Colour Balance

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Marpel
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Joined: September 13th, 2009, 3:19 pm
What is the make/model of your primary camera?: Nikon D810
Location: Port Coquitlam, British Columbia

Colour Balance

Post by Marpel »

Hello,

I have an image of a foggy sunrise (attached if I uploaded correctly...first time with this) that I converted from RAW into a TIFF and opened in PWP. The "Grey - Levels and Colour" transformation gave an initial dynamic range of 2.4 and 98.4. However, after using the Colour Balance transformation, the range now reads .8 and 100. I am wondering why the colour balance transformation would change (widen) the dynamic range of an image, rather than just change the balance and maintain the same DR. The colour balanced image appears to brighten the sun and it looks "whiter" (for lack of a better explanation), but I would have thought that part of the image would be balanced towards a neutral with the same 98.4 reading (like a "greyer" white).

Hopefully, I have explained this correctly and thanks in advance.

Marv
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HanSch
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Re: Colour Balance

Post by HanSch »

Hello Marv,

maybe you used the Colour Balance transformation with the options "enable auto white/black balancing with contrast extension". See the cb-window.

Han
Marpel
Posts: 692
Joined: September 13th, 2009, 3:19 pm
What is the make/model of your primary camera?: Nikon D810
Location: Port Coquitlam, British Columbia

Re: Colour Balance

Post by Marpel »

Thanks Han,

You are quite right. For some reason I thought the default was for auto white and black balance without contrast extension, but it appears contrast extension is part of the default state of the transformation (wonder why that is?). However, when I now disable the contrast extension by clicking the "enable white (and black) balancing" buttons without contrast extension (the middle buttons), the resulting DR is 1.6 and 97.3. I obviously don't know enough about this transformation (or colour editing for that matter!!) to understand what's going on. Do you, or anyone else, know how, or if it's even possible, to colour balance without extending/changing the DR, or if it is even desirable?

Thanks

Marv
den
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Re: Colour Balance

Post by den »

JSachs has written a white paper: "Color Balancing Techniques" that may be helpful in addition to the "F1" Help dialogue when the Color Balance transform is 'active' in PWP5's working space...

The pdf file can be downloaded from here: http://dl-c.com/content/view/34/55/ or linked from the White Paper tab of the electronic manual if it is already in the "Program Files/Digital Light & Color/Picture Window Pro 5/Docs" folder as "Color Balance.pdf".
Marpel
Posts: 692
Joined: September 13th, 2009, 3:19 pm
What is the make/model of your primary camera?: Nikon D810
Location: Port Coquitlam, British Columbia

Re: Colour Balance

Post by Marpel »

Den,

Thanks for the link. I had tried to find that white paper yesterday but couldn't seem to locate it.

Marv
jsachs
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Re: Colour Balance

Post by jsachs »

I'm not sure how you are measuring the dynamic range of the result image, but Color Balance attempts to maintain the dynamic range of the luminance channel which is generally slightly different from the dynamic range of HSV V or HSL L since they are computed using different formulas. Luminance is the channel that most closely matches the visual brightness.
Jonathan Sachs
Digital Light & Color
Marpel
Posts: 692
Joined: September 13th, 2009, 3:19 pm
What is the make/model of your primary camera?: Nikon D810
Location: Port Coquitlam, British Columbia

Re: Colour Balance

Post by Marpel »

Thanks Jonathan,

After doing colour balance, I have usually reviewed the DR by opening the Gray>Levels and Colour transformation and referring to the grayscale DR readout (without making any changes to the transform). Although there is a Colour Space choice of HSV/HSL lower on the transformation box, and it shows HSV as a default, I don't know if it is for the DR or if it is used relative to the Colour Balance tickbox immediately above.

Marv
jsachs
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Joined: January 22nd, 2009, 11:03 pm

Re: Colour Balance

Post by jsachs »

Dynamic range is not a precisely defined term for digital images - for example, if there is a single white pixel and a single black pixel somewhere in the image, then the dynamic range could be considered to cover 0 .. 100%. The color balance transformation uses a complicated algorithm to estimate the black and white point of the image and then maps them to neutral grays of the same luminance. This process is not compatible with measuring the dynamic range using HSV V or HSL L in the Brightness Curve transformation so one would not expect the results to be consistent. The results of the Color Balance transformation are probably about as accurate a method of preserving dynamic range as you are likely to achieve by any other method.
Jonathan Sachs
Digital Light & Color
Marpel
Posts: 692
Joined: September 13th, 2009, 3:19 pm
What is the make/model of your primary camera?: Nikon D810
Location: Port Coquitlam, British Columbia

Re: Colour Balance

Post by Marpel »

Jonathan,

Thanks for the explanation and the comment on the incompatibility of measuring DR using HSV V or HSL L in the brightness Curve. However, what method do you suggest I use to ensure an image has not exceeded the 0% or 100% DR subsequent to the use of Colour Balance, or is that transformation constructed to ensure the image does not reach pure white or pure black with an image that was initially within, but close to the limits, of that range? Perhaps if an original image is so close to the limits to begin with, I should back off the DR a bit at both ends prior to the use of colour balance?

Thanks

Marv
jsachs
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Joined: January 22nd, 2009, 11:03 pm

Re: Colour Balance

Post by jsachs »

You could extract the luminance channel using Transformation/Color/Extract Channel and then check the dynamic range before and after Color Balance. In most cases Color Balance should leave they luminance dynamic range pretty much unchanged. Alternatively you could check HSL L. HSV V maxes out if any of the channels maxes out (e.g. pure red or pure blue) while HSL L and Luminance only max out for pure white.
Jonathan Sachs
Digital Light & Color
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