These notes are for if you want to build OpenGFX2 from the source files. If you just want to download OpenGFX2 then you don't need to worry about these.
Requires git lfs for large file handling. Once git lfs is installed then clone using git as normal.
Requires a system with make, nmlc, git and python3 (3.11+) with PIL, blend-modes, numpy, scikit-image, tqdm. This has been developed using Windows Subsystem for Linux (WSL) and might have peculiarities (eg. incorrect file permissions) on a real Linux install.
Clone the repository, navigate to the repository root directory and run make all. It will take a long time...
The built baseset will be in baseset/, making different versions called OpenGFX2_<variant>-<version>.tar and .zip.
You can (re)build only the "Classic" 8bpp 1x zoom baseset using make baseset or the "High Def" 32bpp 4x zoom baseset using make baseset_highdef.
The built NewGRF(s) will be in newgrf/, making various .grf files.
You can (re)build only the NewGRFs using make newgrf.
You can (re)build only the graphics using make graphics.
Intermediate files can be removed for a clean build using make clean, or make clean_graphics, make clean_baseset or make clean_newgrf for graphics, baseset and newgrf files respectively.
Image processing makes a bunch of intermediate files, particularly *_8bpp.png and things in pygen directories. Others are _bt32bpp.png, _rm32bpp.png. These files are assumed to be temporary intermediates, and any user-modified versions will be blindly overwritten. Make sure you look at .gitignore see which files this applies to.
Files processed by python and encoded by nml are .png files, but those may be derived from other sources. Raw sprites were variously drawn\generated in Paint.NET, Aseprite, Blender and more. Conversion from those raw sources to .png files is not part of the build process - it must be done manually.
All sprite sheets are generated from source png files by graphics/generate_graphics.py. However, OpenTTD-TTF must have first been cloned to graphics/fonts/openttd-ttf. make all or make graphics handles this for you though.