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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 pre-rendered 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).
Advantage and disadvantage
The advantage of pre-rendering is the ability to use graphic models that are more complex and computationally intensive than those that can be rendered in real-time, due to the possibility of using multiple computers over extended periods of time to render the end results. For instance, a comparison could be drawn between rail-shooters Maximum Force (which used pre-rendered 3D levels but 2D sprites for enemies) and Virtua Cop (using 3D polygons); Maximum Force was more realistic looking due to the limitations of Virtua Cop's 3D engine, but Virtua Cop has actual depth (able to portray enemies close and far away, along with body-specific hits and multiple hits) compared to the limits of the 2D sprite enemies in Maximum Force.
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 VIII, 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.
Pre-rendered graphics are used primarily as cut scenes in modern video games, where they are also known as full motion video. In the late 1990s and early 2000s, when most 3D game engines had pre-calculated/fixed Lightmaps and texture mapping, developers often turned to pre-rendered graphics which had a much higher level of realism. However this has lost favor since the mid-2000s, as advances in consumer PC and video game graphics have enabled the use of the game's own engine to render these cinematics. For instance, the id Tech 4 engine used in Doom 3 allowed bump mapping and dynamic per-pixel lighting, previously only found in pre-rendered videos.
One of the first games to use pre-rendering was the Sharp X68000 enhanced remake of Ys I: Ancient Ys Vanished released in 1991. It used 3D pre-rendered graphics for the boss sprites, though this ended up creating what is considered "a bizarre contrast" with the game's mostly 2D graphics. One of the first games to extensively use pre-rendered graphics along with full motion video was The 7th Guest. Released in 1992 as one of the first PC games exclusively on CD-ROM, the game was hugely popular, although reviews from critics were mixed. The game featured pre-rendered video sequences that were at a resolution of 640x320 at 15 frames per second, a feat previously thought impossible on personal computers. Shortly after, the release of Myst in 1993 made the use of pre-rendered graphics and CD-ROMs even more popular; interestingly most of the rendered work of Myst would later be the basis for the re-make realMyst: Interactive 3D Edition with its free-roaming real-time 3D graphics. The most graphically advanced use of entirely pre-rendered graphics in games is often claimed to be Myst IV: Revelation, released in 2004.
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 pre-rendered 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 pre-rendered backgrounds.
Games such as Warcraft III: Reign of Chaos have used both types of cutscenes; pre-rendered for the beginning and end of a campaign, and the in-game engine for level briefings and character dialogue during a mission.
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.