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Author Topic: TUTORIAL: How to use LAB channels in RGB.  (Read 16326 times)
kstruve
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« on: August 15, 2007, 06:49:38 PM »

Here is a short tutorial about how to use the information found in color channels - in this case, how to use LAB color mode channels in an RGB document.  Here is our photo, in RGB space with the Channels palate activated.



First, start by converting a copy of your photo to LAB color mode.  Image>Mode>Lab



Then, highlight the A or B channel (usually you'll get more information from A, but don't write off B altogether!)



As you can see, we can't see a whole lot yet, but wait ... watch what happens when we invert (ctrl+i)



The value relationships now make more sense, but it's too faint to make out much detail yet, so let's adjust the levels.  Adjust your black point so it's just at the start of the histogram curve, and adjust the white point so it's just at the end of the curve.



Now this is the information we want to put in our document.  So just select all (ctrl+A) then copy (ctrl+C), and go to your original document and paste it in.  Now the adjusted A channel is it's own layer in our RGB document.  To get the color back, I've set it's layer blending mode to Luminosity as Phischer mentioned in the other thread.



And that's about it.  Start healing and cloning your heart out from here!

Kurt
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RosyBijou
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« Reply #1 on: August 15, 2007, 08:06:58 PM »

Kurt,
This is wonderful!  I was trying to figure out how to do this from the previous thread and just couldn't get it to work!  Now it all makes sense.  Thank you for taking the time to do all the screenshots and put it together!
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Kerry
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klassylady25
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« Reply #2 on: August 15, 2007, 08:15:18 PM »

I've got you up to the point of pasting it back over your original.  I don't understand where you change the layers... or if you do, back to RGB. and then you show  luminosity on your layer1 when did you change to that.  

It's working all the way up to: "and go to your original document and paste it in.  Now the adjusted A channel is it's own layer in our RGB document.  To get the color back, I've set it's layer blending mode to Luminosity"

Lost you there.  I may have it just want clarification.

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Mhayes
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« Reply #3 on: August 15, 2007, 08:34:54 PM »

Kurt,

Nice tutorial on using Lab. When Photoshop Lab Color first came out by Dan Margulis, I bought it and slugged my way through the book. I had to read it through the first time and then worked my way through the exercises the 2nd time. Half way through the book I was beginning to struggle with some of his lessons. About that time I went online and found a great forum where the members went through each of the chapters and discussed their problems. This is a wonderful site for those who want to learn more about Lab. Here it is: http://www.dgrin.com/showthread.php?t=18203

Lab may not be the answer to a lot of problems, but there are times when it is the perfect answer. If I am not happy with the tonal range on a picture, Lab gives me the chance to go into the lightness channel and tweak in "Shadows and Highlights." It is also a great way to pop the picture without having your colors start to deteriorate like using too much saturation. Another thing I like is sharpening the lightness channel and the ability to use the surface blue to advantage on either the a or b Channel when there is a lot of noise.

Lab is not the answer to everything but sometimes it is the best fix.

Margie
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Margie Hayes
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« Reply #4 on: August 15, 2007, 10:51:01 PM »

Whoops! I meant to type "surface blur," not "surface blue."  Guess I need a spell checker for dummies.  Grin
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Margie Hayes
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« Reply #5 on: August 16, 2007, 01:54:56 AM »

Great tut, thanks Kurt!   OPR Hug

Hannie
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Hannie Scheltema
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Lorraine
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« Reply #6 on: August 16, 2007, 04:54:15 AM »

Thank Kurt,

This a great tutorial.  I've known about the LAB color mode, but never knew what to do with it.

Lorraine
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glennab
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« Reply #7 on: August 16, 2007, 06:31:00 AM »

Hi Kurt -

You're the best for taking the time to "spell out" the method you use for gleaning detail in LAB color.  I wasn't aware of the invert part of using the color channels, so that's getting filed away in my "I know I saw that somewhere, but can't remember where," file, the same place that Kenny's fantastic sandwich technique resides! I think the hard drive in my head is getting full and the partitions are totally fragmented.

I just realized that I should try your method on my current bear.  If that doesn't work, I'll have to post to see if you guys and your eagle eyes can see things I can't.

Thanks for sharing your knowledge with us, guru!

Glenna
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kstruve
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« Reply #8 on: August 16, 2007, 09:49:57 AM »

Thanks for all the comments, guys!  Candice, after I pasted the A channel into my original RGB file, it became "Layer 1".  It's still a black and white image at this point, so what I did is change Layer 1's blending mode to Luminosity.  That way it retains all of Layer 1's detail, but takes on the color of the layer underneath it.  In a way it becomes transparent, but only transparent to color.

This tutorial uses LAB color mode as an example, but most of these same steps can be used for CMYK and RGB too.  Only in CMYK and RGB, it's easier because you don't have to invert the image and increase the contrast.

Between these three color modes, we have up to 10 different "points of view" on our photos.  So keep checking your channels!

Cheers,
Kurt
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glennab
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« Reply #9 on: August 16, 2007, 12:33:42 PM »

Kurt, you're right on about "channel surfing."  It's the first thing I do when I want to find detail.  I use all 3 color spaces, because one never knows which will work best. It's seldom that one of them doesn't give me extra information.

Glenna
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What we do for ourselves dies with us. What we do for others and the world remains and is immortal. ~Albert Pine

(Photoshop CS5 /Mac Pro)
kstruve
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« Reply #10 on: August 16, 2007, 12:40:29 PM »

The only hitch about color channels, is that it only shows you extra information if your photo is in color.

Kurt
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glennab
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« Reply #11 on: August 16, 2007, 12:46:22 PM »

Kurt, somehow I've managed to choose all color photos to restore, so the channels have been invaluable.  Glenna
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What we do for ourselves dies with us. What we do for others and the world remains and is immortal. ~Albert Pine

(Photoshop CS5 /Mac Pro)
klassylady25
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« Reply #12 on: August 16, 2007, 01:12:21 PM »

Thanks for all the comments, guys!  Candice, after I pasted the A channel into my original RGB file, it became "Layer 1".  It's still a black and white image at this point, so what I did is change Layer 1's blending mode to Luminosity.  That way it retains all of Layer 1's detail, but takes on the color of the layer underneath it.  In a way it becomes transparent, but only transparent to color.

This tutorial uses LAB color mode as an example, but most of these same steps can be used for CMYK and RGB too.  Only in CMYK and RGB, it's easier because you don't have to invert the image and increase the contrast.

Between these three color modes, we have up to 10 different "points of view" on our photos.  So keep checking your channels!

Cheers,
Kurt

Salute and thanks for the clarification.  I'll keep playing with it until I lock in the paths, but was able to duplicate your work.  I've tried it on a few of my other pictures and also notice that it doesn't, as you said, work with all things. Great tool however.  Appreciate you taking time to spell it out and I do mean that from my heart.  The visual, for me, was the key.

Hugz,
Candy
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John
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« Reply #13 on: August 17, 2007, 05:21:59 PM »

nice job, Kurt!  Thanks for putting that together.
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Closecrop
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« Reply #14 on: December 08, 2012, 06:52:36 PM »

That's wonderful! haha
Who 'da thunck it?
One more tool for the tool box.
Thank you. Thumbs up
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Ne Obliviscaris
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