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L*a*b* question: why can't it be simulated in RGB?

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  #1  
Old 09-06-2010, 06:58 PM
kkamin kkamin is offline
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L*a*b* question: why can't it be simulated in RGB?

The big benefit I see of Lab is that the luminance is not tied to the color. So you can adjust saturation without clipping the colors as fast. Or darken an image without desaturating the colors as fast.

But why can't adjustments be made in RGB with blending modes such as "color" or "luminosity" to simulate that type of isolated adjustment?
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Old 09-06-2010, 07:14 PM
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mistermonday mistermonday is offline
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Re: L*a*b* question: why can't it be simulated in

You can perform a couple of moves in RGB to emulate certain effects of LAB but LAB is a very different color model and you can not accomplish most of what LAB does in RGB.
Moving back and forth from RGB to LAB is not visibly destructive unless you do it 10,000 times on the same image. The same is not true of other color spaces, particularly CMYK.
LAB has some unique advantages for certain processes and I have done complete high end retouches from start to finish completely in LAB. I especially like it when doing D&B with separate Curve Adj layers because after you have adjusted the luminosity, you add a color correction adj layer and it will be color exact which is not the case in RGB.
And for retouchers who would like to mix RGB and LAB in the same workflow, nested smart object in PS allow you to do that effortlessly and seamlessly.
Regards, Murray
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Old 09-07-2010, 03:13 PM
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Re: L*a*b* question: why can't it be simulated in

You can (more or less) without the time and data damage of converting into and out of Lab. You use the Luminance Blend Mode. For example, the idea of converting to Lab to sharpen data is so 20th century. Use the USM in the RGB color space, then Fade using Luminosity giving you the benefit of only dealing with luminance data AND providing you another control, opacity.

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Moving back and forth from RGB to LAB is not visibly destructive unless you do it 10,000 times on the same image.
Its destructive in 8-bit and depending on the color space. Not a good idea. Every time a conversion to LAB is produced, the rounding errors and severe gamut mismatch between the two spaces can account for data loss, known as quantization errors. The amount of data loss depends on the original gamut size and gamma of the working space. For example, if the working space is Adobe RGB, which has 256 values available, converting to 8- bit LAB reduces the data down to 234 levels for neutrals. The net result is a loss of 22 levels. Doing the same conversions from ProPhoto RGB reduces the data to only 225 values, producing a loss of 31 levels. If you really must convert from RGB to Lab, do so in a high bit file (what Photoshop calls 16-bit).
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Old 09-07-2010, 04:00 PM
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Re: L*a*b* question: why can't it be simulated in

As far as I see it there is nothing during normal image adjustment and retouching that requires LAB. Also worth noting is that many tools only work in RGB.

LAB might have a high gamut; well so does RGB - depending on your profile - but a large gamut is usually doing more harm than good if it is a lot larger than any of your monitors and printers can display (good monitors struggle to display more than ~98% of AdobeRGB). Colors outside this. Well. They are not visible (since they cannot be displayed); and your camera probably didn't capture them either. Those bits are better spent on the important data in your image to reduce banding etc. If you DO work in a high gamut color space, you need (as andrewrodney said) work in 16 bit to avoid the worst issues. Converting to LAB and back does incur a little loss (although it is minor in 16 bit mode).

Some say that LAB is great because you can adjust luminosity separate from color. Well you can do that perfectly well in RGB color mode if you know how to use Photoshop. Luminosity blend mode; as well as hue/saturation/color blend modes let you work with colors and luminosity separately.
Example: a curves adjustment set to Luminosity only affects the Luminosity of the image - not the color/saturation. Likewise if blending is set to color it will affect hue/saturation but not luminosity.

Sure; you CAN perform a high end retouch in LAB with great results. You could also do it in CMYK. Or HSL. With one hand on your back. But why would you? I see a bunch of disadvantages just to make a few adjustment layers behave slightly different...

I would like it if someone was able to show me some good use for the LAB color space. Something that I've missed that allows for a quite useful adjustment that would be really hard in RGB... So far i just see people converting to LAB to do color noise reduction because they have not found blending modes (yet somehow found LAB)... -.-
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Old 09-07-2010, 04:41 PM
lukasdp lukasdp is offline
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Re: L*a*b* question: why can't it be simulated in

any color management book by Dan Margulis might be a good place to start,
I've picked up some great color saturation techniques from him as well as a very streamlined color corrections workflow that utilizes both RGB and LAB.
I find both colour spaces very useful. Removing moire patterns and minimizing noise - two things I think are done easier and better in LAB then RGB.
I recommend his books and Lynda tutorials if you are curious.
~Lukas
lukasdp.com
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Old 09-07-2010, 04:50 PM
kkamin kkamin is offline
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Re: L*a*b* question: why can't it be simulated in

I think there is a definite use for it. I just finished a 6-hour course at Lynda by Deke McClelland.

