It's among the first of your more rudimentary rules relating to the visual language of film.
Basically, if you have two objects interacting with each other (such as two actors in a conversation), you draw an imaginary line between them. In the case of two actors it's relatively easy because their line of sight would be the 180 degree line. Now your camera must choose either one side or the other for its vantage points. In a typical shot-reverse-shot sequence, you might have an over-the-shoulder vantage point behind each actor, but all your camera shots need to stay on that side of the line. Now as the scene progresses, hopefully your actors are moving around (either that or you have a really static scene), and you are constantly redrawing the line. The addition of still more actors walking into a scene results in more complicated spatial relationships, in which the positioning of the line may become more ambiguous or open to interpretation. And there are even numerous instances when you can 'supposedly' get away with crossing the line (though I couldn't tell you what those are, but as with everything else the most skilled directors are usually of the belief that rules are meant to be broken). With enough establishing shots, you can probably skip around the 180 degree line all you want. But do it incorrectly, and even the most untrained eye will recognize that 'something' is very wrong with the spacial continuity.
The rule also applies to moving objects. If your object is a car zooming past, then the road becomes your line. Break the movement into two shots (the car coming, the car going), and you still want to shoot from the same side of the road in both shots. Needless to say, the classic TOS main title sequences -with the Enterprise whizzing past- are a very obvious example of such shots. But whereas 'The Cage' frequently violates the 180 line during its opening/closing titles (and I don't recall whether this is still true with the remastered effects or not), the 1st and 2nd/3rd season TOS titles execute it properly (if pedestrianly).
(I'm not even sure how it was possible to break the line in the first place... wasn't the TOS Enterprise model supposed to be incomplete on one side? Maybe the film got flipped.)
With two starships facing off, the mutally-exchanged phaser fire (or the hypotetical trajectory thereof) makes up your line, so that a ship facing one way is seen from starboard and the other from port. This is unfortunately very elementary when it comes to traditional ST, as the creators seem virtually 'allergic' to imagining the possibility ships coming at each other from multiple plains or trajectories, resulting in some of the most choreographically unsophisticated 'horizon shot' face-offs space opera has to offer. Granted, the saucer-and-wing ships on ST probably don't lend themselves to being shot from very many angles... particularly in relation to each other. I often think it's because of ST that many other space operas (seemingly) go out of their way to avoid horizons and vanishing points in so many of their shots! But I suspect even with the 'cinema verite' documentary style of Galactica, that 180 lines had to be carefully established with each scene before they could be broken.
Um... sorry for this sounding like a lecture...
Last edited by samwiseb : 12-12-2011 at 04:40 AM.