So my guess is this game was a pretty sloppy port from the Xbox. They just did the bare minimum to get it working on PC. So 21:9 might be something only the developer Blind Squirrel Games can do. And they still have yet to fix the mouse issue so if they ever do it, it might be a while sadly.
Replace main.dll in the game folder depending on the desired resolution. Use patcher to disable ghosting and grain effects.
Okami_21x9.7z
Looks like the recent patch on steam breaks the .DLL files. Could you please update the download? I really appreciate your work! Great game so far, bummed ...
^ Seeing that makes me very happy. You are awesome. Also I'm noticing in your screen shots you don't have the canvas filter effect on whatsoever. How did you completely turn it off? The game only lets you set it to "light".
Never mind about disabling the filter a Special K reshade tool was posted ...