L.A.B. Question?

[el_Brujo]

I have been skimming through some text of late and have read on a couple of ocasions that L.A.B. is a better color model to use when retouching and color correcting, i was just wondering if this is true, what are its advantages and or is it just a personal preference, I'd be really interested to hear any comments or arguements on the subject.
Thanks in advance.

[sPECtre]

http://www.ledet.com/margulis/articles.html

If you're interested in lab, it's a must-read....

[el_Brujo]

yeah cool Spectre thanks alot, but i stiil have this question: Do you (you: anyone who reads this) find LAB is a better a color model to use when retouching and color correcting, im quite interested to hear you thoughts on the subject.

once again, thanks Spectre for pointing me in the right direction.

[sPECtre]

I don't use it for every touch-up.

Recently I went to LAB to Blur an halftone pattern from one scan: I severly blur the colors and applied several little blurs on the lightness, to mainain a crisp image.

Some use lab to extract the lightness to get a better Greyscale image than simply unsaturating the pic.

Lab is useful if you wanna play with the contrast only or the colors only, but that sounds like a pleonasm :)

[john opitz]

I use it to blur the color channels,enhance contrast in the image(L channel only). Use the a or b channel to make a mask, instead of using the pen tool. What's that anyway!? If you use the pen tool to make selections. You're just chasing rainbows and setting traps for unicorns.:) :)
Removing color casts. That's what I like about LAB.

[john opitz]

Also lab is good for fast corrections. Set your highlight and shadow to whatever numbers you want them at(L channel) and adjust the a,b channel for color,going by the monitor for that. I know some retouchers,color people that don't use numbers. They use Barcos', what they see on the monitor. This is to do more with rgb than press use.
You have to be careful with lab though. Out of gamut colors.Even for rgb use.

[sh486]

I read LAB is best used to sharpen images in Adobe Magazine when it was out (they stopped making it). Sharpen the L channel and then go back to whatever mode you were working on and it keeps the color better when sharpening.

[Greg Vander Houwen]

Welcome sh486,

Thanks for posting.

Greg

[el_Brujo]

yeah over the past couple of weeks i ve been trying the LAB sharpening technique and i have to say i quite like it and will probably go on to use it regularly
thanks for all your input guys

[sh486]

Hey Greg, thanks. I've been a member for about 2 months but was just always reading the posts.. decided to reply to this one :P

[sPECtre]

I whish personnaly to thank the C.I.E. for creating the lab colorspace that made sh486 switch form spectator to actor!

Welcome!!!

[Greg Vander Houwen]

So that's what did it. Ahh... the power of a large colorspace.

Greg

[sh486]

8)

[Hiroshige]

Thanks sPECtre for that ledet dot com link. Nice intro into the LAB }$


Hiroshige

[Stroker]

I've been playing with L*ab on-n-off for quite some time. When I added Lab to CS Displace 2D, I started getting serious about it. You know me when I get serious.

Believe it or not, but I can do filters directly in Lab mode. I just knocked out a quickie that seems to do a fine job of colour balancing and/or cast removal. A little more time and I'll toss out an extreme beta 1. The Photoshop world could use a few Lab filters, eh?

[markzebra]

Brujo - Ive found myself using LAB for the following things ..

Its great for creating masks and selections, and has the same power as "select color range" but better in some situations- (I've got an action sorted out that duplicates the image, converts to LAB takes the A and B channels out and pastes them into the channels palette of my RGB or CMYK original). I can then use these channels with Levels to create accurate masks

I also use LAB if I need to change the overall saturation of a CMYK image. This is done using the Margulis poineered method of steepening the AB curves. The reason I can't use Hue/saturation safely in CMYK is quite complex, but it causes colour shifts especially in blues.

For sharpening theres no good reason IMHO to use LAB for this really. You can achieve the same thing without mode converting, by using a Luminosity blend mode on a sharpened layer. Or fading to luminosity. Conventional sharpening using Unsharp mask or the new Smart Sharpen doesnt usually cause colour haloes in quite the way that Mr Margulis claims anyway. Unless you choose a particularly unusual situation to illustrate an example in your book %/

Lab color corrections are like playing with fire - tiny swings on the curve have a huge effect. No reason not to do it, but I guess its like using a sledgehammer instead of a chisel. Martin Evening also uses LAB to simulate cross processing effects.

