Auto White Balance

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" wrote in message
ups.com...
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

In article . com,
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 he

http://www.cambridgeincolour.com/tut...te-balance.htm

and I especially recommend:

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

(as well as many of his other articles)

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

"David J. Littleboy" wrote in message
...
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" 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 knowledgeg.

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