Grey World Color Balance

OK - going through the program today, poking into areas I normally ignore. It struck me that I still don't understand Grey World. Can anyone take a stab at explaining what this does?

It corrects color
When would I use it?
When you need to correct color
How it differs from Automatic Colour Balance?
It uses the grey world assumption

For what it's worth, I obviously didn't understand the tool when it was part of the Manual Colour Correction in 7 - but if it warrants being a stand alone operation, I should make the effort?

First, let me say that I'm not going to be explaining how Automatic Color Balance works since that is proprietary information. Let's just say it has an algorithm to correct the color in an image that uses information about the properties of well-adjusted images.

This filter uses a different algorithm. It is based on a very old result from photography that the colors in a sufficiently complex natural scene average to grey. So, what the filter does is to estimate the average color of the image and shift the average color so it becomes grey. "Sufficiently complex natural scene" is somewhat self-defining in that, if the grey world assumption does not produce a good result, you can declare that the scene was not "sufficiently complex" or not "natural" enough :)

The Grey World approach is not nearly as reliable as that in Automatic Color Balance. However, ACB is primarily used for situations where the color is wrong for physical reasons, e.g. improper camera white point, outdoor film used with incandescent light. However, with image editing tools people can make absolutely arbitrary changes to image color, which are not based on any principle and which cannot be predicted. Since it takes some practice and talent to tweak image colors with the various manual color adjustment filters, we added an alternative one step filter that could be used in place of ACB to perform instant correction simply on a try it and see basis. While its is hard to predict exactly when the filter will be most useful, it can be very helpful for getting colors in the right general ballpark when things are very bad.

I've attached two images I posted before. The one of the jumping boy shows that lack of complexity (and a single dominant color) gives bad results with Grey World (i.e. blue skin). However, Grey World does a better job of rescuing the the image of the church interior that was inadvertently photographed with a red filter on the lens. I've also attached a before and after comparison of my correction for a user of a badly faded image. The way the image had faded made Fade Correction filter ineffective. The first step of my correction procedure involved use of Grey World to get the image colors in some sane initial state relative to which I could perform additional enhancements. Without the Grey World option to get me started it would have been hard to make headway with the correction.(images referenced were not included here)

Just like Automatic Color Balance, Grey World has the option to change the color temperature. This is a transformation applied after the balancing to grey and can extend the utility of the filter somewhat, since in many cases color errors due to lighting related to color temperature. If I were to recommend filters for an initial one-step color correction of an image I would suggest these priorities:

1. Automatic Color Balance
2. Black and White Points
3. Grey World Color Balance

Fade Correction is something I would try only when the image has clearly undergone photographic dye fade. Manual Color Correction would be something I would apply after an initial color correction or to get some single specific color (e.g. skintone, Coke logo red) exactly right, possibly at the expense of error in some other image colors. Of course, you can always just play with filters to see what works best in your case. If you do so, I would urge you to look very carefully in shadow and highlight regions for loss of image information, especially when using extreme filter settings. I'm talking here of things like a reflection from someone's nose or forehead that contains little detail originally but becomes a white hole after correction. You have to train yourself to look for such details because normally you react to the overall look of the color in the image. -Kris



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