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Dan Margulis and his curves Philosophy...need help

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  #11  
Old 04-18-2012, 11:18 AM
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Re: Dan Margulis and his curves Philosophy...need

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Originally Posted by Andymania View Post
But can you elaborate in simpler terms to what you mean in the above? What is meant by gamma corrected and linear encoded?
Our digital images have a gamma encoding. For example, if you use sRGB, that encoding of the data is using a 2.2 gamma (actually it isn’t using the gamma formula since there is a tweak to the curve in the shadows but I digress for accuracy of terminology). ProPhoto uses 1.8 as does ColorMatch RGB. Raw files are 1.0 (linear) encoded data. If you come from the film days, think of these curves like H&D curves. Film wasn’t linear. You had to pump more exposure into the shadows and less into the highlights, in-between there was a straight line.

With raw data, half of all the data is within the first stop of highlights. Half of the remaining in the following stop and so forth. You can see an illustration of this in this article:

http://www.digitalphotopro.com/techn...g-for-raw.html

Photoshop deals with what we call gamma corrected images (usually 2.2 or 1.8 but that doesn’t have to be so). For this reason, and the differences where the data falls in a gamma corrected image, typically one neutralizes using the gray eyedroper tool. The black and white eyedropper tools are by default used for clipping. But with raw data, since so much data contained in the first stop of highlights, we white balance (not gray balance) in this area of the tone scale. You can use the WB tool in a raw converter on a gray but you’ll get less than ideal results. You can use the white eyedropper in Photoshop to neutralize but if it is set to the default (255), you’ll also clip everything that was lower than 255 to 255 which isn’t ideal.

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Also since I was talking about color correction, Dan talks about going by the numbers in the info pallete in regards to getting accurate color.
The mindset of an old prepress operator who’s drum scanner only provided CMYK values for a specific output and at the time, probably on a Grayscale monitor. It isn’t 1985 any more <g>. The question becomes, what numbers, in what color space using what color model?

There are numbers you can bank on and lots, lots more you can’t. If you are editing in an RGB working space (sRGB, ProPhoto, ColorMatch etc), these spaces are based on math alone and whenever all three triplets are equal, you have a neutral. That absolutely is not the case with RGB output spaces. Take a gray square that is say 50/50/50 in sRGB and convert it to an output color space for say your Epson printer using a good ICC profile. You’ll get a neutral on the print but the values will not be equal values of RGB.

The use CMYK numbers idea is just as silly as trying to use numbers for any output device. One set of CMYK numbers for a single device that produces a neutral, or a good skin tone will not with another CMYK color space. If you work with one CMYK flavor all the time and you get savvy to the four values to produce a desired appearance, then you can work by the numbers in CMYK. For that one device.

Lab is a device independent color space. So like the RGB working space numbers, you can work with such values without the issues of CMYK above.

In RGB, you know that zero is the blackest black and 255 is the whitest white. The same is true in Lightroom but using what is probably a much more intuitive scale for new users, percentages. 100% RGB is pure white, 0% black.

In Lab, if aStar and bStar are the same, you have a neutral. Lstar is much like the LR percentages but only in terms of Lightness.

So you can work by *some* numbers in a few color models. But you can also just look at the image on a calibrated and profited display. Or use both processes.

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I am practicing some of these concepts he mentions on some professionaly shot raw images. The problem is is that since I was not actually present at the photo shoot, I could not tell you what the right RGB values should be for the model's skin tone.
Neither could the original photographer really. Only Dan can write (as he does so often) what the colors should look like in his opinion but say it as if it were fact. Even with memory colors (sky, green grass, skin), can we really say that after making an image and ending up at the computer we can really ascertain the actual colors? If the colors appear pleasing to you, or the image creator, that is all that matters. Rendering an image (and a print) is part of the photographic process and actual values, colorimetry doesn’t work here. A measured color of the scene (say the models skin), in Lab and the same values transferred within Photoshop on a computer will not visually match when the Lab values are the same. If you want to know why, you need to understand the differences between what is called scene referred (the actual measured color at the scene) and output referred (the numbers we need to represent what we feel is a match). That concept is explained in this article:

http://www.color.org/ICC_white_paper...ent_basics.pdf


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Obviously, if there is significant color cast I wil spot it.
Yup. But what about a model on the beach at sunset? There should be a lovely warm cast.

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I am assuming I have to rely on either my own taste/judgment or the art director's taste/judgement.
Exactly. Or Dan can tell you what cement is supposed to look like on any image he happens to print in a book with cement, despite the environmental conditions or whether he was there to take the picture or not (almost always not). I find it a load of hogwash. If you create the image, you define the color appearance. If you didn’t create the image and you are supposed to ‘correct’ it, then you are the art director and again use your sense of what should appear correct. IF I neutralize the sunset cast on our model at sunset, and you have no other visual clues to the time of day, which is the correct rendering? The photographer or anyone else at the scene would say this is ‘wrong’ but if you are selling the outfit and the cast interfere with the colors, removing the cast may be ‘right’. It is highly subjective.
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  #12  
Old 04-18-2012, 12:45 PM
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Der_W Der_W is offline
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Re: Dan Margulis and his curves Philosophy...need

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Originally Posted by andrewrodney View Post
In Lab, if aStar and bStar are the same, you have a neutral.
I'm guessing you meant if aStar and bStar are 0, you have a neutral?

