Is "White Balance" relative or absolute?

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Originally Posted by george013

Are those figures constants?

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For a particular camera model, yes they are constant. Sigma updates their firmware from time to time and also for each new model and those coefficients per model can and do vary.

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And how does the camera correct for different light sources without knowing what the light source is?

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I did not say that.

The User chooses a WB setting, e.g. sunlight, etc., just like any other camera. 'Auto' does the "gray world" thing and creates a matrix just for that shot. 'Custom' also creates a matrix which can be recalled for later shots, again, just like any other camera.

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Originally Posted by ajax

Cannot find anything like that in the metadata for my (Canon) cameras!

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Someone found some. See http://www.guillermoluijk.com/tutori...b/index_en.htm

Looks like that Canon uses simple multipliers rather than 3x3 matrices, probably because the Bayer CFA is closer to true RGB.

"The multipliers corresponding to my Canon 350D presets are shown here to have an idea of their magnitude:

• 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"

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Originally Posted by xpatUSA

Someone found some. See http://www.guillermoluijk.com/tutori...b/index_en.htm

Looks like that Canon uses simple multipliers rather than 3x3 matrices, probably because the Bayer CFA is closer to true RGB.

"The multipliers corresponding to my Canon 350D presets are shown here to have an idea of their magnitude:

• 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"

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The big question is what happens with auto.

George

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Originally Posted by george013

The big question is what happens with auto.

George

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https://web.stanford.edu/~sujason/ColorBalancing/grayworld.html

There are other, more sophisticated algorithms, but this is the simple one that I mentioned.

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Originally Posted by xpatUSA

https://web.stanford.edu/~sujason/ColorBalancing/grayworld.html

There are other, more sophisticated algorithms, but this is the simple one that I mentioned.

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Interesting. I worked at Kodak a long time ago and from memory they had a reference colour that was a strange neutral khaki rather than grey. It was supposedly the colour you would get if you blended all the colours from thousands of photographs. I have a vague recollection that there were regional variations of the reference. When setting up the printers for a paper change (paper batches had minor colour response variations) a print was made from a reference colour negative that produced this neutral khaki which was measured. Then a printer's colour correction filters were adjusted to try and get all colour channels printed with the shortest and ideally the same timing. Please note that my strongest (also not necessarily accurate) and fondest memories of the sixties have nothing to do with Kodak.

Having a grey cat in all your photographs is still the best approach...:)

https://sno.phy.queensu.ca/~phil/exi...mes/Canon.html
There is a list with canon cameras and their tags. One of them is the color temperature.
I didn't try it.

George

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Originally Posted by ajax

My question arose when trying to edit images that were already developed by the camera and it now seems that WB is primarily a topic that apples to raw file development. Would it be correct to conclude that while the WB tool of the editing software can be applied to previously developed image files that this is not really the purpose of the tool? In that, once the image is developed WB (i.e., color temperature) becomes meaningless. As such, the effect of iteratively revising (i.e., applying the WB tool to) a previously developed image is the same irrespective of starting point.

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Yes some software such as Lightroom and ACR can be used for editing jpegs but by the time you have created a jpeg, the original data in relation to WB is gone. The channel multipliers have been applied and been "baked in". So the WB adjustment can still be applied but there is no reference to as shot color temp etc.

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Originally Posted by ajax

Something I have now recognized subsequent to my original question is that I can take a raw file to my computer and when using the editing software supplied by the camera maker (Canon in my case) the default computer based development produces an image that appears to be the same as the camera developed image. This implies that the raw file supplies all the data needed to reproduce the in camera development. Looking at the metadata for a raw file indicates what looks to me like a lot of parameters pertaining to WB. Most interesting are a few of these values referred to as "ColorTempAsShot", "ColorTempAuto", and "ColorTempMeasured" which all happen to be the same (6339). The camera was likely set to automatically determine WB. None of this metadata is included in the developed jpg or tif file. Also interesting is that Rawtherapee defaults to what it calls the "Camera" supplied value which while close (6282) I cannot find anywhere in the raw file metadata. When I do the same thing with the Canon supplied software (DPP4) the editor opens with a temperature value (5200) that is also the value assigned in the metadata to a parameter called "ColorTempDaylight". While this all leaves me a bit confused I suppose that has mostly to do with the nuance associated with what are obviously pretty complex programs.

