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white balance

 
 
David J. Littleboy
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      06-23-2006

<(E-Mail Removed)> wrote in message
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> Thanks for all the answers.
> I do understand the advantage of using the RAW format to correct the WB
> afterwards, but how does the camera initially decide what color or
> white will be the real white?


By guessing randomly. It turns out that auto white balance is, _in
principle_, impossible.

A camera cannot tell the difference between a white shirt in pink light and
a pink shirt in white light.

Also note that for some things, e.g. sunsets, what you want is a completely
neutral color balance.

David J. Littleboy
Tokyo, Japan


 
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Bob Williams
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      06-23-2006


http://www.velocityreviews.com/forums/(E-Mail Removed) wrote:
> Could someone explain to me:
> - why should white balance adjustment be done by the camera and not by
> a software after the photo is taken. Do the captors behave differently
> depending on the detected color temperature, or is it purely a
> post-treatement?
> - how the white balance captors work and how you can "help" them to get
> the good measure.
>
> Thx
> (a D200/18-200 owner)
>

One reason that cameras make the WB is because many people do not have
photo editors (or don't know how to use them).
They print directly from their memory card
Without some type of automatic WB, many images would look awful even to
the unsophisticated eye of a novice user.
Bob Williams
 
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Dave Martindale
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      06-23-2006
"(E-Mail Removed)" <(E-Mail Removed)> writes:
>I do understand the advantage of using the RAW format to correct the WB
>afterwards, but how does the camera initially decide what color or
>white will be the real white?


It probably assumes that, on average, the scene is approximately grey in
colour. By averaging all the pixels in the scene (or in some smaller
white balance measurement area), then adjusting the relative gain of
red, green, and blue until the average colour of those pixels *is* grey,
you approximately compensate for the light colour.

Of course, this doesn't work well if your scene isn't approximately grey
on average, which is why fixed white balance settings are available, as
well as the ability to white balance on a particular white or grey
object and then save that setting.

Dave
 
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Malcolm Stewart
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      06-24-2006
"Dave Martindale" <(E-Mail Removed)> wrote in message
news:e7hu6s$o8d$(E-Mail Removed)...
>
> It probably assumes that, on average, the scene is approximately grey in
> colour. By averaging all the pixels in the scene (or in some smaller
> white balance measurement area), then adjusting the relative gain of
> red, green, and blue until the average colour of those pixels *is* grey,
> you approximately compensate for the light colour.


Using both Nikon and Canon equipment, I've found problems with both when
shooting predominantly green foliage in other than clear sunlight. Using
custom white balance has worked well with the camera getting a reference
from a grey subject*, but you've got to be aware of changing lighting
conditions as well.
(* I've actually found a use for my old 9 Expodisc.)

--
M Stewart
Milton Keynes, UK
http://www.megalith.freeserve.co.uk/oddimage.htm




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Alan Browne
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      06-24-2006
(E-Mail Removed) wrote:
> Could someone explain to me:
> - why should white balance adjustment be done by the camera and not by
> a software after the photo is taken. Do the captors behave differently
> depending on the detected color temperature, or is it purely a
> post-treatement?
> - how the white balance captors work and how you can "help" them to get
> the good measure.


I ignore camera white balance completely.

I adjust the color temperature per conditions in Kelvin (in 100
increments) and of course expose for desired effect.

Do the above and camera WB is meaningless.

Cheers,
Alan

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-- e-meil: Remove FreeLunch.
 
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Bart van der Wolf
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      06-25-2006

"Dave Martindale" <(E-Mail Removed)> wrote in message
news:e7hu6s$o8d$(E-Mail Removed)...
> "(E-Mail Removed)" <(E-Mail Removed)> writes:
>>I do understand the advantage of using the RAW format to
>>correct the WB afterwards, but how does the camera initially
>>decide what color or white will be the real white?

>
> It probably assumes that, on average, the scene is approximately
> grey in colour. By averaging all the pixels in the scene (or in
> some smaller white balance measurement area), then adjusting
> the relative gain of red, green, and blue until the average colour
> of those pixels *is* grey, you approximately compensate for the
> light colour.


In fact, there are more elaborate schemes than 'grey world' in use.

