What are Render Passes?

When creating computer-generated imagery

Computer-generated imagery

Computer-generated imagery is the application of the field of computer graphics or, more specifically, 3D computer graphics to special effects in art, video games, films, television programs, commercials, simulators and simulation generally, and printed media...

 or 3D computer graphics

3D computer graphics

3D computer graphics are graphics that use a three-dimensional representation of geometric data that is stored in the computer for the purposes of performing calculations and rendering 2D images...

, final scenes appearing in movies and television productions are usually produced by Rendering (computer graphics)

Rendering (computer graphics)

Rendering is the process of generating an image from a model , by means of computer programs. A scene file contains objects in a strictly defined language or data structure; it would contain geometry, viewpoint, texture, lighting, and shading information as a description of the virtual scene...

 more than one "layer" or "pass," which are multiple images designed to be put together through digital compositing

Compositing

Compositing is the combining of visual elements from separate sources into single images, often to create the illusion that all those elements are parts of the same scene. Live-action shooting for compositing is variously called "chroma key", "blue screen", "green screen" and other names. Today,...

 to form a completed frame.

Rendering in passes is based on a traditions in Motion control photography

Motion control photography

Motion control photography is a technique used in still and motion photography that enables precise control of, and optionally also allows repetition of, camera movements. It can be used to facilitate special effects photography. The process can involve filming several elements using the same...

which pre-date CGI. As an example, in motion control photography for a visual effects shot, a camera could be programmed to move past a physical model of a spaceship in one pass to film the fully lit beauty pass of the ship, and then to repeat exactly the same camera move passing the ship again to photograph additional elements such as the illuminated windows in the ship or its thrusters. Once all of the passes were filmed, they could then be optically printed together to form a completed shot.

The terms "Render Layers" and "Render Passes" are sometimes used interchangeably. However, rendering in layers refers specifically to separating different objects into separate images, such as a foreground characters layer, a sets layer, a distant landscape layer, and a sky layer. Rendering in passes, on the other hand, refers to separating out different aspects of the scene, such as shadows, highlights, or reflections, each into a separate image.