Auto White Balance

Discussion in 'Digital Photography' started by pixel_a_ted, Nov 22, 2006.

  1. pixel_a_ted

    pixel_a_ted Guest

    What does a camera auto white balance do when it encounters a scene
    that is basically all one color, or maybe a couple of colors but no
    white in the scene? What assumptions are made in order to set the white
    balance under these circumstances?

    Thanks.
     
    pixel_a_ted, Nov 22, 2006
    #1
    1. Advertising

  2. "pixel_a_ted" <> wrote in message
    news:...
    > What does a camera auto white balance do when it encounters a scene
    > that is basically all one color, or maybe a couple of colors but no
    > white in the scene? What assumptions are made in order to set the white
    > balance under these circumstances?


    Your guess is as good as mine, since the algorithms are proprietary.

    In principle, auto white balance can't work. It can't tell the difference
    between a pink shirt in white light and a white shirt in pink light.

    And it's meaningless if you are taking a picture of a light source, such as
    a sunset.

    David J. Littleboy
    Tokyo, Japan
     
    David J. Littleboy, Nov 22, 2006
    #2
    1. Advertising

  3. pixel_a_ted

    Ken Lucke Guest

    In article <>,
    pixel_a_ted <> wrote:

    > What does a camera auto white balance do when it encounters a scene
    > that is basically all one color, or maybe a couple of colors but no
    > white in the scene? What assumptions are made in order to set the white
    > balance under these circumstances?
    >
    > Thanks.
    >


    Often faulty ones.

    Highly informative information here:

    http://www.cambridgeincolour.com/tutorials/white-balance.htm

    and I especially recommend:

    http://ronbigelow.com/articles/white/white_balance.htm

    (as well as many of his other articles)
     
    Ken Lucke, Nov 22, 2006
    #3
  4. pixel_a_ted wrote:
    > What does a camera auto white balance do when it encounters a scene
    > that is basically all one color, or maybe a couple of colors but no
    > white in the scene? What assumptions are made in order to set the white
    > balance under these circumstances?
    >
    > Thanks.
    >


    I just remain aware of my environment. If I am indoors, I look around
    at the lights. If they are tungsten or florescent, then I set the WB to
    the appropriate setting.

    If I am outdoors, I just check the lighting...sunlight, shade, or cloudy.

    Indoors is easy...the light doesn't change. Outdoors you can go from
    sun to cloudy to shade and you have to remember to change to the
    appropriate setting.


    One other thing unrelated to White Balance, I started shooting in
    Aperture-Priority mode exclusively. This was a suggestion by a
    professional photographer. I checked my old pictures and there are too
    many distracting objects in the background. Now when I look at my new
    pictures, I only see the subject that I want other people to concentrate
    upon.

    Michael
     
    Michael Calverley, Nov 22, 2006
    #4
  5. "David J. Littleboy" <> wrote in message
    news:ek09c0$dpo$...
    SNIP
    > In principle, auto white balance can't work. It can't tell the
    > difference between a pink shirt in white light and a white shirt in
    > pink light.


    In principle, if that is the only item in the image, you are correct.
    However, in practice it does a fair job of estimating an average White
    Point, as a *starting point* for user intervention, based on taste or
    on a controlled reference shot of a truely white subject under the
    same lighting.

    It achieves that by applying a heuristic complemented by "Robertson's
    algorithm":
    <http://www.brucelindbloom.com/index.html?Eqn_XYZ_to_T.html> . The
    heuristic basically takes some average of the x% brightest pixels
    (assuming whites or highlights, potentially excluding skin and or blue
    sky or grass colors) converted to XYZ colorspace, and by using
    Robertson's algorithm, deriving the color temperature to neutralize a
    white object.

    > And it's meaningless if you are taking a picture of a light source,
    > such as a sunset.


    I wouldn't go as far as calling it meaningless, but rather as a stable
    starting point for further user intervention. Taking it as a given
    would be rather meaningless rather soon, indeed.

    --
    Bart
     
    Bart van der Wolf, Nov 23, 2006
    #5
  6. "Bart van der Wolf" <> wrote:
    > "David J. Littleboy" <> wrote:
    > SNIP
    >> In principle, auto white balance can't work. It can't tell the difference
    >> between a pink shirt in white light and a white shirt in pink light.

    >
    > In principle, if that is the only item in the image, you are correct.
    > However, in practice it does a fair job of estimating an average White
    > Point, as a *starting point* for user intervention, based on taste or on a
    > controlled reference shot of a truely white subject under the same
    > lighting.


    Yes. But even as a starting point it's still a rough guess. Consider a room
    with pale blue walls vs. a room with off-white walls both lit by warm late
    afternoon sun through a large window.

    > It achieves that by applying a heuristic complemented by "Robertson's
    > algorithm":
    > <http://www.brucelindbloom.com/index.html?Eqn_XYZ_to_T.html> . The
    > heuristic basically takes some average of the x% brightest pixels
    > (assuming whites or highlights, potentially excluding skin and or blue sky
    > or grass colors) converted to XYZ colorspace, and by using Robertson's
    > algorithm, deriving the color temperature to neutralize a white object.


    Hehe. Stop confusing us with real knowledge<g>.

    Still, I object when people complain that such and such a camera has white
    balance problems. Similarly for matrix metering. Both AWB and matrix AE are
    guesses that are guaranteed to fail some of the time.

    >> And it's meaningless if you are taking a picture of a light source, such
    >> as a sunset.

    >
    > I wouldn't go as far as calling it meaningless, but rather as a stable
    > starting point for further user intervention. Taking it as a given would
    > be rather meaningless rather soon, indeed.


    I'll stand firm on this one. For shots like this

    http://www.pbase.com/davidjl/image/59368818/large

    I want to start from a "neutral" white balance that simply reproduces what's
    there. AWB is very much the wrong thing to be doing here.

    David J. Littleboy
    Tokyo, Japan
     
    David J. Littleboy, Nov 23, 2006
    #6
    1. Advertising

Want to reply to this thread or ask your own question?

It takes just 2 minutes to sign up (and it's free!). Just click the sign up button to choose a username and then you can ask your own questions on the forum.
Similar Threads
  1. jeff liss
    Replies:
    1
    Views:
    509
    Andrew
    Sep 5, 2003
  2. Ralph O'Rourke

    How does AWB (auto white balance) work?

    Ralph O'Rourke, Sep 11, 2003, in forum: Digital Photography
    Replies:
    3
    Views:
    849
    Jimmy
    Sep 12, 2003
  3. Radio913

    Auto White Balance Affected RAW file in Powershot G3!

    Radio913, Sep 17, 2003, in forum: Digital Photography
    Replies:
    4
    Views:
    514
  4. Jan Werbiñski

    White Balance - auto or custom?

    Jan Werbiñski, Oct 4, 2003, in forum: Digital Photography
    Replies:
    9
    Views:
    1,607
    MikeWhy
    Oct 12, 2003
  5. Steve Cutchen

    Custom White Balance: Gray Card or White Card?

    Steve Cutchen, Oct 21, 2005, in forum: Digital Photography
    Replies:
    31
    Views:
    1,622
Loading...

Share This Page