Exactly how does AWB work?

Discussion in 'Digital Photography' started by Bob Williams, Dec 21, 2006.

  1. Bob Williams

    Bob Williams Guest

    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
     
    Bob Williams, Dec 21, 2006
    #1
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  2. Bob Williams

    Guest

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  3. Bob Williams

    Bob Williams Guest

    Bob Williams, Dec 22, 2006
    #3
  4. Bob Williams

    Matt Ion Guest

    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/tutorials/white-balance.htm
     
    Matt Ion, Dec 22, 2006
    #4
  5. Bob Williams

    bugbear Guest

    bugbear, Dec 22, 2006
    #5
  6. Bob Williams

    Matt Ion Guest

    bugbear wrote:
    > Matt Ion wrote:
    >
    >>
    >> http://www.cambridgeincolour.com/tutorials/white-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.
     
    Matt Ion, Dec 23, 2006
    #6
  7. Bob Williams

    Matt Ion Guest

    bugbear wrote:
    > Matt Ion wrote:
    >
    >>
    >> http://www.cambridgeincolour.com/tutorials/white-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.
     
    Matt Ion, Dec 23, 2006
    #7
  8. Bob Williams

    Bob Williams Guest

    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
     
    Bob Williams, Dec 23, 2006
    #8
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