Understanding Separations (RGB vs. Luminosity)

[marsh]

First off, thank you to Richard for a great book. I've recently started working thru it diligently, and know the concepts learned and tools are going to greatly improve my photos. To now, my post-processing attempts have all been "seat of the pants."

I know this is basic, but I'm a little confused on understanding the meaning behind RGB and Luminosity separations. I think a clearer understanding of these will help me better grasp the other concepts in the book.

1) In an RGB channel, do the various tone levels represent the brightness (luminosity) of the channel pixels? Taking the red channel, for example, If you have full tonal range, does this equate to 0=black (0% red light reflected) and 255=full red (100% of red light reflected)?

2) What do the tones in the Luminosity separation actually represent? Is it basically a brightness representation of the composite of all colors together. I've read page 41 over and over, but I'm still confused. I've had no experience with the Lab color model.

Hope my questions are clear. Thank you for the help.

Marsh

[Richard_Lynch]

Interesting way to put that question...Lets see if this helps.

RGB is based on additive light -- the more light you add, the brighter the result. The 3 components act equally: if all are doused you have black, if all are full intensity, you have white, if all are 50%, you have a medium gray. So, intensity in these channels drives tone. You will have to have 100% red to make white, but a 100% red reading in the channel does not mean that area HAS to be white.

The balance between the 3 sources will also drive color. mixing 100% R and G (0% B) makes yellow. It is the interplay between the 3 source colors that drives both tone and color depending on the mix.

LAB is a little different. The Lightness channel displays tone, while a and b channels carry color. The advantage is you can work with color distinctly from tone to make adjustments on one OR the other...using RGB you have to be conscious of both at the same time. Lab can be advantageous when you want the color to change, but not the tone, or tone to change and not color.

So, the color models are essentially different...

Is that helpful?

[marsh]

Yes. Your response is very helpful. Thank you for replying so quickly. Just a quick clarification, and I think I'm set.

You stated....
.

You will have to have 100% red to make white, but a 100% red reading in the channel does not mean that area HAS to be white.

Does this mean the when viewing the tonal representation of the red channel, that Black areas mean 0% red is present in the composite image, and white areas mean that 100% red is present in the composite image?

Also, when you say a 100% red reading (white) in the channel, doesn't necessarily equate to red output in the composite image, is this because the other channels may be outputting different levels, thus creating a non-white color?

In order to have "white" in the composite image, each of the three channels must show 100% of their respective colors (white in the channel view). Right??


Your explanation of Lab cleared up many questions too. Since I've not worked with Lab color before, I was overlooking the color (a-b channels) components created in the separation process. This makes the "Applying Original Color" section (p46) make more sense.

Thank you for your help.

Marsh.

[dpnew]

Hi,

I just read that section in the book, and I think I understand it from a beginner's perspective, so I'll answer your questions until Mr. Lynch has time to respond:

Does this mean the when viewing the tonal representation of the red channel, that Black areas mean 0% red is present in the composite image, and white areas mean that 100% red is present in the composite image?

Yes.

Also, when you say a 100% red reading (white) in the channel, doesn't necessarily equate to red output in the composite image, is this because the other channels may be outputting different levels, thus creating a non-white color?

Yes.

In order to have "white" in the composite image, each of the three channels must show 100% of their respective colors (white in the channel view). Right??

Right.

In your initial post, you say:

Taking the red channel, for example, If you have full tonal range, does this equate to 0=black (0% red light reflected) and 255=full red (100% of red light reflected)?

Unfortunately that's wrong because RGB has nothing to do with reflected light. Read this short description of the differences between RGB and CMYK, and I think you'll understand why.

http://www.adscape.com/eyedesign/photoshop/four/xref/color.html

I hope that helps.