Multiple elements or plates combined into single shot in post — VFX, green screen, matte painting. Shoot clean on set, assemble later.
On set, you quickly realize when a composite shot is sensible: the actor is standing in front of an impossible background situation — be it a historical building that doesn't exist, or a scene where the performers and the environment could never be in front of the camera simultaneously. Instead of shooting everything on location, you film the elements separately and combine them later. This not only saves production days but also the logistical nightmares of location scouting and permits.
The technical implementation proceeds in several layers. Green screen is the classic method — the actor is filmed in front of a monochrome wall, with the background plate shot separately or digitally generated. In editing, the keying process is used: the green color becomes transparent, and the performer suddenly finds themselves in the cockpit of an airplane or on a mountaintop. In parallel, you work with rotoscoping if the edges aren't clean enough — meaning masks that are set frame-by-frame to trace movements. This is time-consuming but necessary for photorealistic results. Some DoPs also work with reflections and light spill to make the integration even more convincing: if the performer is illuminated by the background, the composition doesn't look like a cutout from two different worlds.
On set itself, the rule is: the more consistent your lighting on the performer — color temperature, angle, shadows — the less post-production is needed. Some DoPs even shoot reference spheres on the green screen set so that the VFX supervisors can later see how the lights actually were. The distance to the green wall is also critical — too close, and you get spill, meaning green light on the hair; too far away, and the perspective plausibility suffers.
The disadvantages are real: real backgrounds have atmospheric perspective, particles, depth of field. A digitally created or subsequently composited plate can lack all these qualities. That's why many films work with hybrid methods — real backgrounds, but with the performer filmed in isolation, or even physical partial sets combined with digital extensions. The ideal is often invisibility — the viewer should never notice that it's composited.