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Photoshop Channel concepts: The Power of Ten

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  #11  
Old 08-01-2007, 06:28 AM
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Graphics23 Graphics23 is offline
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Re: Photoshop Channel concepts: The Power of Ten

Quote:
Originally Posted by wallykid View Post
Hi Michael,

Thank you very much for your detailed explanation. I really do appreciate it.
You're welcome! I'm happy to help.

Quote:
My first question is about the term “tag for color management”. What do you mean by this?
In Photoshop, there's only one LAB color mode. But there are many versions of RGB and CMYK.

sRGB, Adobe RGB, ProPhoto RGB, Apple RGB, etc... are ICC Profiles. When an image has an embedded profile it is considered to be tagged for color management.

Color management does two things and only two things; it defines colors and it converts colors.

Without a profile the brightness levels in your image have no meaning. For example; in your RGB image you use the eyedropper tool and place the cursor over a given pixel. Looking at the info palette you see 196r 0g 0b. This is red. But what red; Candy Apple Red, Fire Engine Red, Brick Red, your Red, my Red? That's where the profile comes in. It's the profile that defines what the numbers mean.

Your digital camera captures an image in RGB. Your printer outputs that image in CMYK. At some point there is going to be a conversion. If colors have no definition they can't be converted.

Quote:
My second question is about PhotoShop working spaces. I was under the impression that for web/monitor output the sRGB working space should be used, but for print output Adobe RGB working space should be used. In one of my Photoshop books it even says avoid using sRGB for print output. But you seem to suggest that working in sRGB is okay for printing.
sRGB is not a working space, it's an ICC Profile. Same goes for Adobe RGB. The Photoshop "working space" is simply the ICC Profile which will be assumed, or used by default, if an image is not already tagged or you don't specifically assign one. The term "working space" is something of a misnomer on Adobe's part.

Working in sRGB is OK and has nothing to do with printing. Printing, with some exceptions, is done using CMYK. The trick is to convert your RGB numbers into CMYK numbers which your printer will interpret in such a way that you get acceptable results at output. That's where properly tagging your image comes in.

Again I point out that I'm a prepress technician. As such, I don't work in any one colorspace, nor do I prefer one over another. I use ICC profiles as part of my "trick bag" of image correction/enhancement techniques.

There are two commands in Photoshop under the Edit menu; Assign Profile & Convert to Profile. I almost always start and end my image editing with one or both of these commands. I use them so much that I have them hotkeyed to F2 and F3 respectively. They both allow you to choose an ICC profile from a list of the profiles installed on your machine.

Assign Profile changes the appearance of an image without changing the numbers, by numbers I mean channel structure (RGB, LAB, CMYK) and brightness levels. A brightness level of 196r in sRGB is not the same color red as 196r in ProPhoto RGB.

Convert to Profile changes the numbers while attempting to not change the appearance, because out of gamut colors will change based on your choice of rendering intent.

To put it another way; Assign Profile tells Photoshop what a given "red" means, Convert to Profile tells Photoshop what numbers to use to get that "red" if it is possible to get that "red" using a specific channel structure.

So for me, asking what color space to use is like asking what filter to use. My answer would be to use the one you need to get the job done. Sound familiar?

Adobe RGB is not superior to sRGB, it's different. There is nothing wrong with using sRGB any more than there is in using Unsharp Mask instead of Smart Sharpen.

My advise is to start with sRGB and stay there until you begin to notice that it isn't giving you what you want. That day may never come, but in the meantime you'll get to know what to expect, you won't have to worry about converting for web, and if you give your files to someone else there is very little chance for unexpected results.

This also brings up a very important issue, handing your files off to someone else for output. Unless you know the person creating your prints understands how to deal with ICC profiles, giving an RGB file that is anything but sRGB is asking for an unpleasant surprise.

For an in depth study of this topic you can't do any better than reading Professional Photoshop by Dan Margulis.

Regards,

Michael

Last edited by Graphics23; 08-01-2007 at 08:35 AM.
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  #12  
Old 08-02-2007, 11:15 AM
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Richard_Lynch Richard_Lynch is offline
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Re: Photoshop Channel concepts: The Power of Ten

Michael,
I don't know about pushing off someone to Dan's book as that was a pretty good primer. I use sRGB almost exclusively (largely because of tests in my workflow) and am glad to see you downplay the importance of color space -- too many, I think get riveted to an idea they've heard. Pre-press people don't have that advantage, as they have to deal with reality ;-). Probably the best thing that ever happened for me having to do with imaging was getting thrown under the pre-press train when I worked for a photography book publisher as a jack of all trades...including layout, color correction, and editing. It was a fabulous way to learn all about digital rendering.
I am curious as to your position stated:
Quote:
sRGB is not a working space, it's an ICC Profile.
That may confuse some people as Adobe clearly uses profiles as names for working color spaces. I think I know what you mean, but would you care to clarify?
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  #13  
Old 08-03-2007, 12:24 AM
wallykid wallykid is offline
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Re: Photoshop Channel concepts: The Power of Ten

Hi Michael,

Thanks again for your helpful explanations. I really am grateful.

