This script overlays basic composition guides on your mpv video.

Scripts are usually placed in ~/.mpv/scripts/ (macOS/Linux) or %AppData%\mpv\scripts\ (Windows).
Put the entire downloaded folder in this directory. Do not rearrange individual files or directories.
The name of the directory is irrelevant, as long as ...\scripts\whatever\main.lua is preserved, it should work.
See: https://mpv.io/manual/master/#lua-scripting and https://github.com/mpv-player/mpv/wiki/User-Scripts

g           cycle through the guides
G           reset back to no guides
CTRL g  cycle between 16:9 mode and 4:3 mode

Everything breaks when you're not full screen btw, and that's not really a fixable issue; that's just how MPV / this script works.
The 4:3 ones may look a tad wonky because of the scaling, but everything lines up appropriately

To remove the feedback in the top left, in main.lua change variable verbose to false.

To change what key the script uses, in main.lua set the variable key to another character.
To disable automatic keybinding, in main.lua set the variable key to nil

To add hooks to input.conf, use the key-bindings comp_guide_cycle, comp_guide_clear, and comp_guide_43. For example:
h script-binding comp_guide_cycle
Alt+q script-binding comp_guide_clear
Ctrl+m script-binding comp_guide_43

If you want to add more guides, add the filename and guide name to the arrays and leave a bgra formatted file in the "guides" folder.

Open an issue or bother MasterofAres#1755 on discord if something breaks