http://www.lynda.com/home/DisplayCourse.aspx?lpk2=616

But I'm still having a hard time wrapping my head around the color space. I hope Dan's book will be thorough. The video's I've seen of Dan working on Lab have impressed me.

http://revision3.com/pixelperfect/labcolor

http://www.peachpit.com/podcasts/epi...7-d2cdd6fc9d60
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Old 09-07-2010, 06:05 PM
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Re: L*a*b* question: why can't it be simulated in

Quote:
Originally Posted by kkamin View Post
I think there is a definite use for it. I just finished a 6-hour course at Lynda by Deke McClelland.
But I'm still having a hard time wrapping my head around the color space.
If Deke has you confused, Dan will positively make your head explode!
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Old 09-07-2010, 07:09 PM
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Re: L*a*b* question: why can't it be simulated in

Wow, get stuck on a plane for a day and look what happens!
Andrew and Chain, if you argue that the conversion to LAB and back is "very destructive" it ought be easy for you to produce a real photograph showing convincing signs of this serious destruction. That is, something that a skilled person would, with careful observation, perceive as damage, either at the moment after reconversion to RGB or thereafter with any plausible attempt to correct. This should be a real-world demonstration, with a real photograph. Not a histogram. Not a gradient or other computer-generated graphic. Not a demonstration that the file is not pixel-for-pixel identical with the original.
The LAB conversion myth has been circulating for 15 years, and scores of people have done their own careful testing and are unable to come up with such a demonstration on even a single image. It's a safe prediction that if you ask people for an example all you'll get back is a passle of worthless histograms and gradients, together with links to sites that say that conversions to LAB cause catastrophic damage but lack example images to back the claim up.
I have had the discussions with Dan Margulis and am convinced he is right. You can check out the proof and arguymants in chapter 6 of his book.
Whatever the merits of the theory, however, it's contradicted by the practical results with real images.
Kevin, I watched Deke's LAB course at Lynda.com a few years after I learned LAB from Dan Margulis. The Deke course was a big disappointment. Dan's LAB book for me was better written and easier to understand than all of his other books on Color.
Andrew, have you actually read / worked your way through Dan's LAB Color book or are you assuming it is similar to his other works?
Andrew and Chain (forgive me for jumping around) yes, as I pointed out there are some RGB work arounds like Fade to Luminosity, and setting blend modes to Luminosity. I actually never go into LAB if it is just to sharpen.
And yes, there are a number of things you can do in LAB that you can not do in RGB as well as many things that you can not do as quickly or as efficiently in LAB - Try CTRL 4 followed by Ctrl I, followed by Ctr 2
Try creating an asymetric spatial frequency split of an imge RGB.
I am out of time now but will try to post a couple of examples over the next day.
Regards, Murray

Last edited by mistermonday; 09-08-2010 at 02:40 PM.
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Old 09-07-2010, 07:25 PM
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Re: L*a*b* question: why can't it be simulated in

Quote:
Originally Posted by mistermonday View Post
Wow, get stuck on a plane for a day and look what happens!
Andrew and Chain, if you argue that the conversion to LAB and back is "very destructive" it ought be easy for you to produce a real photograph showing convincing signs of this serious destruction.
At what stage in the files life? The beginning? Towards the end? Shown on output to what device? After how much additional editing?

Throwing away good data, for no reason is a bad idea. Working with an 8-bit file and making such conversions leads to data loss, its simple math. Simply work in high bit, and now you’re just converting for little reason and pretty much wasting your time, but the data loss argument is now moot.

Quote:
That is, something that a skilled person would, with careful observation, perceive as damage, either at the moment after reconversion to RGB or thereafter with any plausible attempt to correct. This should be a real-world demonstration, with a real photograph. Not a histogram.
Are you channeling Dan Margulis now? Are you aware of his bogus “16-bit challenge?” If not, start here:
http://www.brucelindbloom.com/index....nMargulis.html

Quote:
I have had the discussions with Dan Margulis and am convinced he is right.
Well then lets move on and agree to disagree.

Quote:
You can check out the proof and arguymants in chapter 6 of his book.
I have the book, its not proof.

Quote:
Andrew, have you actually read / worked your way through Dan's LAB Color book or are you assuming it is similar to his other works?
Yup, I find his still confusing and very long winded. I’ve sat in on his seminars on several occasions as well at PSW. I find his style of wiggling out of specifics (this will work unless it doesn’t), irritating. I find his analysis of images (this area of the image is supposed to be neutral) silly (yes, cement is always neutral right? Unless its lit by sunset).
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Old 09-07-2010, 07:28 PM
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Re: L*a*b* question: why can't it be simulated in

Quote:
Originally Posted by mistermonday View Post
Moving back and forth from RGB to LAB is not visibly destructive unless you do it 10,000 times on the same image. The same is not true of other color spaces, particularly CMYK.
Now why would that be?
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