Color/Luminosity separation

Started 1 month ago | Discussions thread
Tom Axford Veteran Member • Posts: 8,410
Re: Color/Luminosity separation

Pictus wrote:

Tom Axford wrote:

Unfortunately, the problem is more fundamental than that and changing the colour space doesn't solve it.

It occurs because mathematically Lab colour has a much larger gamut than RGB.

Are you sure?

However, the difference is not because some colours in Lab cannot be represented in RGB. Instead, it is because Lab can represent saturated colours on a much wider brightness scale than is possible in RGB (whatever RGB colour space is used).

"Keep in mind that CIELab was just an attempt to create a perceptually uniform color space where equal steps correlated to equal color closeness based on the perception of a viewer. The CIE didn't claim it was prefect (because its not). Most color scientists will point out that Lab exaggerates the distance in yellows and consequently underestimate the distances in blues. Lab assumes that hue and chroma can be treated separately. There's an issue where hue lines bend with increase in saturation perceived by viewers as an increase in both saturation and a change in hue when that's really not supposed to be happening. Further, according to Karl Lang, there is a bug in the definition of the Lab color space. If you are dealing with a very saturated blue that's outside the gamut of say a printer, when one uses a perceptual rendering intent, the CMM preserves the hue angle and reduces the saturation in an attempt to make a less saturated blue within this gamut. The result is mathematically the same hue as the original, but the results end up appearing purple to the viewer. This is unfortunately accentuated with blues, causing a shift towards magenta. Keep in mind that the Lab color model was invented way back in 1976, long before anyone had thoughts about digital color management."

Anyway, in Photoshop by staying in 16 bits+RGB color mode+ProPhoto RGB color space when doing this to separate color/luminosity, I see no loss of colors.

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.

An example:

Original image

Luminance set to maximum in GIMP

HSV Value set to maximum in GIMP - this truly preserves colour (hue and saturation) while increasing luminosity to the maximum possible for each pixel individually

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