Bloom is a computer graphics effect used in video games, demos, and high dynamic range rendering to reproduce an imaging artifact of real-world cameras. The effect produces fringes of light extending from the borders of bright areas in an image, contributing to the illusion of an extremely bright light overwhelming the camera or eye capturing the scene. It became widely used in video games after an article on the technique was published by the authors of Tron 2.0 in 2004.
Theory
The physical basis of bloom is that, in the real world, lenses can never focus perfectly. Even a perfect lens will convolve the incoming image with an Airy disk. Under normal circumstances, these imperfections are not noticeable, but an intensely bright light source will cause the imperfections to become visible. As a result, the image of the bright light appears to bleed beyond its natural borders. The Airy disc function falls off very quickly but has very wide tails. As long as the brightness of adjacent parts of the image are roughly in the same range, the effect of the blurring caused by the Airy disc is not particularly noticeable; but in parts of the image where very bright parts are adjacent to relatively darker parts, the tails of the Airy disc become visible, and can extend far beyond the extent of the bright part of the image. In HDRR images, the effect can be re-produced by convolving the image with a windowed kernel of an Airy disc, or by applying Gaussian blur, before converting the image to fixed-range pixels. The effect cannot be fully reproduced in non-HDRR imaging systems, because the amount of bleed depends on how bright the bright part of the image is. As an example, when a picture is taken indoors, the brightness of outdoor objects seen through a window may be 70 or 80 times brighter than objects inside the room. If exposure levels are set for objects inside the room, the bright image of the windows will bleed past the window frames when convolved with the Airy disc of the camera being used to produce the image.
Some of the earliest games to use the bloom effect include Ico and Outcast. Bloom was later popularized within the game development community in 2004, when an article on the technique was published by the authors of Tron 2.0. Bloom lighting has been used in many games, modifications and game engines such as Quake Live, and the Spring game engine. The effect was popular in 7th generation games, and was used heavily in PC, Xbox 360 and PlayStation 3 games as well as Nintendo GameCube and Wii. Games such as the MMORPGRuneScape and games in the Hitman series have made use of the bloom effect as well.
