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What does No WB mean in the Raw dialog?
Posted: August 27th, 2011, 8:29 pm
by Charles2
One of the options in the Color tab of the Raw dialog of Picture Window Pro is No White Balance. Is that literally true? Or does it mean a kind of standard white balance, applying a fixed triple of ratios to the (R, G, B) values or a standard temp and tint like 6500K and 1.0?
The answer will presumably help me think about adjusting the colors I see when I start work with No WB, which is sometimes the best choice I see.
Re: What does No WB mean in the Raw dialog?
Posted: August 27th, 2011, 10:21 pm
by ksinkel
The base raw image essentially consists of rgb values as recorded by the sensors. If a white balance option is selected, these values are adjusted according to the selection. If the selection is no white balance, these values are presented as is. Obviously since no adjustment is made, this setting is NOT likely to produce good balance and is therefore not recommended for routine use. If you do use it, you will generally have to use some subsequent process to acieve balance.
In my experience you defintiely need a starting point for color adjustment. Most of the time, camera white balance provides a good starting point. But cameras are not foolproof. If it turns out that Camera WB is not good for a particular image, I use the color probe. Click on any area that should be neutral, such as clouds, whites of eyes, teeth, foam in water. Click around in a couple of adjacent locations since there are usually subtle color differences even in neutral areas. This should bring you very close if not right on. If the image does need further balance tweaking, it is now fairly easy to make small adjustments to tint and temp to achieve the look you want.
Of course, you will also want to adjust saturation.
Kiril
Re: What does No WB mean in the Raw dialog?
Posted: August 27th, 2011, 11:21 pm
by den
Kiril....
Does the Raw Dialog color probe's algorithm generate "-r m1 m2 m3 m4" values of the "dcraw" development portion of PWP's Raw Dialog?
From GL's dcraw tutorial -- see last paragraph....
http://www.guillermoluijk.com/tutorial/ ... dex_en.htm
WHITE BALANCE
The way DCRAW implements the white balance interface is not the usual Temperature/Tint but a lower level set of 4 multiplying factors which will scale linearly each of the RGBG image channels of the Bayer matrix (white balance is applied before demosaicing). In general factors number 2 and 4 will be the same as they correspond both to the green channel, just in different locations of the Bayer distribution.
We must point that a particular white balance does not mean an absolute set of values for all those multipliers, but the relative proportions between them. In this way the same white balance can be achieved with different sets of factors as long as they keep their relative proportions.
The multipliers may have 3 different origins depending on the option used:
-w White balance is set according to camera settings at the moment of the shot
-a Automatic white balance calculated by DCRAW over the whole image
-r m1 m2 m3 m4 Custom white balance by chosing the individual multiplying factors
If none of these options is used, DCRAW will take by default a white balance preset corresponding to lighting a grey card with a standard D65 light source.
The correct linear values to be applied will vary from one camera to another. To try to guess which values are to be used to get a correct white balance is not an easy task. However it is very simple to obtain the factors that correspond to each of the camera white balance presets which is a good start point. To do this we just need to take a shot with each of the presets and develop the resulting RAW file with the -v -w commands that will display those numbers.
Find here a table of the RGB multipliers to achieve different white balance presets in the Canon 350D:
Default (D65 lamp): multipliers 2.395443 1.000000 1.253807
Tungsten: multipliers 1.392498 1.000000 2.375114
Daylight: multipliers 2.132483 1.000000 1.480864
Fluorescent: multipliers 1.783446 1.000000 1.997113
Shade: multipliers 2.531894 1.000000 1.223749
Flash: multipliers 2.429833 1.000000 1.284593
Cloudy: multipliers 2.336605 1.000000 1.334642
To specify that we do not want any white balance at all applied we will use the -r 1 1 1 1 option. If doing so, we will be able to adjust later the white balance making use of the linearity of the histogram being this the best way to test different white balance values until we reach one that satisfies us. However this is not a recommended way to apply the white balance, just to calculate the multipliers. We should go back and use the calculated factor with the -r command as interpolaton Bayer algorithms are optimised to work on already balanced images.
Re: What does No WB mean in the Raw dialog?
Posted: August 28th, 2011, 4:16 pm
by Charles2
Thank you, Kiril. Yes, the occasions to select No WB are rare. (Odd reversal here concerning who deprecates a PWP option.)
Occasionally, No WB is the easiest start when you want to keep a desired color cast, like a late afternoon "golden hour."
Also, when you have a shot of a scene and another shot there with a Color Checker, it is easy to pass both through the Raw dialog with No WB, then use Match Reference or Color Balance in the main menus.
I guess my question was, does No WB apply standard ratios to reflect the fact that the R, G, and B sensors are not equally sensitive to light, or to reflect the fact that the sensors are sensitive to light at different ratios than the human eye. Your answer appears to be no and no, their readouts are used at 1:1:1.
And I assume all this is independent of demosaicing, so that the fact that typical Bayer filters have two green sensors for one red and one blue sensor becomes irrelevant.
Thank you.
Re: What does No WB mean in the Raw dialog?
Posted: August 28th, 2011, 10:12 pm
by ksinkel
To Den's question, PWP does not use the -r dcraw option but does its own conversion. However the principle used is the same.
The tint/temp controls and multipliers are actually different ways of presenting the same information. In fact temp and tint values can be converted to multipliers and vice versa. Thus each of the lighting types (tungsten, daylight, etc) have equivalent temp/tint values.
Incidentally, the multiplier values in the table Den cites should be taken as 'typical' values rather than exact ones. After all, 'warm' and 'daylight' fluorescents are quite different from each other. So is noon and evening daylight. There is a range of 'tungsten' lights. So there is no single set of multipliers that can be used to adjust for all fluorescent light or any of the other lighting types in the table. However, the types can serve as useful starting points for adjusting color balance. PWP incudes a number of such typical lighting presets for just such a purpose.
Kiril
Re: What does No WB mean in the Raw dialog?
Posted: August 28th, 2011, 11:31 pm
by den
Kiril.... Thank you... so many ways to manipulate image data to reach preferences!!!