Color/Luminosity separation

Started 1 month ago | Discussions thread
Tom Axford Veteran Member • Posts: 8,423
Re: Color/Luminosity separation

Pictus wrote:

Tom Axford wrote:

We are talking about somewhat different things.

If you go increase the luminosity up to the maximum for all pixels, when you convert back to RGB (whatever colour space), the colours will often be washed out, possibly even to pure white. Exactly what happens on conversion from Lab to RGB will depend on the software, but usually the colour is not preserved if the luminosity is greater than RGB can represent for that colour.

Try it for yourself, I'd be interested to know what result you get. Create a pure white luminosity layer (i.e. maximum luminosity) and then combine your original colour layer with the white luminosity layer. When I have done that in GIMP, the result has not preserved the colours. There is no guarantee that Photoshop will do exactly the same.

In RGB color mode, a color layer above a white layer results in a 100% white image
In LAB color mode, a color layer above a white layer results in this

Thanks for posting that.  It is not the same as what GIMP does, but similar in that it certainly doesn't preserve the colours accurately if the luminosity is higher than can be represented in RGB.

Of course, this example is very extreme, but even much more modest operations on the luminosity layer can result in colour changes.  The changes do not normally affect the whole image, but only those areas where the colours are highly saturated.  Highly saturated blues are the most susceptible.

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