Very interesting discussion, I always love to read you contributions, Andrew!
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  #13  
Old 04-18-2012, 12:46 PM
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Re: Dan Margulis and his curves Philosophy...need

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Originally Posted by Der_W View Post
I'm guessing you meant if aStar and bStar are 0, you have a neutral?
Yes, sorry.
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  #14  
Old 04-21-2012, 01:49 PM
Andymania Andymania is offline
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Re: Dan Margulis and his curves Philosophy...need

Those articles were great Andrew. Thank you. Now in regards to different RGB spaces: If I am working in Adobe 1998 RGB and I define a specific red as, lets say R=138 G=77 B=50.
Those same numbers in say, for example, ProPhoto RGB could be a different red? I am wondering since I just finished reading a chapter in Dan's book in regards to assigning profiles, converting to profiles and embedding profiles. And since I am on the topic, maybe you guys can explain to me what exactly are the pros and cons of these things. Better yet, how they exactly work. Dan mentioned to never even bother embedding a profile since the printer on the other end will usually disregard it and use his own profile. That is hard to believe because isn't the whole point of embedding a profile is so that the person on the other end can see what Im trying to achieve? I am still relatively new to this guys so bear with me.

Kav knows already that I'm trying to make a career change from the federal government to retouching so Im trying to acquire all this knowledge. Sometimes I wonder how much of the CMYK and printing knowledge do I really need to know to get started.
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Old 04-21-2012, 02:35 PM
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Re: Dan Margulis and his curves Philosophy...need

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Originally Posted by Andymania View Post
If I am working in Adobe 1998 RGB and I define a specific red as, lets say R=138 G=77 B=50.
Those same numbers in say, for example, ProPhoto RGB could be a different red?
Yes/

Quote:
Dan mentioned to never even bother embedding a profile since the printer on the other end will usually disregard it and use his own profile.
Generally speaking, not true. As a generic statement, it is silly. All a profile does is define the scale of the numbers either someone or something (a RIP) see and uses it or ignores it. The numbers do not change.

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That is hard to believe because isn't the whole point of embedding a profile is so that the person on the other end can see what Im trying to achieve?
Exactly. Without the profile, what color should R=138 G=77 B=50 be or look like?
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  #16  
Old 04-21-2012, 03:07 PM
Andymania Andymania is offline
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Re: Dan Margulis and his curves Philosophy...need

Why would a RIP ignore it?????
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  #17  
Old 04-21-2012, 03:12 PM
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Re: Dan Margulis and his curves Philosophy...need

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Originally Posted by Andymania View Post
Why would a RIP ignore it?????
Depending on the RIP, it can be setup to see and use the profile or ignore it. Really, really old RIP’s would sometimes crash if they encountered a profile but this is hardly something I’d expect to see in the 21st century. But there was a time when old timers would recommend one not embed a profile for this reason. I suspect Dan ran into this and maybe that is why he suggests it.

There is another reason to keep files untagged. Say you have 300 images going into a book, all in output CMYK ready for print. Each profile could be 2mb in size. You’re talking 600mb to define CMYK numbers that you could just tell someone about. The data is in output color space so hopefully the RIP or other people in the chain will simply send the numbers, as is, to the output device. The profile in this case provides no benefit although I’m just opposed to providing untagged RGB or CMYK mystery meat to just anyone. If I knew for a fact that no one would open and view, edit or alter the data going to the printer, the profiles are not necessary. But if anyone needs to understand what the numbers mean, they need a profile.
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  #18  
Old 04-21-2012, 04:39 PM
Andymania Andymania is offline
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Re: Dan Margulis and his curves Philosophy...need

Oh wait.....you mean embedding a profile increases a file size?????? I didn't know that lol.

But wouldn't each image from the 300 have it's own CMYK values? How can one set of CMYK values be used to define the color of all 300 images??

AHHH....I see. Embedding a profile is useful only if the next person might alter it in some way, shape or form.
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  #19  
Old 04-21-2012, 04:46 PM
Andymania Andymania is offline
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Re: Dan Margulis and his curves Philosophy...need

Well hold on...another question. If I'm working in an Adobe 1998 or Prophoto RGB working space, then one would have to be certain that whoever receives the file has to be working in those spaces as well right? Or I should I know beforehand what working space the person who is going to receive the file is working in and set my space to his......I'm a little confused on this.

I never did understand the "convert to current working space option" in Photoshop. I thought the whole point of color management is to have consistent color from the time one pushes the shutter button to the time it gets on paper........why would you convert?
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  #20  
Old 04-21-2012, 06:24 PM
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andrewrodney andrewrodney is offline
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Re: Dan Margulis and his curves Philosophy...need

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Originally Posted by Andymania View Post
But wouldn't each image from the 300 have it's own CMYK values?
They might but the values are being described by a single profile. The profile describes the output conditions (the press for example). So 300 images, piles of CMYK values, one profile.

Quote:
AHHH....I see. Embedding a profile is useful only if the next person might alter it in some way, shape or form.
The profile defines the scale of the number. Just as telling you we are 300 miles apart gives you information that simply saying “300” does not.

Quote:
If I'm working in an Adobe 1998 or Prophoto RGB working space, then one would have to be certain that whoever receives the file has to be working in those spaces as well right?
Correct. Giving someone ProPhoto and they assume sRGB (or anything but ProPhoto) would be BAD. That is why you have to embed the profile.

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I never did understand the "convert to current working space option" in Photoshop.
NOT a good setting in the Photoshop Color Settings. It forces a conversion from the original color space to whatever color space a user has selected in those Color Preferences.
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