Beware, WB presets don't have a fixed behaviour - RAW Shooting and Processing - CHDK Forum
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Beware, WB presets don't have a fixed behaviour

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Offline wap4

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Beware, WB presets don't have a fixed behaviour
« on: 15 / July / 2008, 03:15:16 »
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I have just discovered that in A720IS the so called preset WB 'sunlight' (and maybe other 'presets' also) is NOT actually a preset, but acts as corrective guide to AWB.

Experiment:  I've a CRW file. I set my monitor's color temp very high, point the cam to it, and use raw develop function
to get a jpg out of the CRW. Now again I make monitor's color temp very low and repeat the subsequent steps to get another jpg out of the CRW. In both the cases, I use sunlight WB.

Observation: The jpgs have different color balance. (See the uploaded pictures, if A720 owners don't believe me, please upload them to 100CANON folder of your SD card & see the same sunlight WB icon from the camera!)

Also I've found that the behaviour of AWB is also not fully determined from the CRW file, but also depends greatly on surrounding light during the raw developing. Don't know whether custom WB also behaves this way!
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Offline LjL

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Re: Beware, WB presets don't have a fixed behaviour
« Reply #1 on: 15 / July / 2008, 07:55:58 »
Well, I'd say this is one point in favor of implementing white-balance fine-tuning as recently requested. We'd need to find a propcase, or something, that is[/url] writable, though.
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Offline lukg

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Re: Beware, WB presets don't have a fixed behaviour
« Reply #2 on: 15 / July / 2008, 08:54:21 »
Afaik if you set WB to daylight the camera choose the "right" white in a range, it is not a fixed value (like eg. D65)...but maybe I am wrong  ::)
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Offline PS

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Re: Beware, WB presets don't have a fixed behaviour
« Reply #3 on: 15 / July / 2008, 19:47:43 »
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Offline vit40

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Re: Beware, WB presets don't have a fixed behaviour
« Reply #4 on: 17 / July / 2008, 08:09:52 »
Yes, I also noticed this. When I was "calibrating" color matrix for my A620 (by hand), I got close match on most (real life) pictures (using WB Daylight), but on some pictures colors were significantly different. Also, I remember there were some differences in highlights (sometimes, highlights are additionally saturated, sometimes desaturated, depending whether they are blown or not) etc

Well, these are point and shoot cameras, made to make most attractive pictures with minimum efforts, so it's to be expected that there is some "additional logic" to correct user's mistakes


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Offline wap4

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Re: Beware, WB presets don't have a fixed behaviour
« Reply #5 on: 17 / July / 2008, 10:02:01 »
Actually I wanted to attract attention of the calibration peoples so that when they compute the Matrix, they must develop
the raw in a 'darkroom' to get the jpeg, to remove any bias.

Edit: Even this plan is flawed as I discovered that the AWB understands the color of the surrounding light during raw develop, no matter how less it is & applies correction to the presets. Now I know that AWB algorithm is totally independent of the CRW data, it only depends on outside light color during raw develop.
 
« Last Edit: 18 / July / 2008, 14:29:03 by wap4 »
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Offline Howdy

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Re: Beware, WB presets don't have a fixed behaviour
« Reply #6 on: 01 / August / 2008, 05:03:13 »
Hello!

I do not know if http://gauss.ffii.org/PatentView/EP1339239#head-9e7f90282dc60aee711351ee145959741484566d is of any use but I think that it confirmes your observations.
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Offline wap4

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Re: Beware, WB presets don't have a fixed behaviour
« Reply #7 on: 01 / August / 2008, 10:13:25 »
Howdy,

Thanks for giving me the link, which confirms my claim. But the problem is, Canon's correction is mostly on the cooler side
and too greenish, which I find irritating. Good news is that custom WB really has a fixed behaviour.  :)
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