the level, including the camera editor, the gizmo, and the view frustum culling. Finally, he created a race path for Sonic, so he does not succumb to the level’s many bottomless pits.
You can download the mod for Sonic Adventure DX here.