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regardless of platform, which was the first that did '3d' effects?

it depends what you consider as 3d i guess..

3d starfields have been around for as long as there are demos/intros (well, more or less) :)

Speedway by Panic

universe by god in the seven days democompo

101 dalmatians from disney in 1961 :D

oh wait it's not a demo it's a movie

Depends on what you call '3d effect', an effect using 3d perspective (it could be just dots) or the usual rotating 3d objects with polyfiller or wireframe mode.

The oldest I can remember on Atari ST is this TNT Demo, made in 1987. It features a 3d computer in wireframe (realtime afair, not some precomputed animation).
http://www.pouet.net/prod.php?which=22933


An amazing breakthrough on Atari ST was the 3D part in the Union demo (january 1989) that featured complex (concave) 3D objects
http://www.pouet.net/prod.php?which=11060


There is some earlier realtime 3d stuff (not the usual precomputed Boink or Juggler demos) on other platforms but I just can't remember. If anyone has examples, that's an interesting topic :)

I found that one Elite 1983 on Acorn BBC B
http://www.thelegacy.de/Museum/4252/

yeah but it's a game, not a demo :)

http://www.pouet.net/prod.php?which=4077
as for PC i think this one :)