How does auto white balance work, and how reliable is it?
Asked 12/9/2010
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I understand that auto white balance (AWB) tries to estimate the correct color balance for a scene, but I’m not sure how a camera can determine that from the image alone. Does it compare the RGB channels or use assumptions about what parts of the scene should be neutral? How is AWB typically implemented in modern cameras, and in what situations does it work well or fail?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
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The original assumption is that the average scene should be color neutral and therefore by computing the average color in the scene and then applying the same correction to every pixel you would get a scene whose average color is neutral which should have the correct white-balance. This will fail when there is a dominant color and the scene.
Algorithms got more sophisticated over the years with lots of technical papers and patents written on the subject. They added more intelligence like clamping to the set of known illuminants.
The exact algorithm differs between cameras and it seems to work extremely well outdoors during the day, where there is little variation. Under artificial light there is much more variance and it is rather hit or miss. Older digital cameras were particularly bad but it has been improving on average.
The very best white-balance performance I've ever seen was on the HP Photosmart R967. DC Resource noticed this and commented that they should win the Nobel prize! Several recent compact cameras also do an excellent job. The advantage of a mirrorless camera over a DSLR for this is that it can read data from all over the sensor. DSLRs can now do that in Live-View mode.
Some DSLRs use an entirely different approach which is to measure white-balance instead. This is the case for the Olympus E-5. It has a dedicated 'external' sensor which measures the light falling on the camera. You can turn this off for cases when you are shooting from a different lighting than your subject.
Originally by user1620. Source · Licensed CC BY-SA 4.0
user1620
15y ago
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Auto white balance doesn’t truly “know” the correct setting—it estimates it from the scene. A common approach is to assume the overall scene, or the brightest areas, should be roughly neutral (white/gray), then shift color so those areas look neutral. More advanced systems add rules such as limiting results to likely light sources or using extra scene analysis, and exact methods vary by camera maker and model.
AWB usually works very well in consistent daylight and many normal scenes. It becomes less reliable under artificial lighting, where light sources vary more, or in scenes dominated by one strong color. For example, a photo of a blue wall may be interpreted as a neutral wall under blue light, so the camera may remove the blue cast incorrectly.
So yes, AWB is essentially making an educated guess from image data, sometimes helped by other sensors depending on the camera. In practice it is often close, sometimes spot-on, and occasionally very wrong. If you shoot RAW, white balance is easy to fine-tune later, which is why many photographers leave AWB on and correct only when needed.
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