BLOOM (SHADER EFFECT)

'Bloom' (sometimes referred to as 'light bloom') is a computer graphics effect used in computer games and high dynamic range rendering (HDR) to reproduce an imaging artifact of real-world cameras. The effect produces fringes (or feathers) of light around very bright objects in an image.
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 disc (the diffraction pattern produced by passing a point light source through a circular aperture) ^[1]. Under normal circumstances, these imperfections aren't 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 (actually, infinitely 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 HDR images, the effect can be re-produced by convolving the image with a windowed kernel of an Airy disc (for very good lenses), or by applying Gaussian blur (to simulate the effect of a less perfect lens), before converting the image to fixed-range pixels. The effect can't be fully reproduced in non-HDR imaging systems, because the amount of bleed depends on how bright the bright part of the image is.

As an example, if 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, windows will be bright enough, when convolved with the Airy disc of the camera being used to produce the image, to cause the image of the windows to bleed past the frames of the window.
The effect is popular in current generation games, and is used heavily in Xbox 360 and Playstation 3 games. Current generation gaming systems are able to render 3D graphics using floating point frame buffers, in order to produce HDR images. To produce the bloom effect, the HDR images in the frame buffer are convolved with a convolution kernel in a post-processing step, before converting to RGB space.
The Afterimage effect is sometimes used to enhance the Bloom effect.
Bloom became very popular after ''Tron 2.0'' and is used in many games, modifications, and game engines, including ''Sauerbraten'', ''Nexuiz'', and the OGRE Rendering Engine. ''Ico'' was one of the earliest games to use the bloom effect.^[2]
Bloom lighting is now sometimes considered overused and clichéd, reminiscent of the overusage of lens flare effects in early 3D-accelerated computer games.
Bloom also refers the unrelated effect that is produced when a CCD camera is pointed at a bright light. Bright lights will temporarily oversaturate the CCD, producing oddly colored light trails as the image of the bright light moves across the CCD imaging device.

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References

1. Airy disk
2. Bittersweet Symphony

See also

★ High dynamic range rendering

★ Tone mapping