I have one last question and then I will leave you in peace.

In your last post your compared the 2 PhotoShop commands “Assign Profile” and “Convert to Profile”. I have used “Convert to Profile” many times before, but have never used “Assign Profile”. And after reading your explanation I still can’t see why you would need to use this command. Why would you want to change how your image looks? You mentioned that you use BOTH very often. I found this surprising. Can you please give me some examples of situations when the “Assign Profile” command would be useful.

Take care,

Conor
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  #14  
Old 08-03-2007, 03:33 AM
Michel B Michel B is offline
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Re: Photoshop Channel concepts: The Power of Ten

Imagine someboby has posted an aRGB picture for the web, and the profile cannot be read in the file exif info, you'll get dull and flat colours. Then you have to assign aRGB profile, and perhaps convert it to sRGB before posting for the web.

Michel B
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  #15  
Old 08-03-2007, 10:10 AM
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Graphics23 Graphics23 is offline
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Re: Photoshop Channel concepts: The Power of Ten

Quote:
Originally Posted by wallykid View Post
Thanks again for your helpful explanations. I really am grateful.
Glad to do it!

Quote:
I have one last question and then I will leave you in peace.
Famous last words?

Quote:
In your last post your compared the 2 PhotoShop commands “Assign Profile” and “Convert to Profile”. I have used “Convert to Profile” many times before, but have never used “Assign Profile”. And after reading your explanation I still can’t see why you would need to use this command. Why would you want to change how your image looks? You mentioned that you use BOTH very often. I found this surprising. Can you please give me some examples of situations when the “Assign Profile” command would be useful.
ICC Profiles are typically used for color management, one of the most vexing and overrated issues in image manipulation. However, for color correction, purposeful use of ICC Profiles can be a tremendous boon to the prepress technician on a tight schedule.

Figure 1a would be a wonderful holiday shot, if only we could see it better. Due to the auto exposure feature of most digital cameras the picture is much too dark. As usual, the harried prepress technician is charged with quickly turning this amateur capture into something that looks like it was shot by a professional.

1a.jpg

Figure 1b was created using Photoshop’s Assign Profile command and choosing a custom ICC profile created for just such an occasion, the occasion being the need to quickly adjust a dark picture… not the Christmas holiday.

1b.jpg

As you can see, with one quick command I've significantly improved this image and made subsequent edits much easier, if they're even needed at all. Now it's a simple matter of using Convert To Profile to prepare the image for output. In this case I converted to sRGB for posting to the web.

Another example is that very often I get images which are not tagged. They open in Photoshop using my default profile, which Photoshop calls the "working space". But this may not be the correct profile for the image. I use Assign Profile to preview various profiles until I feel the color is correct, usually this just means trying sRGB, Apple RGB, Adobe RGB, etc.., after which I do whatever editing is needed, then convert to the output profile. This is very similar to what Michel B outlines above, only in the print world.

Regards,

Michael
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  #16  
Old 08-03-2007, 10:49 AM
wallykid wallykid is offline
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Re: Photoshop Channel concepts: The Power of Ten

Hi Michael,

As promised I'll leave you in peace now. Your posts have been very helpful. I really do appreciate the time you took to answer my questions. I owe you a beer or two!

Take care,

Conor
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  #17  
Old 08-03-2007, 02:50 PM
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Graphics23 Graphics23 is offline
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Re: Photoshop Channel concepts: The Power of Ten

Quote:
Originally Posted by Richard_Lynch View Post
Michael,
I don't know about pushing off someone to Dan's book as that was a pretty good primer.
Thank you!

I always recommend Dan to anyone serious about image editing. In the prepress world, Dan is the Man!

Quote:
I use sRGB almost exclusively (largely because of tests in my workflow) and am glad to see you downplay the importance of color space -- too many, I think get riveted to an idea they've heard. Pre-press people don't have that advantage, as they have to deal with reality ;-).
Yes, indeed! I deal with far too many "Artists" who get hung up on their precious colors. If they only knew what I do to their files prior to output...