I'm not at liberty right now to discuss in much more detail, but
Robertson's algorithm
(http://www.brucelindbloom.com/Eqn_XYZ_to_T.html), allows to
characterize Kelvin temperatures for an accurately 'masked' image.

> Of course, this doesn't work well if your scene isn't approximately
> grey on average, which is why fixed white balance settings are
> available, as well as the ability to white balance on a particular
> white or grey object and then save that setting.


Correct, an approximately correct Auto-white balance will require to
mask-out non-contributing elements like sky, grass and human skin.
Even then, it'll require a well 'composed' image to allow reliable
approximation of Kelvin Color temperature.

Bart

 
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David J. Littleboy
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      06-25-2006
"Bart van der Wolf" <(E-Mail Removed)> wrote:
>
>> Of course, this doesn't work well if your scene isn't approximately
>> grey on average, which is why fixed white balance settings are
>> available, as well as the ability to white balance on a particular
>> white or grey object and then save that setting.

>
> Correct, an approximately correct Auto-white balance will require to
> mask-out non-contributing elements like sky, grass and human skin. Even
> then, it'll require a well 'composed' image to allow reliable
> approximation of Kelvin Color temperature.


Another irritation here is that the one setting I want, colorimetrically
neutral*, doesn't appear in any camera or software I've ever seen. There are
cases, e.g. sunsets and sunrises, where the whole concept of "white balance"
makes no sense at all.

*: Presumably there's a defined K temperature that corresponds to this.
Although it probably differs with the camera. (Hmm. If the K temperature
scale is calibrated correctly, the colorimetrically neutral setting should
be the same across cameras. But betting that RSP gets this right for
different cameras strikes me as unlikely.)

David J. Littleboy
Tokyo, Japan



 
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Scott W
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      06-25-2006
David J. Littleboy wrote:
> "Bart van der Wolf" <(E-Mail Removed)> wrote:
> >
> >> Of course, this doesn't work well if your scene isn't approximately
> >> grey on average, which is why fixed white balance settings are
> >> available, as well as the ability to white balance on a particular
> >> white or grey object and then save that setting.

> >
> > Correct, an approximately correct Auto-white balance will require to
> > mask-out non-contributing elements like sky, grass and human skin. Even
> > then, it'll require a well 'composed' image to allow reliable
> > approximation of Kelvin Color temperature.

>
> Another irritation here is that the one setting I want, colorimetrically
> neutral*, doesn't appear in any camera or software I've ever seen. There are
> cases, e.g. sunsets and sunrises, where the whole concept of "white balance"
> makes no sense at all.
>
> *: Presumably there's a defined K temperature that corresponds to this.
> Although it probably differs with the camera. (Hmm. If the K temperature
> scale is calibrated correctly, the colorimetrically neutral setting should
> be the same across cameras. But betting that RSP gets this right for
> different cameras strikes me as unlikely.)


I believe that sRGB using D65 as the illumination standard, and
sunlight is pretty close to D65 so setting the white balance to a good
white surface under sunlight would come close to what you are looking
for. And since you can cut and past WB setting in RSE you only need
one raw photo of a white surface to set others with.

Adobe RGB also uses D65 so this would work there as well.

I will admit I don't know all the ins and outs of these color spaces
so I might be missing something.

Scott

 
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Bart van der Wolf
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      06-25-2006

"David J. Littleboy" <(E-Mail Removed)> wrote in message
news:e7kqsm$9g4$(E-Mail Removed)...
> "Bart van der Wolf" <(E-Mail Removed)> wrote:

SNIP
>> Correct, an approximately correct Auto-white balance will require
>> to mask-out non-contributing elements like sky, grass and human
>> skin. Even then, it'll require a well 'composed' image to allow
>> reliable approximation of Kelvin Color temperature.

>
> Another irritation here is that the one setting I want,
> colorimetrically neutral*, doesn't appear in any camera or software
> I've ever seen.


It is possible to get quite an accurate White balance reading if you
include a reference in the image (or a test image for the sole purpose
of providing a reference for other images shot under the same lighting
conditions).
Tools like WhiBal
(http://www.rawworkflow.com/products/whibal/index.html) or the
BabelColor White target
(http://www.babelcolor.com/main_level/White_Target.htm) are very
usable.

