Save this file as autoexec.cfg in the doom 3/base folder.If you have a 16:10 monitor, use “2” instead of “1” for the aspect ratio. Be sure to use the native resolution of your monitor for the height and width.
Open Notepad or your preferred text editor and add the commands from my example above.Keep the anti-aliasing to a maximum of 8x (there is a bug that some people reported when using 16x with custom resolutions). Once the game is installed, start it up and set your desired graphics settings.This means if it’s been installed before and you’ve tried tweaking configuration files or used mods, uninstall the game and any mods, then delete any leftover files from them in the Steam folder and any other folders where they create file. Make sure you start with a clean, fresh installation of Doom 3.
Also, to ensure a fool-proof widescreen experience, follow these steps: