Two actors, A and B, face each other; the imaginary line drawn through both of them is the axis of action (the "180° line"). A director must keep every camera in the same 180° half of the circle around that line for a scene to cut together cleanly:
screen_dir(camera) = sign( cross(B−A, camera−A) )
+1 → actor A reads screen-left → B
−1 → actor A reads screen-right → B
Both actors' apparent screen position and eyeline direction flip the instant a camera crosses to the far side of that line. Cutting between a shot from each side ("crossing the line") makes the audience feel the actors have swapped places or are suddenly facing the same way, breaking spatial continuity.
- Angle slider — swings the camera in a full circle around the two actors, 0°–360°.
- Distance slider — dolly in/out; distance doesn't affect which side of the line you're on, only framing.
- Shoot This Angle — commits the current camera as a storyboard frame; two consecutive shots from opposite sides are flagged as a line-cross error.
- Side A / Side B presets — jump to a canonical safe angle on each half, mirroring how a real 180° diagram is drawn on set.
This is the actual rule taught in every film-school cinematography course and enforced on set by the script supervisor — it is why shot/reverse-shot dialogue scenes read as spatially coherent instead of disorienting.