Re: [Gimp-docs] Color Temperature

[Elle Stone <ellestone ninedegreesbelow com>]

On 07/07/2018 02:25 PM, Julien Hardelin wrote:
> Hi all,
>
> I don't understand why we can change the original temperature in 
> colors/Color Temperature. If this is the color temperature of the light 
> source the image was taken with, it should be a fixed value that can't 
> be changed.

Hi Julien,

I don't think "What color temperature was the original image" is a 
well-defined question. If the image was color-balanced by making a 
neutral object in the image read as neutral, at least in ICC profile 
software the color temperature of the image is now D50, as the original 
color cast from whatever light was actually used, has been 
color-balanced away. But maybe the user actually color-balanced to 
retain the impression of the color of light in the scene, for example 
capturing the colors of the sunset instead of white-balancing them away, 
or to impart a creative look via color-balancing.

Anyway, as long as the user dials in the same starting temperature for 
the Original and Intended temperature, the result is "no change". But 
for any given amount of change, the resulting color depends on the 
starting color.

For example, let's say the user wants to lower the temp by 2000. 
Starting from 6500 and lowering to 5500 produces a different result than 
starting from 5000 and lowering to 3000. Which results are "the right" 
results is basically an aesthetic judgement call.

If the user has something more precise in mind (like actually 
replicating a color change based on changing "colors of the light"), 
they'd be better off using RawTherapee's totally awesome CIECAM02 model.

Best,
Elle

> Julien
>
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