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David I had a look at a Canon raw file in EXIFTool Gui and post a screen shot below. The raw channel multipliers are shown in a slightly different way to other camera makers in the tag WB_RGGBLevelsAsShot .

To get the actual multipliers, you divide the values shown in this tag by the green value (1024). This gives a red multiplier of 2.22 (2275/1024) and a blue multiplier of 1.607 (1646/1024). These are the values used by the raw processing software. Many other makers do not have a ColorTempAsShot tag in their raw files.

https://c1.staticflickr.com/1/898/40...94432e74_o.jpg

The screenshot below is from Darktable software with the same raw file opened as for the previous screenshot. This shows the channel multiplier values used in the WB adjustment section. You will see that this software is using the exact multiplier values that were embeded in the raw file. Note also that their calculation of color temp is a bit different to Canon's.

Dave

https://c1.staticflickr.com/1/922/40...92a6db0fdc.jpg

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Originally Posted by dje

Yes some software such as Lightroom and ACR can be used for editing jpegs but by the time you have created a jpeg, the original data in relation to WB is gone. The channel multipliers have been applied and been "baked in". So the WB adjustment can still be applied but there is no reference to as shot color temp etc.




David I had a look at a Canon raw file in EXIFTool Gui and post a screen shot below. The raw channel multipliers are shown in a slightly different way to other camera makers in the tag WB_RGGBLevelsAsShot .

To get the actual multipliers, you divide the values shown in this tag by the green value (1024). This gives a red multiplier of 2.22 (2275/1024) and a blue multiplier of 1.607 (1646/1024). These are the values used by the raw processing software. Many other makers do not have a ColorTempAsShot tag in their raw files.

https://c1.staticflickr.com/1/898/40...94432e74_o.jpg

The screenshot below is from Darktable software with the same raw file opened as for the previous screenshot. This shows the channel multiplier values used in the WB adjustment section. You will see that this software is using the exact multiplier values that were embeded in the raw file. Note also that their calculation of color temp is a bit different to Canon's.

Dave

https://c1.staticflickr.com/1/922/40...92a6db0fdc.jpg

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Interesting to know how to use those figures.
In CaptureNx2, I can only talk about that, adjusting wb wit a gray point only changes the R and B channels. If I just adjust the colors, not the wb, than the 3 channels are averaged.

George

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Originally Posted by george013

Interesting to know how to use those figures.
In CaptureNx2, I can only talk about that, adjusting wb wit a gray point only changes the R and B channels. If I just adjust the colors, not the wb, than the 3 channels are averaged.

George

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Darktable is the same. When WB is adjusted either by color temp, presets or the white sampler tool, the G channel multiplier stays at 1. With Darktable, you can actually change the R,G and B multipliers individually but that is frought with danger IMO!

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Originally Posted by ajax

"Is "White Balance" relative or absolute?" This question pertains to the editing (development) of digital images. . . .

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White Balance is absolute.

I pretty much agree with all that Ted ('xpatUSA') has written.

However, I add that the 'absolute' of White Balance for mostly, for all practical measures of 'Photography' and specifically as you are asking with reference “to the editing (development) of digital images” – is worth forth-fifths of five-eighths of bugger-all.

When the ‘absolute’ matters as ‘an absolute’ is for forensic assessment.

For (one) example, as reference for Art Works recorded for Insurance Purposes – and then it is critical that the reference image is viewed on a Calibrated Monitor and that Monitor is viewed in a room with lighting sufficient as to not throw a Colour Cast.

I note that there are not many situations when such is either required, or available.

