I'm a tad confused on when trying to use emulators of various consoles such as GBA/GameCube/Wii/PS2/Xbox/90's early 00's PC games whether I should be selecting OpenGL or DirectX when trying to use something such as DgVoodoo2/ReShade. Is it just on a per game basis? How do I find out this information? Thank you in advance.
Well...
With ReShade you can add shader effects to games and programs (like emulators) that are using OpenGL or Direct3D 9 or higher.
dgVoodoo 2 can translate Direct3D instructions (DirectX 3D instructions) from versions up to Direct3D 8.1 to current Direct3D versions (with often better compatibility than the translation Windows does itself). It also adds nice features like e.g. render the 3D graphics in resolutions that weren't originally supported by the game/program you use dgvoodoo 2 with.
If you have an old game/program that doesn't support OpenGL and only supports a Direct3D version lower than Direct3D 9 you can't use ReShade directly. But in this case you can often use dgVoodoo 2 to translate the Direct3D instructions to a current Direct3D version. This lets you use ReShade also (choose "Direct3D 10+" during ReShade setup). Depending on how old the game/program is not all shaders will work though.
It depends on the game/program which 3D APIs (3D application programming interfaces) are supported. Some do only support OpenGL. Some only a certain version of Direct3D. Some support more than one API and lets you choose. Some support more than one API and autodetect and choose an API that hopefully works best. In case that the game/program don't let you choose the API you either have to do some research (often you can find the info easily somewhere in the internet or in a Readme.txt that came with the game/program) or you can try to find out yourself.
One approach to find out what API a game/program supports (when it don't let you chose one) is ReShade itself. Try to install ReShade with one of the supported 3D APIs (Direct3D 9, Direct3D 10+ and OpenGL) and looks if it works. If it doesn't work uninstall it and install it with one of the other APIs and so on. If it doesn't work with all 3 supported 3D APIs the game may use an Direct3D version lower than Direct3D 9. In this case try dgVoodoo 2. If this also doesn't work then probably something doesn't work with the "hooking" mechanism. Then you can ask at the forum in a fitting section and provide some info about your system and the game/program you're trying to play/use.
If ReShade/dgVoodoo 2 doesn't work with all your games/programs you're probably doing something wrong installing the tools.
Thank you SO much for that in depth response! I have been able to use ReShade and DgVoodoo2 with various versions even having to use WIP's for certain bugs with certain games it is absolutely incredible. I was hoping to just be able to somehow easily detect what API a specific game is using to determine the correct course of action for ReShade/DgVoodoo2 but it looks like It'll just have to be trial and error of each option. Thanks for the idea of checking the Readme.txt!