I found the ruling to be an interesting read which went to lengths to distinguish idea from expression. It even referenced an older case of Atari vs. Amusement World (Asteroids vs. Meteors) which Atari lost:
It seems clear that defendants based their game on plaintiff's
copyrighted game; to put it bluntly, defendants took plaintiff's
idea. However, the copyright laws do not prohibit this. Copyright
protection is available only for expression of ideas, not for ideas
themselves. Defendants used plaintiff's idea and those portions of
plaintiff's expression that were inextricably linked to that idea.
The remainder of defendants' expression is different from plaintiff's
expression. Therefore, the Court finds that defendants' "Meteors"
game is not substantially similar to and is not an infringing
copy of plaintiff's "Asteroids" game.
The Munchkin ruling also references an older case and quotes:
Thus, "if the only similarity between plaintiff's and defendant's
works is that of the abstract idea, there is an absence of substantial
similarity and hence no infringement results."
It later goes on to say:
Plaintiffs' audiovisual work is primarily an unprotectible game, but
unlike the bee pin, to at least a limited extent the particular
form in which it is expressed (shapes, sizes, colors, sequences,
arrangements, and sounds) provides something "new or additional
over the idea."
This was to be North American's downfall:
North American not only adopted the same basic characters but also
portrayed them in a manner which made K. C. Munchkin appear
substantially similar to PAC-MAN. The K. C. Munchkin gobbler
has several blatantly similar features, including the relative
size and shape of the "body," the V-shaped "mouth," its distinctive
gobbling action (with appropriate sounds), and especially the way in
which it disappears upon being captured. An examination of the K. C.
Munchkin ghost monsters reveals even more significant visual
similarities. In size, shape, and manner of movement, they are virtually
identical to their PAC-MAN counterparts. K. C. Munchkin's monsters,
for example, exhibit the same peculiar "eye" and "leg" movement.
Both games, moreover, express the role reversal and "regeneration"
process with such great similarity that an ordinary observer
could conclude only that North American copied plaintiffs' PAC-MAN.
Had they only copied the idea the case would have gone the way of the Asteroids vs. Meteors case. Where they went wrong is that they copied the expressions used to such a degree that it was found to be infringing their copyright.
Yep. And in the case of Battlezone, a game that is about its appearance, I would argue that the expression can't be separated from the abstract idea. But I doubt we'll ever see a court's opinion on that.
To me, the abstract idea is a 3D wireframe world where you accumulate points and avoid being shot by wireframe enemies, much in the same way that the Munchkin ruling says:
Other games, such as "Rally-X" (described in Dirkschneider)
and North American's own "Take the Money and Run," illustrate
different ways in which a basic maze-chase game can be expressed.
The idea could still have been a 3D wireframe world where things try and shoot you, but it didn't have to be tanks. Atari's own Star Wars: Empire Strikes Back from 1985 shows how the idea can have a radically different expression.
http://scholar.google.com/scholar_case?case=6441518363892064...
A notable quote from that case:
The Munchkin ruling also references an older case and quotes: It later goes on to say: This was to be North American's downfall: Had they only copied the idea the case would have gone the way of the Asteroids vs. Meteors case. Where they went wrong is that they copied the expressions used to such a degree that it was found to be infringing their copyright.