Wrapping your head around the L*a*b* histogram

[kkamin]

It's Lab day at the forum apparently. : )

In the RGB color mode, I understand what the histogram is telling me for the 3 color channels. I can adjust it with curves or levels with comprehension.

The Lab color channels, a & b, are confusing me greatly. All the histograms I've seen are sharp spikes at the midpoint. I don't understand what that spike is really representing. :S

1. To saturate an image in Lab, the end points of color channels a & b are moved inward uniformly. What the heck is this really doing under the hood? How is this redistributing the color tones or creating more saturated colors?

2. What does the white point and black point (end points) represent in the Lab, a & b channel histogram?

3. Any other insights into how the Lab histogram is representing color data would be appreciated. Or any other Lab related insights.

Links to resources are great too!

Thanks everyone!

[mistermonday]

LAB's gamut is much greater than all of the other color model. In order to represent what you see in a RGB or a CMYK color space the numbers will not need to be too far from the A and B center points along the X axis which is why you see narrow histograms. By moving the curve endpoints you are redistributing those low value A and B pixel values to higher numbers. When you do this you are increasing the intensity of those colors.
If you would like to learn more about LAB, the best book evr written on it is called "Photoshop LAB Color: The Canyon Conundrum and Other Adventures in the Most Powerful Colorspace". The book is authored by Dan Margulis a reknowned and respect expert in Photoshop and Color Theory.
Regards, Murray

[kkamin]

Thanks much, Murray, for both thread replies! I'll get that book for sure; it's been on my Wishlist for a while. : )

[mistermonday]

Kevin, you are very welcome. The value I got out of the book far exceeded the cost.
Regards, Murray

[amica999]

Just for snooping around while waiting: http://www.dgrin.com/showthread.php?...light=margulis

[kkamin]

Thanks so much Amica999! This is be a great supplement while reading the book!

[andrewrodney]

You might also want to ask yourself if you even need to worry about Lab. Its a great color space for scientific work, color management analysis but for image processing, its often confusing, there are no capture or output devices that handle Lab so that means at least two more conversions of your data. If you are working from original data captured raw, then the need for Lab is nearly moot, you’d do all the heavy lifting rendering the raw data into RGB color. And a lot of the older techniques that required Lab in Photoshop can be replaced using the luminosity mode.

Lab was designed for a certain task long, long before Photoshop was even an idea in the Knoll brothers head.

[kkamin]

Thanks, Andrewrodney,

I am a photographer and I don't think I'll use it that much for my own images since ACR and LR work in a 16-bit Lab-like space. But nonetheless I might still want to do some final color adjustments in PS further down the retouching road on those images.

But I also do some freelance motion design work, and often work with jpegs or pads supplied by clients and I think it will come in useful there. It seems like Lab can move around graduated tones in better ways.

[andrewrodney]

Originally Posted by kkamin

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I am a photographer and I don't think I'll use it that much for my own images since ACR and LR work in a 16-bit Lab-like space.

Actually there is zero Lab used in ACR or Lightroom. Its all an RGB processing engine.

But I also do some freelance motion design work, and often work with jpegs or pads supplied by clients and I think it will come in useful there. It seems like Lab can move around graduated tones in better ways.

If the JPEGs are of decent quality, you may find you’ll end up with better data, and less work, especially if you have to apply multiple corrections to many images, doing this either in ACR or Lightroom. There are some major advantages to parametric editing as opposed to pixel editing later in Photoshop, especially if you are doing a lot of global tone and color work.

For precise pixel editing (what most here would consider retouching), nothing beats Photoshop.