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Author Topic: Exposing to the Right  (Read 15209 times)
Guillermo Luijk
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« Reply #40 on: August 24, 2009, 11:27:20 AM »
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Quote from: papa v2.0
If for example I ETTR and say clip all channels in the RAW file ( too much ETTR), how will that effect the WB.

what are the main methods used for white balancing , ie Gray world, Retinex, Colour by Convolution etc or are the channels balanced to a set white point ( daylight, tungesten etc)

When I open a RAW in ACR the white balance 'as shot' is displayed. how is calculated? And how would it be affected if the RAW is overexposed as mentioned above.

WB is independent of how much you exposed (clipped) the RAW channels. It just affects how the JPEG thumbnail displays on your camera, and how WB is applied when developing the RAW file in case you choose 'as shot'. The 'as shot' WB factors will be those you chose in the camera, whatever they were (auto, preset or custom WB).

For white balancing a clipped RAW file the same methods apply as with any other RAW file (auto, any preset, eyedropper over a neutral area of the scene,...). You simply won't be allowed to use the eyedropper (is this the name in English?) on the blown areas.
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JeffKohn
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« Reply #41 on: August 24, 2009, 04:10:28 PM »
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Since the red and blue multipliers are less than unity for daylight and most other illuminatioin, one can use the RGB histograms on most of the more adavnced cameras to check for clipping.
With which cameras? I don't think this is true for any of the Nikon's I've shot with over the last several years.
« Last Edit: August 24, 2009, 04:10:42 PM by JeffKohn » Logged

madmanchan
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« Reply #42 on: August 24, 2009, 07:49:25 PM »
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papa v2.0, the WB methods you discuss are automatic estimation methods. They are only relevant if you wish to understand how a raw conversion software performs its "automatic" WB mode. Most of the time, however, I suspect you are simply going to click-WB or choose one of the standard presets and then tweak from there. Ultimately you are just shoving the native pixel coordinate values through a 3x3 matrix. Simple white balance without color optimization just means multiplying each of the R, G, and B values by a separate constant -- i.e., the matrix is a diagonal matrix. You can refer to Chapter 6 of the DNG specification if you wish to learn the specific math that Camera Raw and Lightroom use.
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papa v2.0
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« Reply #43 on: August 25, 2009, 06:11:34 AM »
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Quote from: madmanchan
papa v2.0, the WB methods you discuss are automatic estimation methods. They are only relevant if you wish to understand how a raw conversion software performs its "automatic" WB mode.

I am interested to find out what methods are use for calculating the auto WB, either in ACR or even by camera  manufactures, although I suspect that they are proprietary and there is little information available. If anyone has any ideas...




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bjanes
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« Reply #44 on: August 25, 2009, 02:20:20 PM »
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Quote from: JeffKohn
With which cameras? I don't think this is true for any of the Nikon's I've shot with over the last several years.
Less than unity was a typo and I should have said more than unity. Obviously, if the green channel is intact (multiplier = 1.0), the red and blue channels could clip only with multipliers greater than 1.0.
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BernardLanguillier
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« Reply #45 on: August 25, 2009, 11:16:15 PM »
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Quote from: evogel99
It is ironic that given all the EEs working on the design of these things, such an obvious design optimization has never been included. Perhaps they tried too hard to mimic film cameras as a design spec, and there is nothing like that even possible in a film camera. In digital, you know everything there is to know about the "film" (sensor). You surely should leverage that information!

They have been trying super hard to make DSLR behave like film cameras, and now you are telling them that the DSLR should behave in a digital specific way?

Cheers,
Bernard
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A few images online here!
digitaldog
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« Reply #46 on: August 26, 2009, 08:55:29 AM »
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Quote from: BernardLanguillier
They have been trying super hard to make DSLR behave like film cameras...

Which is kind of silly considering they are not. The underlying capture process is as we all know, vastly different. So I can only assume they do this to comfort the masses to make more sales, not necessarily targeting the professional or those hopping for the bet capture data. But then if they all cared for our well being, we wouldn’t be dealing with the messiness of all these proprietary files every time a new camera comes out.
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Andrew Rodney
Author “Color Management for Photographers”
http://digitaldog.net/
Guillermo Luijk
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« Reply #47 on: August 26, 2009, 06:50:20 PM »
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Quote from: papa v2.0
I am interested to find out what methods are use for calculating the auto WB, either in ACR or even by camera  manufactures, although I suspect that they are proprietary and there is little information available. If anyone has any ideas...
I think there is not much secret to tell. Typical Auto WB tries to get an image which is gray in average, i.e. where the R, G and B averaged values are the same. And to achieve that it calculates the 3 appropiate relative coefficients (2 in practice) by which each RAW channel must be scaled.

Auto WB will never be able to achieve a correct WB in all situations because it ignores the photographer's intentions, which cannot be guessed by a piece of software. That is why sometimes it works fine and sometimes horrible. The success depends on the scene and user's perception rather than on the calculation method.

This scene was developed under 4 different WB coefficients. See how Zero Noise Auto WB nearly provides the same result as DCRAW Auto WB. The slight changes are due to subtle differences in the algorithms, but the goal was the same and to achieve it is very simple. The camera (Canon 350D) Daylight preset was too bluish for my taste. The last sample uses a round patch over the USA flag for WB, although I prefer the Auto WB in this case.





Regards.
« Last Edit: August 27, 2009, 02:02:53 AM by GLuijk » Logged

bjanes
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« Reply #48 on: August 27, 2009, 11:14:41 AM »
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Quote from: GLuijk
I think there is not much secret to tell. Typical Auto WB tries to get an image which is gray in average, i.e. where the R, G and B averaged values are the same. And to achieve that it calculates the 3 appropiate relative coefficients (2 in practice) by which each RAW channel must be scaled.
Regards.

As per Eric Chan's previous post, multiplying by the red and blue coefficients merely gives a first order approximation to the white balance (diagonal matrix). To obtain a more accurate white balance, one has to use a full matrix derived from the correlated color temperature of the set white balance. ACR interpolates between daylight and tungsten profiles to derive the matrix. The precise methodology used by ACR is described in Chapter 6 of the DNG specification, but many of these matters are beyond my expertise. For my Nikon D3, I don't know exactly what white balance information is recorded in the raw file. It could by multipliers, a correlated color temperature, or a CIE xy coordinate. Perhaps Eric can elaborate.

If you have a well defined white as in Guillermo's image, one can achieve an approximate WB relatively simply by using linear curves for the red and blue so as to equalize the RGB values in the whites and neutral areas. This is equivalent to using multipliers. This is shown in the following image rendered by Iris into a gamma one space without white balance. The white balance curves are shown. Additional curves were needed for gamma, contrast, and saturation.

[attachment=16247:WhiBal.png]

The image is hardly optimized, as shown by comparison to the ACR rendering.

[attachment=16248:2009Jul16_0014.jpg]
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marcmccalmont
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« Reply #49 on: August 28, 2009, 05:27:00 AM »
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And I thought this was the beginners section     I'd hate to read the advanced section!
Marc
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Marc McCalmont
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