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Author Topic: L*a*b users?  (Read 6910 times)
Czornyj
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« Reply #20 on: April 19, 2009, 12:37:38 PM »
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Quote from: digitaldog
And its been oversold for a long time. Here we have Bruce Fraser discussing this way back in 1999 (in only the way Bruce could do so)
Thanks a lot, Andrew, that's very interesting (as usual)

Quote from: Graeme Nattress
Perceptual uniformity has nothing to do with colour shifts.
Maybe I miss something... So what has to do with blue-purple, or red-orange shifts?
http://www.brucelindbloom.com/MunsellCalcH...html#BluePurple

Quote from: JeffKohn
A/B curves are still the best way to get color separation that I've found, the PS/ACR 'Vibrance' controls are just not in the same league.
Vibrance control is something different. You can get a/b curves color separation effect with HSL sliders, plus they're more intuitive, fast and convinient.
« Last Edit: April 19, 2009, 12:40:12 PM by Czornyj » Logged

digitaldog
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« Reply #21 on: April 19, 2009, 12:42:27 PM »
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Quote from: Czornyj
Maybe I miss something... So what has to do with blue-purple, or red-orange shifts?
http://www.brucelindbloom.com/MunsellCalcH...html#BluePurple

I don't think you're missing anything. Both Bruce's explain the issues well.

Quote
Vibrance control is something different. You can get a/b curves color separation effect with HSL sliders, plus they're more intuitive, fast and convinient.

And non destructive, parametric, being applied in an optimal processing order (in Adobe converters), in high bit, from the highest gamut data etc. Its interesting how old school some of the Photoshop processing techniques are for actual pro photographers capturing Raw data and using decent Raw converters. Not as sexy or controversial I guess as moves into and out of Lab.
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Andrew Rodney
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http://digitaldog.net/
Graeme Nattress
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« Reply #22 on: April 19, 2009, 01:52:56 PM »
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Quote from: Czornyj
Maybe I miss something... So what has to do with blue-purple, or red-orange shifts?
http://www.brucelindbloom.com/MunsellCalcH...html#BluePurple

Ok, I see what you're getting at now. In that case, Bruce is saying that if you treat Lab as perceptually uniform, when it is not, you'll get errors when you're using that space for gamut mapping.

Graeme
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Czornyj
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« Reply #23 on: April 19, 2009, 04:50:11 PM »
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Quote from: Graeme Nattress
Ok, I see what you're getting at now. In that case, Bruce is saying that if you treat Lab as perceptually uniform, when it is not, you'll get errors when you're using that space for gamut mapping.

Graeme

Gamut mapping, huh?

Let's say I'm editing such a picture:


...and I want to make it lighter:


Very convinient editing space, indeed.
« Last Edit: April 19, 2009, 04:50:54 PM by Czornyj » Logged

Graeme Nattress
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« Reply #24 on: April 19, 2009, 04:56:18 PM »
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Quote from: Czornyj
Gamut mapping, huh?

Let's say I'm editing such a picture:


...and I want to make it lighter:


Very convinient editing space, indeed.

The images that illustrate what you're saying don't work here - can you fix / repost in some way so that I can understand?

Graeme
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Czornyj
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« Reply #25 on: April 19, 2009, 05:04:20 PM »
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Quote from: Graeme Nattress
The images that illustrate what you're saying don't work here - can you fix / repost in some way so that I can understand?

Graeme

Pardon, it's fixed
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Graeme Nattress
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« Reply #26 on: April 19, 2009, 05:33:08 PM »
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Quote from: Czornyj
Pardon, it's fixed

Thanks. I see your point now!

Graeme
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madmanchan
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« Reply #27 on: April 19, 2009, 06:07:59 PM »
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CIE L*a*b* was designed to measure color differences. The original Delta E metric (DE 1976), which is just the Euclidean distance, has many shortcomings which is why the CIE subsequently proposed DE 1994 and more recently DE 2000 to deal with the non-uniformities in the L*a*b* math. As Bruce Lindbloom explains on his site with examples, there are cases when moving along a straight line of constant hue angle in L*a*b* causes noticeable perceived hue shifts. This is not to say it can't be used successfully for image processing, but rather that it was not the original intent; furthermore, there have been more recent models that are better suited for image processing if "perceptual uniformity" or consistency is desired.

In some cases, image processing in a perceptually-uniform space makes sense, and other cases where it really doesn't. For example, some image processing operations assume (or work best on) linear light in which case using native camera RGB, GMCY, or some linear transformation thereof is the right thing to do.
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Jack Flesher
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« Reply #28 on: April 19, 2009, 08:43:44 PM »
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Or you can open your image in RGB, add a Levels adjustment layer and slide the middle slider to like 2.00, or add a Brightness/Contrast adjustment layer and dial up brightness as desired, or even a curves layer and bump the middle -- the key is to change the blend mode of that adjustment layer to luminosity so it only affects the uh, luminosity...  What's really cool is you end up with an image that is lighter like your Lab smaple, yet the red and blue still look like red and blue and not orange and lavender.

Lab has it's uses, but frankly I have never needed it for editing images.

Cheers,
« Last Edit: April 19, 2009, 09:59:19 PM by Jack Flesher » Logged

rovanpera
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« Reply #29 on: April 19, 2009, 10:11:45 PM »
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You have to take care when using curves in luminosity mode, bright saturated colors and deep saturated colors get clipped easily. I'm generally sticking with the good ol' normal mode, and saturation mode if I want to change saturation for hi-mid-shadow tones separately. And to stay on topic, I'm not using lab apart from rare occasions I need a or b channel for masking.
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Guillermo Luijk
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« Reply #30 on: April 20, 2009, 06:45:50 AM »
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Quote from: Jack Flesher
the key is to change the blend mode of that adjustment layer to luminosity so it only affects the uh, luminosity...
But again Luminosity blend mode is just another model. In particular the well known HSB colour model already mentioned in the thread and derived from the RGB numbers of the colour profile being used.
In Luminosity blend mode, Hue (H in the HSB model) is preserved but Saturation (S in HSB) strongly changes in a non perceptually uniform way. For example a S curve strongly desaturates the deep shadows and highlights.

As digitaldog suggested, there is not only one way to do things and there is not the perfect tool. Or we'd rather say the perfect tool is that one that helps us to reach our goal in image edition as simply as possible.

BR
« Last Edit: April 20, 2009, 06:46:38 AM by GLuijk » Logged

ejmartin
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« Reply #31 on: April 20, 2009, 09:28:02 AM »
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As part of my standard workflow (in RGB color space), I use two curves layer adjustments, an S curve for contrast with the blending mode set to "Luminosity" and steepened straight line with the blending mode set to "Color" to boost color some.  However, I've always wondered what are the math formulae behind these blending modes -- what specific modifications to color space coordinates are they implementing?  Is this documented or explained anywhere?
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emil
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