> > I think the difficulty is how to decide where the mid-grey point is.
> > Once decided upon then action is easy.
> I thought the tutorial might have had the answer to that by using the
> 50% gray layer with Mode: Difference.
> http://digital-photography-school.com/precise-color-cast-correction-with-gray-fill-layers
> snippet:
>  "This simple tutorial solves this problem by finding neutral gray in
>  the image, thereby removing the need to guess the color. In order for
>  this trick to work however, your image MUST contain 50% gray tone
>  somewhere in the image; otherwise the color correction will be off.
>  ...
>  The ‘Difference’ mode is a comparative layer mode that will invert
>  pixels containing 50% gray, turning those pixels black."
> I know there are many ways to achieve the end result, but part of
> these tutorials is about learning and experimenting, so the knowledge
> can be passed on. :-)

I agree with you 100% and an important part of that learning is being
able to discuss the various points raised. It does look as though that
tutorial should help in finding a 50% grey area but, if there is a
colour cast, then that area will never be a pure black and, of course,
the grey area will not be a pure grey.


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