In the last five years I can recall loosing only one job due to not being able to sufficiently match color, and that was because the "Artist" wouldn't supply me with hardcopy to see what I was supposed to match!

Quote:
Probably the best thing that ever happened for me having to do with imaging was getting thrown under the pre-press train when I worked for a photography book publisher as a jack of all trades...including layout, color correction, and editing. It was a fabulous way to learn all about digital rendering.
I've said many, many times that spending a year as an entry level prepress technician is an invaluable education for someone looking to work in any aspect of the graphics industry.

One major peeve of mine is that college level graphics courses don't really prepare the student to work in the real world. Too much emphasis is placed on having a pretty portfolio and not enough on production.

Quote:
I am curious as to your position stated:
That may confuse some people as Adobe clearly uses profiles as names for working color spaces. I think I know what you mean, but would you care to clarify?
As used by Adobe, the term "working space" refers to an application level setting which determines the default ICC Profile for RGB and CMYK documents. This default profile, or "working space", is used whenever one changes color mode via the Image>Mode menu commands. It is also the assumed profile when one opens any document which does not already have an embedded ICC Profile. Even, confusingly enough, when one chooses "Leave as is (don't color manage)" in the Missing Profile dialog or "Discard the embedded profile (don't color manage)" in the Embedded Profile Mismatch dialog.

I took exception to the term "working space" as used by the original poster, because it implies that one is changing the application level color settings on a per document basis, a practice I don't recommend. Converting your Adobe RGB image to sRGB for posting to the web is not the same as changing the "working space".

I'm nitpicking, certainly. But I always make it a point to use the terminology as precisely as possible when explaining these rather complicated subjects.

Now how about elaborating on that channels as layers topic?

Regards,

Michael

Last edited by Graphics23; 08-03-2007 at 05:58 PM.
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  #18  
Old 08-17-2007, 11:41 AM
dahved dahved is offline
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Re: Photoshop Channel concepts: The Power of Ten

I read your Powers of Ten, and this thread, and I say "Thanks!" for all the great info.
I'm wondering what you meant when you wrote about a quick blend of the A into the L using Overlay mode. What steps did you take to do that? (I understand that it was in LAB mode, but I don't get how you overlaid a channel on another.)

<<<For example, the other day I was working on an image where I wanted to increase the contrast between subject and background. After running the Ten Channel Action I noticed that in the A channel of LAB the subject was light and the background dark. After a quick blend of the A into the L using Overlay mode I got just what I was looking for.>>>

David
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  #19  
Old 08-18-2007, 08:40 PM
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Graphics23 Graphics23 is offline
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Re: Photoshop Channel concepts: The Power of Ten

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Originally Posted by dahved View Post
I read your Powers of Ten, and this thread, and I say "Thanks!" for all the great info.
You're welcome!

Quote:
I'm wondering what you meant when you wrote about a quick blend of the A into the L using Overlay mode. What steps did you take to do that? (I understand that it was in LAB mode, but I don't get how you overlaid a channel on another.)
I almost always use the Apply Image command when doing channels blends. One may also use Channel Mixer and Calculations, but I tend to use those more for creating masks.

Here are the steps:

Open the channels palette and convert to LAB.
Duplicate the background layer.
Click on the L channel. Then hit the tilde key (~) to view the composite so you can see the results.
Go to Image>Apply Image.
The "Source" will be the image you're working on.
Set "Layer" to Background.
Set "Channel" to A.
The "Target" is always the active channel(s) and layer of the working image.
Set "Blending" to "Overlay".
Set "Opacity" to 100%.
Click "OK".

ApplyImageScreenShot.jpg

Here's the results of the actual image I was referring to:

Before

tiger-before.jpg

After

tiger-after.jpg

Regards,

Michael
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  #20  
Old 08-22-2007, 04:45 AM
Michel B Michel B is offline
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Re: Photoshop Channel concepts: The Power of Ten

Trying to replicate this Lab correction in Elements with Benny Pedersen's Lab add-on:
- Extracting A channel in a duplicate file
- add a luminosity layer on original, ie duplicating background, new 50% gray layer under duplicate. Duplicate set to luminosity, merge down, set mode to luminosity
- copy A channel from duplicate file to new layer, link to luminosity layer and mode to overlay.

The result was not enough, so I duplicated the A layer and did minimal colour and levels adjustments.

Thanks for this tutorial (and to Benny Pedersen...)

Michel B
Attached Images
File Type: jpg tiger-beforew.jpg (87.1 KB, 71 views)

Last edited by Michel B; 08-22-2007 at 04:48 AM. Reason: adding image
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