I do color correction every day now as part of my job.

[Kevin Connery]

I just finished Dan Margulis' Photoshop LAB Color : The Canyon Conundrum and Other Adventures in the Most Powerful Colorspace (http://www.amazon.com/exec/obidos/tg/detail/-/0321356780/qid=1127069007/), and have to say it's as well done as his Professional Photoshop books are, and with as much depth. Not, perhaps, as much as Stroker would want :] but far more than just about anyone else would.

It's the only Photoshop-oriented LAB book on the market that I know of, and I'm glad it's an excellent one. I'm going to reread it a few time in the near future, and keep it on hand, just as I do with PP.

[Stroker]

For me, Lab is easy. It's basically Displace in colour space. I'm used to thinking identity 128 and working the halves seperately. This shared idea is why I absolutely adore manipulating Lab with CS Displace 2D. Rawr.

Power of Lab in a Nutshell:
- based on human perception, major bonus
- *complete* seperation of colour information from Lightness, which means no more sat!=0 woes, no more Hue <> Lum messing with junk, and a few other little things

Now, I don't know much about CMYK and print, so I can't really say anything about how Lab relates to them. Good or bad - I don't know. But the bad that I do know of is the toolset in Photoshop is so lacking for Lab work. This is not the fault of Lab, but trying to use 'old' paradigm tools for 'new' paradigm colour space. They just don't mix well.

[kevinf]

Agreed, I look forward to more robust Lab tools in future photoshop versions. I very frequently use lab for color adjustment work nowadays.

[pixel8]

Margulis also pointed out, somewhat surprisingly to me, that while converting from RGB to Lab and back produced no information loss, there is a small loss when going from CMYK to Lab and back. Worth remembering.

[MrJelly]

CMYK <> RGB loses info too. CMYK is a bit of a colour-space pariah... of course it has the advantage of being printable, so we put up with it's little eccentricities :P

[edgework]

CMYK <> RGB loses info too. CMYK is a bit of a colour-space pariah... of course it has the advantage of being printable, so we put up with it's little eccentricities :P

However, CMYK offers a kind of subtlty over LAB and RGB in that the CMY separations, having no shadow detail, don't produce the drastic "battering ram" effect of RGB curves and the lightness channel in particular. A contrast curve to the magenta channel in luminosity mode can work wonders on just about any face imaginable, a trick that seems to be unique to CMYK. And Dan Margulis has written extensively about the use of the Black channel for sharpening and enhancing shadow detail.

Sometimes if I'm in LAB or RGB and find myself with a case of Black Channel envy, I'll dupe the image and convert to CMYK. Then I borrow the black channel as the basis of a layer mask in the original image. A luminosity curve (or curve to the lightness channel) with such a layer mask does a far better job of targeting shadows, or allowing a steep sharpening in the way that Dan sharpens the black plate, without without sacrificing any colors that might be out of the CMYK gamut.

A powerful advantage of LAB is the way in which layers, adjustment or image, can be targeted with the blending options sliders. Having the four distinct colors available makes it easy to target specific colors quickly by excluding the unwanted tones. I find the lightness slider to be more powerful than the grey sliders in RGB and CMYK; images with multiple casts can be dealt with quickly in LAB through targeted curves.

Since reading Margulis' new book, I've found myself jumping into lab more and more for various fixes.

[markzebra]

All photoshop color conversions go via LAB anyway. LAB is the intermediate space that Photoshop uses to say "this is how colors appear"

So if you convert from RGB to CMYK for example - there are actually 2 conversion tables being referenced, one to LAB and then another to CMYK. So RGB/LAB/RGB is equivalent (in terms of loss only) to one RGB/RGB conversion. It has gone through two space conversions.

[hydrospell]

This discussion spans three years!

*bump*

I'm finding out more about Lab now thanks to this thread. Yay!