OpenGL

OpenGL must break purity due to the need for hardware-specific linkage. Intel, AMD, and Nvidia have different drivers for example. On NixOS, these libraries are symlinked under

 /run/opengl-driver/lib

and optionally (if hardware.opengl.driSupport32Bit is enabled)

 /run/opengl-driver-32/lib

When a program is installed in your environment, these libraries should be found automatically. However, this is not the case in a `nix-shell`. To fix, add this line to your shell.nix:

 LD_LIBRARY_PATH="/run/opengl-driver/lib:/run/opengl-driver-32/lib";

Testing Mesa updates

To avoid a lot of rebuilds there's an internal NixOS option to override the Mesa drivers: hardware.opengl.package

It can be used like this: hardware.opengl.package = (import /opt/nixpkgs-mesa { }).pkgs.mesa.drivers;

However, since Mesa 21.0.2 this doesn't necessarily work anymore and something like the following might be required:

system.replaceRuntimeDependencies = [
  ({ original = pkgs.mesa; replacement = (import /opt/nixpkgs-mesa { }).pkgs.mesa; })
  ({ original = pkgs.mesa.drivers; replacement = (import /opt/nixpkgs-mesa { }).pkgs.mesa.drivers; })
];

Note: Both of these approaches are impure and only work to a certain degree (many limitations!). If you want to use a different version of Mesa your best option is to use an overlay or a Git worktree where you use the same Nixpkgs revision and only alter pkgs/development/libraries/mesa/ for one of the two approaches mentioned above.

Debugging Mesa issues

There are a lot of useful environment variables for debugging purposes: https://docs.mesa3d.org/envvars.html

The most important one is LIBGL_DEBUG=verbose and helps with debugging error like:

libGL error: MESA-LOADER: failed to open $DRIVER (search paths /run/opengl-driver/lib/dri)
libGL error: failed to load driver: $DRIVER

Related

https://nixos.wiki/wiki/Nixpkgs_with_OpenGL_on_non-NixOS