My advice is to Colour Balance your images to suit your taste and Artistic Impression: if you sell them or give them away as either Digital Files or Prints the chances are that none will ever be viewed on a Calibrated Monitor nor ever viewed under Controlled Lighting.

WW

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Originally Posted by William W

White Balance is absolute.

I pretty much agree with all that Ted ('xpatUSA') has written.

However, I add that the 'absolute' of White Balance for mostly, for all practical measures of 'Photography' and specifically as you are asking with reference “to the editing (development) of digital images” – is worth forth-fifths of five-eighths of bugger-all.

When the ‘absolute’ matters as ‘an absolute’ is for forensic assessment.

For (one) example, as reference for Art Works recorded for Insurance Purposes – and then it is critical that the reference image is viewed on a Calibrated Monitor and that Monitor is viewed in a room with lighting sufficient as to not throw a Colour Cast.

I note that there are not many situations when such is either required, or available.

My advice is to Colour Balance your images to suit your taste and Artistic Impression: if you sell them or give them away as either Digital Files or Prints the chances are that none will ever be viewed on a Calibrated Monitor nor ever viewed under Controlled Lighting.

WW

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Yes , 19.2 deg C is an absolute value but having everyone in a room wanting the air conditioning set to it is a completely different matter...:)

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Originally Posted by William W

White Balance is absolute.

I pretty much agree with all that Ted ('xpatUSA') has written.

However, I add that the 'absolute' of White Balance for mostly, for all practical measures of 'Photography' and specifically as you are asking with reference “to the editing (development) of digital images” – is worth forth-fifths of five-eighths of bugger-all.

When the ‘absolute’ matters as ‘an absolute’ is for forensic assessment.

For (one) example, as reference for Art Works recorded for Insurance Purposes – and then it is critical that the reference image is viewed on a Calibrated Monitor and that Monitor is viewed in a room with lighting sufficient as to not throw a Colour Cast.

I note that there are not many situations when such is either required, or available.

My advice is to Colour Balance your images to suit your taste and Artistic Impression: if you sell them or give them away as either Digital Files or Prints the chances are that none will ever be viewed on a Calibrated Monitor nor ever viewed under Controlled Lighting.

WW

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Well said Bill.

Corporations want their logos and product colours to exactly match the Pantone Matching System colours. Some high end photo reproduction requirements are looking for an extremely high degree of colour fidelity, These images are taken in extremely tightly controlled conditions, which high end equipment and processed to exacting standards in highly controlled environments. That accounts for a tiny fraction all the images taken and there are a few specialty suppliers who have cornered the market for this type of work.

For the rest of the world, Paul's example is bang on. Even those high end ProPhoto D1 flash units are at best, ±150K over the entire power range, so theory falls down to reality quite quickly.

And that's before we even think about personal taste.

Dave, thanks for providing a very precise answer to a question that arose in my mind before I had a chance to post it. This resulted from reading some of the excellent articles referenced above by Guillermo Luijk, where he indicated such multipliers as having come from a Canon (in his case 350D) but that I was unable to identify for my Camera. The screen shots from your Canon raw file match mine.

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Originally Posted by William W

White Balance is absolute.

I pretty much agree with all that Ted ('xpatUSA') has written.

However, I add that the 'absolute' of White Balance for mostly, for all practical measures of 'Photography' and specifically as you are asking with reference “to the editing (development) of digital images” – is worth forth-fifths of five-eighths of bugger-all.

When the ‘absolute’ matters as ‘an absolute’ is for forensic assessment.

For (one) example, as reference for Art Works recorded for Insurance Purposes – and then it is critical that the reference image is viewed on a Calibrated Monitor and that Monitor is viewed in a room with lighting sufficient as to not throw a Colour Cast.

I note that there are not many situations when such is either required, or available.

WW

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Good points for 'most of us', Bill.

Being perhaps more into color than most, I am known on another forum for testing color accuracy which also involves some absolute values.

