Some Japanese Pachislo machines, the highly sought after, rarer ones had a feature where they played these holographic animations on the glass over the score reels. They’re incredible. It could definitely be done and adds a literal entire new level to the game!

It's been done.

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Many commercial slot machines have this feature. It's called a clear LCD. I believe
WMS patented it and licensed it to others. Now that Scientific Games owns both WMS and Bally, they both have the feature without a fee per machine.

Damn it, I’ve been pointing this out for 2 years . There is a Crossy Road redemption game which is the best demo examole for this. The cab needs to be ultra lit (paging Pinstadium!) but otherwise it works great

We experimented with this back in the early days of Heighway (2012), but there were three real drawbacks. One, it was incredibly easy to break the screen. Two, being LCD the screen could only darken, not lighten, and was completely incompatible with any art below it no matter how much light there was in the game. Three, it created a kind of 'prism' effect which rendered the game unplayable to anyone with dodgy eyesight who wore glasses.

A transparent OLED screen might have worked better had they been at all cost effective, but ultimately the only real solution is to wait for a cheap enough screen you can cut through and shape, and use it like a playfield protector with all the mechanics and fixings placed through it, and on which all the playfield art, lamps, inserts and interactive effects can be displayed.

The slot technology is known as "transmissive reels", which is basically an LCD with the back removed, and the backlighting comes from the general lighting that also lights the reels. It is correct that WMS patented and licensed this technology.

Heighways first game, Circe's Animal House had a concept with graphics projected onto the glass, I recall it damn near blinding the player in order to get enough light to get a clear picture. Though with the advent of pinstadium and stuff, I imagine players' eyes are gradually adjusting to greater amounts of light on a playfield Sonny_Jim can tell you more about it.

-- Jimmy

Ahh, what EalaDubhSidhe said!