Sven Neumann wrote:
> Hi,
> On Tue, 2007-11-27 at 22:04 +0100, Stephane Chauveau wrote:
>> Nice! I never noticed that the grey point picker in the level tool could 
>> be used to adjust the  WB.  It is  unfortunately limited to the mid-tones.
> How is it limited to the mid-tones?
> Sven

The black point picker is of course totally useless for adjusting the 
white balance.

The white point picker can be very useful for white balance but, since 
it also set the white point, it can only be used you have a true white 
point in you image. By that, I mean a point that is both neutral-grey 
and of maximal brightness. This is usually not the case.

So was it left is the 'middle-point' picker. This one sets the 
middle-point of the RED, GREEN & BLUE levels but without affecting the 
'right-point'  of those levels. In practice that means that  the  white 
balance of the current 'white point' is absolutely not affected by that 
picker. More generally, the more you are the highlights, the less you 
are affected by the 'middle-picker'.
That  was I mean by 'limited to the mid-tones' though  I should have 
written 'mid-tones and shadows' since the 'middle-point' picker also 
affect the shadows (but in practice they are less interesting).

If you are not convinced (or just don't understand me) then think about 
that: you cannot change a true white point (255,255,255) using the 
middle-point picker.

However, after playing a bit with the middle-point picker, I came to the 
conclusion that the results are usually quite good. This is because, in 
Gimp, the WB is usually applied on an image that was already adjusted 
(AWB and exposure in the camera, ...) so the WB coefficients tend to be 
small (-15% to 25% on each channel), most of the image is already in the 
mid-tones or shadows and the parts that are in the highlight are often 
already white (typically the clouds). Not having to care about clipping 
the highlights is also nice.

The situation in a tool like UFraw is quite different: The WB is usually 
the first transformation applied to the raw data. The WB coefficients 
can be huge (-50% to +100% on each channel) and are followed by some 
complex adjustments (ICC profile, exposure, gamma, highlight clipping 
and recovery, ...).

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