Dan Margulis Applied Color Theory

Desaturation and the L Channel


   Date: Sat, 3 Jan 2004 20:27:28 -0400
   From: "Michael Cervantes
Subject: Desaturate should be equal than L channel?

Does Desaturate command eliminates color?

If it is true, then the resulting desaturate image should be equal than L channel in Lab mode, where color and luminosity are separated. Right?

Compare both method.

Duplicate an image
Copy L channel to a new Layer

In original image duplicate layer, and apply Desaturate.

Set both new layers to 50% opacity.

Happy and prosper 2004.

Michael Cervantes
MC Design Studio
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   Date: Sat, 3 Jan 2004 16:48:28 -0800
   From: "Mike Russell"
Subject: Re: Desaturate should be equal than L channel?

Michael Cervantes wrote:

Does Desaturate command eliminates color?
[does not match the L channel in Lab mode]

No - it uses an HSL based calculation, which depends only on a geometric calculation in RGB space.

In CMYK mode, desaturate assumes C=M=Y is neutral, ignoring the K channel - so the results can be quite weird, neutral areas with a heavy K component end up very light.

Even in Lab mode, Image>Adjust>Desaturate does not simply map to the L channel - nor does it match the desaturate of the same image converted to RGB.

Other tools - sponge, and the layer modes, are affected by the same issues. For example, a CMYK layer in color mode results in interesting shades of cyan and brown.

Mike Russell
www.curvemeister.com
www.geigy.2y.net
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  Date: Sun, 4 Jan 2004 12:49:46 -0400
   From: "Michael Cervantes"
Subject: Re: Desaturate should be equal than L channel?

Mike, thanks for your answer, it is clear and concise. Now, I'm asking myself, what should be truly Desaturation. Copying "L" Channel or Photoshop Desaturation command? I think that "L" channel shows really a desaturate image. What do you think?

Best regards

Michael Cervantes
MC Design Studio
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   Date: Sun, 4 Jan 2004 12:40:55 -0800
   From: Mike Russell
Subject: Re: Re: Desaturate should be equal than L channel?

Michael Cervantes wrote:

Mike, thanks for your answer, it is clear and concise. Now, I'm asking
myself, what should be truly Desaturation. Copying "L" Channel or
Photoshop Desaturation command? I think that "L" channel shows really
a desaturate image. What do you think?

My take on this is that desaturation, as when using the sponge tool, is useful in small amounts, as when a single color is out of gamut and you want to knock it down a bit.

Applying a CMYK in luminance mode is also a relatively unusual occurence, but allowing colors to leak through is probably a bug, and I could see an option in Photoshop's future specifying something like "color modes in Lab" or some such.

The true nature of desaturation I leave to philosophers.  As a practical matter, there are other ways to accomplish desaturating that get around these limitations.  For desaturating the entire image, you're probably better off mixing channels as Dan describes in his book, simulating the effect of colored filters in B&W work.

Mike Russell
www.curvemeister.com
www.geigy.2y.net
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   Date: Sun, 4 Jan 2004 19:07:51 EST
   From: Dan Margulis
Subject: Re: Re: Desaturate should be equal than L channel?

Michael Cervantes writes,

Now, I'm asking myself, what should be truly Desaturation. Copying "L"
Channel or Photoshop Desaturation command? I think that "L" channel shows really a
desaturate image. What do you think?

They are two versions of the same thing, but it is possible that you will like the L better, because its encoded in a way that its darknesses don't correspond to those of any channel in RGB or CMYK.

If you have two RGB files that look alike but one is in Apple RGB and the other is in sRGB, the individual channels will not look alike. The sRGB channels will be lighter, because sRGB defines all colors as being darker than in Apple RGB, therefore the channels have to be lighter to compensate if you want the same result.

In the L channel the darkness is generally defined as being even darker than sRGB, so the channel has to be even lighter than the corresponding sRGB channel. But also, L devotes more space to midtones and less to highlights and shadows than RGB and CMYK.

Therefore, if you have two copies of the same RGB, and you use Luminosity mode or else convert to grayscale on one, and you convert to LAB and use the L channel on the other, the L channel will look lighter and also will have more midtone contrast. Often that makes it look better than the first version, sometimes worse. You get to decide which one you prefer.

Dan Margulis
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   Date: Mon, 05 Jan 2004 14:30:19 -0800
   From: Steven Barton
Subject: Re: Re: Desaturate should be equal than L channel?

When R=G=B the color is desaturated. This can be achieved any number of ways. But if you are trying to produce a quality black and white version of a full color image, converting to grayscale will generally give you good results, although you may choose to adjust the midtone placement. The L channel will give you similar results, but usually brighter. Simply using Photoshop's desaturate function will average the RGB channels and is, therefore, not usually the best approach. For example, desaturating the petals of an RGB yellow sunflower will produce a heavy gray, 61% tone compared to 21% achieved by taking the L channel.

Steven Barton

Adobe Photoshop training classes are taught in the US by Sterling Ledet & Associates, Inc.