Pre-rendering is the process in which video footage is not rendered in real-time by the hardware that is outputing or playing back the video. Instead, the video is a recording of a footage that was previously rendered on a different equipment (typically one that is more powerful than the hardware used for playback). Pre-rendered assets (typically movies) may also be outsourced by the developer to an outside production company. Such assets usually have a level of complexity that is too great for the target platform to render in real-time.
The term pre-rendered describes anything that is not rendered in real-time. This includes content that could have been run in real-time with more effort on the part of the developer (e.g. video that covers a large number of a game's environments without pausing to load, or video of a game in an early state of development that is rendered in slow-motion and then played back at regular speed). The term is generally not used to describe video captures of real-time rendered graphics despite the fact that video is technically prerendered by its nature. The term is also not used to describe hand drawn assets or photographed assets (these assets not being computer rendered in the first place).
The advantage of pre-rendering is the ability to use graphic models that are more complex and computationally intensive than what can be rendered in real-time, due to the possibility of using multiple computers over extended periods of time to render the end results.
The disadvantage of pre-rendering, in the case of video game graphics, is a generally lower level of interactivity, if any, with the player. Another negative side of pre-rendered assets is that changes cannot be made during gameplay. A game with pre-rendered backgrounds is forced to use fixed camera angles, and a game with pre-rendered video generally cannot reflect any changes the game's characters might have undergone during gameplay (such as wounds or customized clothing) without having an alternate version of the video stored. This is generally not feasible due to the large amount of space required to store pre-rendered assets of high quality. However, in some advanced implementations, such as in Final Fantasy VII, real-time assets were composited with pre-rendered video, allowing dynamic backgrounds and changing camera angles.
Another problem is that a game with pre-rendered lighting cannot easily change the state of the lighting in a convincing manner.
The use of pre-rendered backgrounds and movies also was made popular by the Resident Evil and Final Fantasy franchises on the original PlayStation, both of which use prerendered backgrounds and movies extensively to provide a visual presentation that is far greater than the console can provide with real-time 3D. These games include real-time elements (characters, items, etc.) in addition to pre-rendered backgrounds to provide interactivity. Often a game using pre-rendered backgrounds can devote additional processing power to the remaining interactive elements resulting in a level of detail greater than the norm for the host platform. In some cases the visual quality of the interactive elements is still far behind the prerendered backgrounds.
Recently Prerendered graphics are becoming rare in games with stronger real time rendering ability, making prerendered graphics unneeded.
CG movies such as Toy Story, Shrek and Final Fantasy: The Spirits Within are entirely pre-rendered. Pre-rendered graphics are used primarily as cut scenes in modern video games, where they are also known as full motion video.
Another increasingly common pre-rendering method is the generation of texture sets for 3D games, which are often used with complex real-time algorithms to simulate extraordinarily high levels of detail. While making Doom 3, id Software used pre-rendered models as the basis for generating normal, specular and diffuse lighting maps that simulate the detail of the original model in real-time.
Pre-rendered lighting is a technique that is losing popularity. Processor-intensive ray tracing algorithms can be used during a game's production to generate light textures, which are simply applied on top of the usual hand drawn textures.