hows graphics done on things like slot machines

[stav]

so i know graphics on normal computers are done using something like opengl or direct x to control the graphics card

but how's it done on things like bill boards and slot machines?

is it just a small computer like a raspberry pi running linux and using something like opengl to draw the graphics?
or is it much more advanced with their own operating system and custom hardware?

[helios]

Billboards come in many different forms. The simpler ones are matrices of light bulbs controlled by some microcontroller that probably receives data through a very simple serial interface, or something along those lines. The color LED ones are basically giant computer monitors. Sometimes the controller programs crash and you can see they're literally connected to commodity hardware running commodity software:
http://photos1.blogger.com/blogger/353/70/1600/psp_billboard_crash.jpg
http://i.imgur.com/YdsAV.jpg
http://d28mt5n9lkji5m.cloudfront.net/i/VJs6HXeoGj.jpg
http://www.arageek.com/wp-content/uploads/bsod-mall_3471433k.jpg
Slot machines and ATMs are the same deal. Unless there's some specific form factor or compatibility requirements, you can bet that any stationary display is connected to desktop hardware running a desktop OS and a custom application, simply because that kind of hardware is really cheap and programmers for it are very common (and thus cheap).