> There are cases, e.g. sunsets and sunrises, where the whole concept
> of "white balance" makes no sense at all.


Well, since we usually want to convey a sensation rather than accurate
color balance, it's a valid procedure to deviate from correct into
pleasing color-balance. Having a 'correct' starting point takes a lot
of guesswork out of the equation, even if we deliberately want to
deviate form it.

> *: Presumably there's a defined K temperature that corresponds to
> this. Although it probably differs with the camera. (Hmm. If the K
> temperature scale is calibrated correctly, the colorimetrically
> neutral setting should be the same across cameras. But betting that
> RSP gets this right for different cameras strikes me as unlikely.)


RSE/RSP has an issue with the displayed number for Kelvin, when
compared to other methods of determination. White-balance clicking
works as it should though, so my suggestion is to include a reference
in one image and copy that balance (K + tint) to others taken under
similar conditions (also works with other Raw converters).

Bart

 
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David J. Littleboy
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      06-25-2006
"Bart van der Wolf" <(E-Mail Removed)> wrote:
> "David J. Littleboy" <(E-Mail Removed)> wrote:
>> "Bart van der Wolf" <(E-Mail Removed)> wrote:

> SNIP
>>> Correct, an approximately correct Auto-white balance will require to
>>> mask-out non-contributing elements like sky, grass and human skin. Even
>>> then, it'll require a well 'composed' image to allow reliable
>>> approximation of Kelvin Color temperature.

>>
>> Another irritation here is that the one setting I want, colorimetrically
>> neutral*, doesn't appear in any camera or software I've ever seen.

>
> It is possible to get quite an accurate White balance reading if you
> include a reference in the image (or a test image for the sole purpose of
> providing a reference for other images shot under the same lighting
> conditions).
> Tools like WhiBal (http://www.rawworkflow.com/products/whibal/index.html)
> or the BabelColor White target
> (http://www.babelcolor.com/main_level/White_Target.htm) are very usable.


In the cases under discussion here, I don't want the white balanced; I want
an _uninterpreted_ rendition of the color in the scene as seen by the
camera.

>> There are cases, e.g. sunsets and sunrises, where the whole concept of
>> "white balance" makes no sense at all.

>
> Well, since we usually want to convey a sensation rather than accurate
> color balance,


This is unlike you: you've missed the point. A sunset is a light source, so
"white balancing" makes no sense whatsoever. None, zilch, zero, nil.

> it's a valid procedure to deviate from correct into pleasing
> color-balance. Having a 'correct' starting point takes a lot of guesswork
> out of the equation, even if we deliberately want to deviate form it.


That's why I want to know what the "colorimetrically neutral" color
temperature is...

>> *: Presumably there's a defined K temperature that corresponds to this.
>> Although it probably differs with the camera. (Hmm. If the K temperature
>> scale is calibrated correctly, the colorimetrically neutral setting
>> should be the same across cameras. But betting that RSP gets this right
>> for different cameras strikes me as unlikely.)

>
> RSE/RSP has an issue with the displayed number for Kelvin, when compared
> to other methods of determination. White-balance clicking works as it
> should though, so my suggestion is to include a reference in one image and
> copy that balance (K + tint) to others taken under similar conditions
> (also works with other Raw converters).


Ah. That's interesting. So far I've been ignoring tint. Sounds as though I
can't. In the general case, assuming a completely arbitrary light source, a
single slider isn't enough, of course.

By the way, I've never seen "white balance clicking" (or any other clicking
with an eydropper, for that matter) do anything other than produce a
completely insane color (or white/black point, contrast, or whatever)
setting. So I don't click. Ever.

What I do is to find something close to a neutral gray in an image and
adjust the white balance manually while watching the RGB values. (Living in
Tokyo, there's usually a lot of neutral gray.)

Of course, much of the time, there will be parts of an image that are in
full sun, parts in open shade, and parts in deep shade, and each will need a
different color balance, so the final color balance almost always is a
compromise of some sort.

David J. Littleboy
Tokyo, Japan


 
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