How does artwork work?

[Gradius]

Hi. I'd like to know how artwork works. I mean: is is some kind of semitransparent sheet over the monitor?

[Generic Eric]

well in the old days, maybe even still, it was a piece of glass that was painted.

some mame cabinet builders tend to use plexi glass or lexan.  or reuse a bezel from their favorite game.

[Gradius]

Yes, but if it's painted, how on earth can we see the bitmaps on screen? Because when I play Space Invaders in MAME I can see the ships clearly over the painted background.

[Snarbald]

Didn't some versions of space invaders have transparent cellophane give the illusion of a color game?

[krick]

Space Invaders used a mirror that reflected the monitor image.
You weren't looking directly at the monitor...

http://www.campusprogram.com/reference/en/wikipedia/s/sp/space_invaders.html

[spriggy]

In relation to 'Snarbalds' comment...

I am currently restoring a 1979 Cosmic Gueriila UR. This game was just an interesting spin on the Space Invaders platform (Quite good actually).  The early cabinets used a B&W monitor, however had different colored strips of acetate overlayed directly onto the monitor to give color to the characters on screen.

[danny_galaga]

snarbold might not be old enough to remember. but i am  
space invaders was one of the first machines i played. i remember wondering why they would bother with the cellophane when the colour tv had been invented. didn't know about economics when i was a little lad  

[RayB]

Yes, but if it's painted, how on earth can we see the bitmaps on screen? Because when I play Space Invaders in MAME I can see the ships clearly over the painted background.

A Bezel is artwork that goes AROUND the sides and top/bottom of a monitor. Therefore, there's no paint in the area where a monitor picture is displayed.

The Space Invaders art you're talking about is "background art". And the way THAT works is as described above. It's a painted mirror and the video graphics are then reflected off of it, giving the illusion of graphics mixed with artwork.