I shoot the X-rite MacBeth 24-patch color card under known lighting; open in my converter and adjust patch #22 to be 120/255 in sRGB.

Save and open that in RawTherapee (RT), which has an L*a*b* color-picker.

Every card comes with a list of L*a*b* values for each patch. By comparing what RT gives with those given values I can calculate the delta-E for the patches of interest (usually the six in row three: blue to cyan), where delta-E is a measure of color difference.

https://en.wikipedia.org/wiki/Color_difference

It could be said that delta-E values are truly relative but gained via absolute measurements. ;)

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Originally Posted by xpatUSA

I shoot the X-rite MacBeth 24-patch color card under known lighting; open in my converter and adjust patch #22 to be 120/255 in sRGB.

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The methodology certainly makes sense and is parallel to the approach that I use with the x-Rite software.

The assumption in the methodology is the quality and consistency of the light source. Is your daylight or flash really 5500K and is your tungsten light source really 2850K? Studio flash colour temperature does vary by power level setting and this can be several hundred K from the published "nominal" values. Likewise, even at the same power setting the colour temperature output can vary by a few 10's K.

Back when I was doing colour printing in my home darkroom; I could never work between about 5:00PM and 9:00PM because of changes in line voltage on the nominal 120V circuit. Even outside of that window, if a high power draw appliance turned on; the line voltage would drop and I would have to throw out the paper the light from the enlarger would shift to a much more yellow light for close to a second. I would have loved to have been able to have a line voltage regulator, but they were definitely out of my budget range in the early 1970s.

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Originally Posted by Manfred M

The methodology certainly makes sense and is parallel to the approach that I use with the x-Rite software.

The assumption in the methodology is the quality and consistency of the light source. Is your daylight or flash really 5500K and is your tungsten light source really 2850K? Studio flash colour temperature does vary by power level setting and this can be several hundred K from the published "nominal" values. Likewise, even at the same power setting the colour temperature output can vary by a few 10's K.

Back when I was doing colour printing in my home darkroom; I could never work between about 5:00PM and 9:00PM because of changes in line voltage on the nominal 120V circuit. Even outside of that window, if a high power draw appliance turned on; the line voltage would drop and I would have to throw out the paper the light from the enlarger would shift to a much more yellow light for close to a second. I would have loved to have been able to have a line voltage regulator, but they were definitely out of my budget range in the early 1970s.

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Either the wiring in your house was rubbish or the delivering of the power. I hardly can believe that last in a city as Ottawa.

The point here is that when one wants to calculate with the color temperature or whatever else one has to use figures. A programmer can't work with his feelings.

George

I've come to recognize that in my 18 April 2018 post I erred when saying the following:

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Looking at the metadata for a raw file indicates what looks to me like a lot of parameters pertaining to WB. Most interesting are a few of these values referred to as "ColorTempAsShot", "ColorTempAuto", and "ColorTempMeasured" which all happen to be the same (6339). The camera was likely set to automatically determine WB. None of this metadata is included in the developed jpg or tif file. Also interesting is that Rawtherapee defaults to what it calls the "Camera" supplied value which while close (6282) I cannot find anywhere in the raw file metadata. When I do the same thing with the Canon supplied software (DPP4) the editor opens with a temperature value (5200) that is also the value assigned in the metadata to a parameter called "ColorTempDaylight".

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When the "Color Temperature" option is selected in the WB tool for the Canon DPP4 Raw Editor it seems to always have an initial value of 5200. This seems to be an alternative to using something the camera deduced and relayed to the raw processing software.

With that said, I've taken the new knowledge I've gained form this discussion regarding multipliers and gone back and reviewed prior articles I'd read on the subject of WB. Of particular interest is this one found on the Cambridge in Color website. Interestingly it makes no mention of multipliers.

