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Exactly how does AWB work?
I think I understand how custom WB works wherein one displays a white
card to the camera in the same light as the subject. This "tells" the camera to make all areas of the subject that look exactly like the test card, appear white The camera must then adjust the gain of R,G,and B amplifiers so the output of each color is equal ,e.g. 255,255,255. The assumption is then made that if White is correct, then all colors are correct. Even if the camera sees a surface that reflects pretty close to equal amounts of RGB, e.g. somewhere around 128,128,128, it must assume that it is being shown a color-unbiased gray card and adjusts the amplifier gains accordingly. But what does the camera "home in" on when it is in Auto WB Mode? Does it assume that a typical scene "averages" out as medium gray? I think not because I've taken LOTS of flower pictures where the scene is very strongly biased toward a single color. So, how does the camera know where to set the color balance in AWB Mode? Bob Williams |
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Exactly how does AWB work?
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#4
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Exactly how does AWB work?
Bob Williams wrote:
I think I understand how custom WB works wherein one displays a white card to the camera in the same light as the subject. This "tells" the camera to make all areas of the subject that look exactly like the test card, appear white The camera must then adjust the gain of R,G,and B amplifiers so the output of each color is equal ,e.g. 255,255,255. The assumption is then made that if White is correct, then all colors are correct. Even if the camera sees a surface that reflects pretty close to equal amounts of RGB, e.g. somewhere around 128,128,128, it must assume that it is being shown a color-unbiased gray card and adjusts the amplifier gains accordingly. But what does the camera "home in" on when it is in Auto WB Mode? Does it assume that a typical scene "averages" out as medium gray? I think not because I've taken LOTS of flower pictures where the scene is very strongly biased toward a single color. So, how does the camera know where to set the color balance in AWB Mode? Bob Williams http://www.cambridgeincolour.com/tut...te-balance.htm |
#5
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Exactly how does AWB work?
Matt Ion wrote:
http://www.cambridgeincolour.com/tut...te-balance.htm Wow. Great science, clearly explained. I can see much reading ahead of me on the rest of that site. BugBear |
#6
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Exactly how does AWB work?
bugbear wrote:
Matt Ion wrote: http://www.cambridgeincolour.com/tut...te-balance.htm Wow. Great science, clearly explained. I can see much reading ahead of me on the rest of that site. Yeah, I actually just found that site in a Google search, but I've bookmarked it now... quite well-done. |
#7
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Exactly how does AWB work?
bugbear wrote:
Matt Ion wrote: http://www.cambridgeincolour.com/tut...te-balance.htm Wow. Great science, clearly explained. I can see much reading ahead of me on the rest of that site. By the way, one trick I've read of (haven't tried it yet, but it sounds sensible) for a custom-WB target, is to just snap the lid of a Pringles can or something similar over your lens - it's a pretty even white, and translucent enough to be well-backlit. Note that the target used doesn't have to be bright-white, as WB is looking only at the BALANCE of color, not the brightness of it (thus the name); all that's required is an even area of an even mix of all colors. Most common is a light grey, which is less likely to overwhelm the camera in bright light, and also less likely to show dirt, smudges, etc. that could throw the reading off. |
#8
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Exactly how does AWB work?
Bob Williams wrote: I think I understand how custom WB works wherein one displays a white card to the camera in the same light as the subject. This "tells" the camera to make all areas of the subject that look exactly like the test card, appear white The camera must then adjust the gain of R,G,and B amplifiers so the output of each color is equal ,e.g. 255,255,255. The assumption is then made that if White is correct, then all colors are correct. Even if the camera sees a surface that reflects pretty close to equal amounts of RGB, e.g. somewhere around 128,128,128, it must assume that it is being shown a color-unbiased gray card and adjusts the amplifier gains accordingly. But what does the camera "home in" on when it is in Auto WB Mode? Does it assume that a typical scene "averages" out as medium gray? I think not because I've taken LOTS of flower pictures where the scene is very strongly biased toward a single color. So, how does the camera know where to set the color balance in AWB Mode? Bob Williams After Googling on a bunch of sites explaining Automatic White Balance, it appears that it is a very complicated subject. Apparently different manufacturers use different proprietary algorithms to accomplish AWB. Some companies do it better than others. One method that seems reasonable to me is for the camera to look for the brightest group of pixels in the entire image. In most images the brightest spot will be a specular highlight, i.e. a direct reflection of the light source itself. These can be found, for instance, reflected in the eye of a person or animal or from a shiny curved surface as in jewelry. If there ARE specular highlights, the camera can tell from the color distribution in the highlight, the color temperature of the light source. Now, for a uniform matte surface, there probably would not be ANY specular highlights. So some other method (algorithm) must be used. If the matte surface is extremely biased to one color and has no specular highlights, I have no clue how a correct AWB can be created. Yet from my own experience I have taken many macro shots of leaves or flowers in which the image is extremely biased toward a single color and the results were quite accurate. Perhaps, in real world subjects, unnoticed by me, there were a few little specular reflections that the camera recorded and from which it could determine the color distribution. An interesting discussion on AWB is the following site. I don't follow all the math but the concepts are pretty clear. See: www.iis.sinica.edu.tw/JISE/2006/200605_02.pdf Bob Williams |
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