A digital clock for a Raspberry Pi with a framebuffer display, with festive snow that gathers on the display.
Disclaimer: This is pretty much made just for me as a weekend(s) project, and is thrown online in case it's handy (or cool) to someone else. Expect to have to hack it about a bit to make it work well for your case. (Likewise it's not the cleanest codebase in the world, ahem.)
Note: During less-festive times (like, the time of writing at this commit) I tend to reconfigure it to do something, well, less-festive. It's currently doing a cool thing with clock segments popping into rainbow sand as they change instead of snow. Fiddle with the factory functions in main to pick something else.
There is an SDL 2 build you can select via the Makefile that has a menu that lets you switch between the two clocks. This is suitable more modern (and wasteful :P ) setups with 3D-accellerated X or Wayland, rather than framebuffer, such as a Pi 3 or 4 with a HyperPixel display.
Snow gathers on the digits, and falls off them when they change. It also builds up at the bottom of the screen, and falls away from that during the first fifteen seconds of each hour. The color of the time gently fades between red and green every other minute. Snow in the background flutters gently in response to simulated breezes (but there's no fluid dynamics for getting kicked up by snowfalls, etc.).
I targetted this for the Tontec mz61581-pi-ext on an original Raspberry Pi B+ I got as a gift (which managed to snare me into buying far too many Pis, apparently), which is a neat little GPIO-attached screen and transparent case...and is for an older header format that current Pis and has had its Amazon product page replaced with a different one.
(If you have the hardware, these links might help set it up: 1, 2)
Still, SDL means this should target anything. It's deliberately SDL 1.2 code so it can support raw framebuffers, so you don't have to run X11 (although it'll work with that too, and opens in a window on x64_64 for development). It's also fairly simplistic in its SDL usage, so migrating to 2 was easy enough should be easy if you want to---as should be adapting it to other embedded displays.
You want Debian/Rapsbian packages clang libsdl1.2-dev, then SDLVERSION=1 make.
The default is now SDL version 2, libsdl2-dev, and also uses libconfuse-dev.
This does not support framebuffers and won't work with the Tontec, which is why SDL 1 is still supported.
But in theory it works in ways more amicable to modern graphics setups if you're using something with a 3D chip.
Visual Studio Code can run builds, using whichever version is set in c_cpp_properties.json (there's some hacky overrides that pixmas{,2}.cpp will always assert the right version for themselves).
make run will build (if necessary) and run the binary.
Note there's a really sloppy check in the Makefile that sets a DESKTOP compiler define that instead launches in windowed mode. If you're doing development on a laptop/desktop that's not x86_64, you'll need to change that.
For the Tontec GPIO display make runonpi will set the right environment variables to target the framebuffer, and run via sudo to get permissions for it.
You can trade off having to run as root for opening permissions on /dev/console. Unfortunately on Raspbian it's root:root, so there's no simple group to join.
You don't need to be root if you're not using the framebuffer (e.g. SDL2 build).