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is it better to start retouch with a flat image?

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  #21  
Old 04-10-2015, 04:25 AM
captain_j_hook captain_j_hook is offline
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Re: is it better to start retouch with a flat imag

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Originally Posted by klev View Post
Everything you learned is incorrect. I'm not sure where you read it, but you picked bad sources of information.
[...]
If you're interested in this I can suggest some reading material.
I read the two books Dan Margulis wrote. I don't think I picked bad sources of information. But yes please do suggest some reading material because I only seriously started with LAB one year ago and I still have a long way to go.

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Originally Posted by klev View Post
For the LAB problem, you can see it by scaling the A and B channels.
Could you please be more precise?

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Originally Posted by klev View Post
As for it being a bad idea, we don't have any good way to model a lighter version of the same color without variation.
Why not? A lighter version of the same color is just a lighter color. Like... Talking about LHS values, only L is changed. H and S stay the same. I can't get why.

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Originally Posted by klev View Post
Part of the reason is that color models do a poor job of modeling various phenomena such as absorption, most of these models use non-linear computation, and non of them guarantee spectral alignment. They use the rule where if two colors appear the same under the same conditions by their measurement standard, they are equivalent..
You say most of these models. What's the difference between one model and another? I am really interested in the topic.

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Originally Posted by andrewrodney View Post
Did you see what Thomas wrote about doing it your way rather than his way?
Yes I did he says "The saturation effects that Simon considers a defect is actually something that most users actually want". So to reply to your

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Originally Posted by andrewrodney View Post
You say the ACR engine can't separate H from L from S yet as I asked, how do you think HSL in both products operates?
I guess they operate with a non-totally pure luminance curve as he states.
He simply says the curve delivers what "most users actually want" and the results with a pure luminance curve were "visually worse looking". I get from a commercial point of view where he is coming from but I couldn't agree less. It's not a problem if they are visually worse looking because it's just a temporary thing: you have to go through color correction and enhancement before you can actually see some visually nice things.

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Originally Posted by andrewrodney View Post
Are you familiar with what Lab is and how it was created?
I've read a lot about it but thanks for your color history anyway, very nice of you I didn't know about the experiment. But I knew it came before Photoshop and computers. It's more a mathematical abstract model.

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Originally Posted by andrewrodney View Post
Lab colorimetry is a reliable tool for predicting whether two given solid colors will match when viewed in very precisely defined conditions. It is not, and was never intended to be, a tool for predicting how those two colors will actually appear to the observer or for image editing.
I wouldn't say that LAB is a tool for predicting how colors will actually appear. LAB helps you with color shifts and mixes that couldn't otherwise be obtained in RGB or CMYK. You "mathematically", "theoretically" get those colors and then go back to RGB and let PS do the calculation. It's like sharpening in LAB. LAB can conceive very bright yet saturated colors (conversely to RGB, where high saturation and high brightness just convey to a very high number which means "nearly white"). You sharpen in LAB and then go back to RGB and you won't have white halos but colored ones. PS calculates the shift and manages to translate those bright saturated colors in RGB. And it's magical

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Originally Posted by andrewrodney View Post
Keep in mind that CIELab was just an attempt to create a perceptually uniform color space where equal steps correlated to equal color closeness based on the perception of a viewer. The CIE didn't claim it was prefect (cause its not).
I can understand what you say but I would not refer to LAB as a perceptual color model; it's more about theoretically defining colors in numbers and then try to achieve them in a most convenient color space for output (RGB and CMYK).

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Originally Posted by andrewrodney View Post
Most color scientists will point out that Lab exaggerates the distance in yellows and consequently underestimate the distances in blues. Lab assumes that hue and chroma can be treated separately. As for the blue shifting you ask about, there is an issue where hue lines bend with increase in saturation perceived by viewers as an increase in both saturation and a change in hue when that's really not supposed to be happening.
I would be interested to have a reading about it! It seems an interesting issue to delve into.

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Originally Posted by andrewrodney View Post
If you are dealing with a very saturated blue that's outside the gamut of say a printer, when one uses a perceptual rendering intent, the CMM preserves the hue angle and reduces the saturation in an attempt to make a less saturated blue within this gamut. The result is mathematically the same hue as the original, but the results end up appearing purple to the viewer. This is unfortunately accentuated with blues, causing a shift towards magenta.
So you mean it's a flaw of perceptual rendering intent in LAB mode?
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  #22  
Old 04-10-2015, 08:06 AM
skoobey skoobey is offline
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Re: is it better to start retouch with a flat imag

Don't over think.
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  #23  
Old 04-10-2015, 11:56 AM
klev klev is offline
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Re: is it better to start retouch with a flat imag

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Originally Posted by captain_j_hook View Post
I read the two books Dan Margulis wrote. I don't think I picked bad sources of information. But yes please do suggest some reading material because I only seriously started with LAB one year ago and I still have a long way to go.
I like Margulis' work. I lost my previous response where I mentioned a number of books. Thinking about it again, it's difficult to say which ones are practical. Mark Fairchild's book has a lot of good information about chromatic adaptation models. If you're looking for an overview of practical knowledge, I would suggest one of Reinhard's books. They're quite a bit more modern and not as specific to printing industries.

LAB is a different matter. You still have a defined light source, and it attempts to describe things by their appearance under that specific light source. Note the word appearance. It is not a function of radiometric intensity. RGB is a little different. It's depicted by convolution. Essentially each channel is a sum over all wavelengths where the contribution of any light of that wavelength to a specific channel is defined by the product of the intensity of light matching that wavelength and the defined scale factor for the contribution of that wavelength to that specific channel. This means that some wavelengths (in theory) contribute to more than one channel.