I've also gone back and made a comparison of the WB tooling on each of the 3 raw processors I've used. The WB tool [shown here] for Rawtherapee (which has been my preferred raw processor) makes no mention of multipliers. However, unlike DPP4 [shown here] it does seem to deduce a displayed value for the temperature based on the raw data. In that, different raw files shot with the same camera and settings show a different value for different shots. The DPP4 tool does contain something identified as "Fine Tune" which is a 2 dimensional grid that allows for producing 2 parameters based on East|West (Amber|Blue) or North|South (Green|Magenta) positioning within the grid. I'm suspecting that the resulting parameters may be multipliers of some kind but could not presume such to be the same as what's been discussed herein. Lastly the WB tool for UFraw (which I think utilizes DCraw) is most interesting because it seems to match the discussion herein. As shown here it deduces a value for temperature which, by the way, is quite different than the temperature deduced by Rawtherapee but it displays corresponding multipliers and when you adjust the temperature slider the multipliers change accordingly.

Based on this evidence I'm thinking that while a value for temperature is being deduced based on the raw data the method for doing this is differs from one raw processor to another and I must conclude is not precisely defined. Insofar as the Cannon DPP4 software does NOT reveal a deduced temperature value, at least that I've been able to find, one can only assume that it might be the "ColorTempAsShot" value (6339 in this case) found in the metadata.

When considering the initial question that I posed, as I instinctively thought temperature is absolute. When doing raw processing (i.e., raw data from camera exists) that temperature (i.e., corresponding multipliers) can be applied to the raw data data and result will be consistent (i.e., absolute) from one use (i.e., execution) of the raw processor to the next. However, when editing an already developed image (e.g., jpg, tif, png) which has already undergone this absolute rendering of temperature but the temperature value is at this point unknown (i.e., been lost) the WB tool can only assume some starting point (it looks like Rawtherapee may default such to 6490) and then compute differences from that starting point which is what I had in mind as being relative. To the extent that the raw development is absolute it seems to me that the resulting developed image has a well known specific value for temperature (or the multipliers if you prefer). Is there a good reason for excluding this information from the metadata for a developed image? It would seem to me that having that data when editing a previously developed image would allow the WB tool to adjust to that point and then when the temperature is changed it could come to the same point that would have happened if that temperature where applied to the original image. This would make the tool much more consistent and remove any doubt about the absolute verses relative concept.

Possibly the answer is that the WB computations are but one of several that can be applied in the raw development process and that accurately making such an adjustment is not possible. I also recognize that when using integer math (e.g., divide by 1024 is nothing more than shift right) data is lost that cannot be recovered (i.e., rounding error). While current computer software is moving to floating point math, I suspect that there are lots of cameras still performing integer math.

Another thought I've had is that conceptually WB is something that only applies to raw processing and that I'm guilty of misusing the software when trying to apply the tool to previously developed images. I suspect that other color management tools could be used to achieve the same result but that it may not be as simple as using the WB slider.

Finally, do experienced photographers who know what they are doing use the WB tool on previously developed images?

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Originally Posted by ajax

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Finally, do experienced photographers who know what they are doing use the WB tool on previously developed images?

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In CaptureNx it's disabled, as mentioned before. As it should be.
George

What do you know, DPP4 also disables it. Sorry to have missed that.

I should have mentioned that GIMP which cannot process raw files makes no mention of WB. Prior to V2.8 it offered a "Color Balance" tool which worked differently and makes no mention of temperature or multipliers. GIMP V2.10 has added what it calls a "Color Temperature" tool that always has a value called "Original Temperature" set to 6500 and also displays something called "Intended Temperature" which may be set to a different value. It looks to me as though they may be being careful not to use the term "White Balance" (WB).

I'm hoping that most people here mean "correlated color temperature" (CCT), not black body temperature. I'm also hoping that most people are not thinking that two lamps of the same CCT have the same chromaticity (color):

https://en.wikipedia.org/wiki/Color_...re#Calculation

Also:

http://www.lrc.rpi.edu/programs/nlpi.../whatisCCT.asp

See lamps A and B, each with a CCT of 3000K ... it is quite obvious that images shot under each lamp would need different color balancing in post.