Fork the Zombies- quick Halloween project

Like the title, this is a project completed over 3 weeks of spare time with Halloween as a goal. Our neighborhood is spaced out just about right for Halloween, so we get kids from the surrounding neighborhoods trick-or-treating through here. I set up an outdoor movie screen, built a “fork the zombies” game (my 13yo son’s idea) and a laser guided (wireless) crossbow. I’ll get some technical details up in a bit but let me post some pictures first.


The crossbow has a class 1 red laser built into the front, and there’s a webcam up there with a red filter in front of it. This means I have a calibration routine that I run so that the webcam can correlate where the red dot is on the screen (much like a mouse pointer). I couldn’t run the calibration routine until it got dark and here (just outside Atlanta) it got dark enough at about 6:45. I think this pic was taken at about 7:00pm.

Here’s some close-ups of the crossbow:



It’s basically a 2x4 stud that I freehand carved into a rough crossbow shape. I wanted to avoid guns, but you’ve got to fling forks at zombies. Few technical points:

  1. There’s an esp32 wifi board in there, it reads trigger pulls (trigger is a microswitch up there with a 3D printed cap) and sends a UDP packet off to the hit detect.
  2. Hit detect is ALWAYS looking for that red laser, and normalizes it (0-1 in X and Y) from the lower left hand corner of the screen.
  3. When hit detect gets a UDP ‘Trigger’ packet, it sends a message (I’m using ZMQ pub/sub) to the game, which then spawns a fork in the same direction reported from hit detect.

Game stayed pretty busy all night- my sounds and graphics are fairly cartoonish (flinging a fork is actually a mouth-harp sproing
I got some great comments from the kids- a few wanted to know how the whole thing worked, one actually asked “How did you know zombies are allergic to forks?” Here’s one of my favorite pics- a pack of fairies playing:

For technical details, there’s a projector connected to a Jetson Nano running the game, and another nano connected to a webcam doing hit detect. Besides that it’s just the laser and the esp32 in the crossow sending UDP packets, all are connected to a wifi router. Honestly you could skip the nanos and just do it on a single laptop, this is definitely more portable. I will release the source with instructions to build, but I need to get some credits in order first- the zombies aren’t mine (inexpensive turbosquid purchase, I modified the skins) and most of the sounds came from freesound.org. I need to attribute these to their proper authors before I can release. I do have the list, hopefully I can get that in order this week. The game plays fine on a PC, you just mouse click to fire a fork, but it’s not as challenging because I have the crossbow limited to 1 shot every 1/2 second. The game is designed to last 1 minute or less.
The hit detect stuff is python and openCV, I’ll probably write (and link to) a separate article on how all that works. Meanwhile for the Urho stuff- what an awesome engine! This was a ton of fun. I started with example 6- skeletal animation, and imported the zombies. Then I brought in elements of example 11- Physics for hit detect and fork flight. From there it was just steering things where I wanted them to go! I think the hardest part was getting ZMQ to work with cmake.

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