Both cases leverage the idea of a neutral value. It's just one of convergence. Note that it's supposed to correlate to the color of unfiltered light, the composition of which produces a visual match to the defined reference (typically a black body temperature value).

In both of these cases, definitions are somewhat perceptual in that they don't directly consider spectral composition of returned light. This isn't all that important unless you're studying effects of metamerism, but for the sake of discussion here, it's incorrect to suggest that one gives you the impact of directly changing the intensity of a light source, or that scaling L definitively gives you a lighter version of the same color either as it would be perceived or tied to some physical state (again hard to define here, because even raising the intensity of a light can result in different appearances due to metamerism).

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Originally Posted by captain_j_hook View Post
So you mean it's a flaw of perceptual rendering intent in LAB mode?
I don't know what he was referring to there. The CMM is just a color engine. It defines transformations between color spaces as described by color profiles written to ICC standards. Perceptual mode has been practically broken since the beginning, so I don't know why anyone would use it over relative colorimetric. Version 4 profiles and CMM updates were actually supposed to fix that, but I don't see it.
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  #24  
Old 04-10-2015, 12:13 PM
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andrewrodney andrewrodney is offline
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Re: is it better to start retouch with a flat imag

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It's more a mathematical abstract model.
Yes but the same could be said of RGB Working Spaces like sRGB, ProPhoto RGB etc.

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So you mean it's a flaw of perceptual rendering intent in LAB mode?
Itís a flaw in Lab, itís not based on color appearance as Klev writes.
LAB as supposed to be perceptual color model.
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  #25  
Old 04-10-2015, 12:23 PM
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andrewrodney andrewrodney is offline
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Re: is it better to start retouch with a flat imag

Danís book is very, very useful if you receive butt-ugly rendered images and have to fix someone elseís mistakes. Danís no photographer and the examples in his book are mostly just awful. If handed lemonís, make lemonade. His use of Lab does that although itís questionable if the work by and large couldnít be done in RGB. For example, weíre told to sharpen in Lab but the Fade luminosity blend mode used will produce the same net results (itís not identical but it does what the Lab group suggest is needed).

If youíre working with raw data, if you end up with a Lemon, itís largely your own fault. I suspect 90% of the issues Dan fixes in his book using Lab or any other technique could be done before the image is even rendered. But Dan has an incorrect view of ACR and AFAIK, he has never used any other raw converter (Iíve never seen him mention it). Raw is like digital clay. In the wrong hands you get an ugly RGB vase and in the right hands you get really good RGB imagery. Or you can take his suggestion which I think is silly; zero everything out in ACR, render a lemon and then fix it in Photoshop using Lab or something else. When all you know is a hammer, everything has to be nailed in Photoshop proper.

IF you charge by the hour, zeroing out the raw converter and fixing the lemon you just rendered makes a lot of sense. Otherwise itís not very sensible; GIGO:Garbage In Garbage Out.
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  #26  
Old 04-10-2015, 12:52 PM
rudym rudym is offline
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Re: is it better to start retouch with a flat imag

Renato if you're still around just follow Skoobey post.
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  #27  
Old 04-10-2015, 02:09 PM
klev klev is offline
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Re: is it better to start retouch with a flat imag

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Originally Posted by andrewrodney View Post
IF you charge by the hour, zeroing out the raw converter and fixing the lemon you just rendered makes a lot of sense. Otherwise itís not very sensible; GIGO:Garbage In Garbage Out.
That's more of a design thing. What you see in the raw processor window is a representation of how the image will appear mapped into a gamma corrected space with a smaller volume than the source gamut. It's much more than just a change of coordinate basis, so it's not possible to completely reproduce the behavior of adjustments made by the raw processor.

I suspect what he wants is a set of tools made to do extensive retouching on data that retains a set of information closer to that produced by the raw, but I don't know of any options there.
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  #28  
Old 04-10-2015, 02:25 PM
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andrewrodney andrewrodney is offline
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Re: is it better to start retouch with a flat imag

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Originally Posted by klev View Post
That's more of a design thing. What you see in the raw processor window is a representation of how the image will appear mapped into a gamma corrected space with a smaller volume than the source gamut.
Well at least in the ACR engine, that doesnít have to be so, itís processing color space uses ProPhoto RGB primaries with a 1.0 TRC, you can encode in ProPhoto RGB itself or, itís rather simple to create a ProPhoto RGB 1.0 TRC profile out of Photoshop, select that in ACR and be done.
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I suspect what he wants is a set of tools made to do extensive retouching on data that retains a set of information closer to that produced by the raw, but I don't know of any options there.
He being Dan? No, heís totally confused by the product and doesnít understand how it functions or how it works. He stated many years ago ďitís unfit for professional useĒ. See Jeff Scheweís text:http://forum.luminous-landscape.com/...4234#msg624234
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  #29  
Old 04-10-2015, 03:21 PM
skoobey skoobey is offline
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Originally Posted by andrewrodney View Post
Well at least in the ACR engine, that doesnít have to be so, itís processing color space uses ProPhoto RGB primaries with a 1.0 TRC, you can encode in ProPhoto RGB itself or, itís rather simple to create a ProPhoto RGB 1.0 TRC profile out of Photoshop, select that in ACR and be done.
This. .
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  #30  
Old 04-10-2015, 03:30 PM
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andrewrodney andrewrodney is offline
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Re: is it better to start retouch with a flat imag

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This. .
This... what?
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