Image rendering is when an image is created via a computer from a computer model that contains all the information the image is to contain, including shadows and textures. In video rendering, this refers to the generation, via computer, of each individual frame, which when strung together form a fluid video. This is used primarily for the application of special effects. For example, if a special filter is added to a video using a computer software program, then the video must be rendered by the computer, with each frame generated and compacted into a video output.
Rendering, as we've seen, is used with videos for applying visual effects in post production and enhancing certain imagery via simulation. For example, a scene shot in a cemetery can be given a darker, gloomier atmosphere through the application of a digital filter that may darken the frames and a digital effect that adds a distinctive fog to the scene, all of which must be rendered frame-by-frame. There are many features that may be rendered digitally, including some that are almost universal among modern films.
In modern film, visual effects are found almost universally. Even films that are not heavy on special effects employ some type of digital alteration, whether it be color grading or title sequences. Other effects that must be rendered include shading, addition of shadows, addition of motion blurs and non-photorealistic effects, which are used to replace a photographic image with an abstract visualization that literally warps the image, turning the characters into cartoons or paint-like images. This technique can be seen in the Keanu Reeves film, A Scanner Darkly, whereas each frame is posterized, meaning the details are washed away, resulting in a cartoon-like picture.
Some films and videos are almost entirely digital, meaning that every element in the films must be digitally simulated via rendering. These films--like Toy Story, Shrek and Wall-E--are composed almost entirely of pre-rendered frames strung together into a working film. Rendering each frame allows for much greater addition of detail, while at the same time makes these type of films extremely tedious to make. Toy Story, for example, the very first feature film that was fully animated using CGI, required over 114,000 frames, with each individual one taking an average of eight hours to render. While modern technology has sped up the process, even more recent films, like 2009's Up, took over five years to create.
Rendering used for a variety of applications, including many outside of film. Modern videogames use real-time rendering to render the images on the screen in real time, with machines like the Xbox360 and Playstation 3 able to render extremely detailed images in comparison to consoles a decade ago. Rendering is also found in architecture and simulation programs.