Assign Profile VS Convert to Profile, when to use?


(From a discussion in a forum november 2003, regarding above question

So after 3 years of banging my head against the wall to learn how to get correct colors out of my printer, I have finally made some strides towards getting accurate colors. I believe most of the confusion that came from color management for me is in the "Assign Profile", and "Convert to Profile" commands in Photoshop and thinking I needed to use them in my workflow. I have just figured out that using these two commands where what was causing me so many hassles for so many years. But I would like to know the use of each command in a real world environment and the advantage disadvantage of using them. I have been pouring over the Photoshop documentation (it states this: "When using the Assign Profile command, you may see a shift in color appearance as color numbers are mapped directly to the new profile space. Convert to Profile, however, shifts color numbers before mapping them to the new profile space, in an effort to preserve the original color appearances") as well as numerous color management books and it is never really explained clearly. Am I the only one that gets confused by these? Could someone please clear up my confusion and give me a breif overview of situations and uses for each command so I can sleep better at night.


1.) Assign Profile: Use a profile to describe how the RGB numbers in your image should be interpreted, but without converting them based on this new description. This would only be useful in applications that understand a file tagged with a profile - like PhotoShop, but not for i.e. Internet Explorer. In PS you are then previewing how the image will become if you select to convert it into i.e. your current real working space, but the RGB values you see in the Info palette still shows the unconverted, or untagged, RGB values. If you assign i.e. a Camera profile, you should convert into a working space before doing any major adjustments, since (as the name implies - working space) they are more suitable for working with your images. Convert to profile: Convert the RGB values in your image from their current "Assigned State" into another gamut, i.e. into Adobe RGB as a working space, into sRGB for displaying on the web or sharing with others, or into a Printer profile Space for printing on that specific printer.

If you don't use profiles in PS, your "Assigned State" is interpreted as your current working space. In PS, select >Window, >Show Status Bar, and then from the triangle menu at the bottom Status Line select >Document Profile, and you will always know how the current image is interpreted. When saving images, especially when they are not in sRGB on the PC (or not in ColorMatch on the Mac), always check the option "ICC Profile: xxx" in the "Save dialog" if possible. This ensures that both you and others opening the image at a later stage have a fair chance of understanding how the image is to be interpreted...
Any help?
Magne Nilsen
Software Development


2.) You can find a tutorial on the differences at http://www.digitaldog.net .
Once you run through this, the differences will be quite clear.
Andrew Rodney.


3.) Ok let me see if I understand this in different terms,

Assign profile is only to be used if you open a image that does not have a tag to let you know where it came from. For example you open an untagged picture that you know came from an sRGB source, if you try to convert to aRGB it will incorrectly convert do to not knowing what color space the source file came from. You would then open the image tagging it with sRGB then convert to aRGB once opened so that the colors will be mapped correctly. I have found that if I assign profile inside of Photoshop from say a aRGB image to an sRGB image the colors go crazy do to it being incorrectly tagged since assigning profile is not doing anything but telling Photoshop what colorspace the file came from and would then be describing the color mapping incorrectly. Basically there is little use for assign profile unless you are getting images that do not have the profile attached.

Convert to profile is to be used if you are sending a file to a printer or computer that does not support color profiling, or to convert into your working space from the native colors in the file. Example: I send a lot of images to a lab that uses a Fugi Frontier. The Frontier does not support color profiling although I have a profile to describe it, so I can proof it using adobe soft proofing choosing the profile that was created for it, adjust the out of gamut colors and then convert to that profile. I then save it untagged and then send it to the printer.

I guess the confusion was coming from the "Assign Profile" function and what to use it for. I basically do not need to use it due to the fact that I create my own files and rarely work with pictures from an outside source.

Thanks for the help! If anyone really understand color management and sees a flaw in my understanding, please comment. Please only if you are experienced at color management and use a workflow everyday. It can be confusing enough for people to understand without a ton of misinformation floating around. Thank you
lardawg


4.) lardawg,
You weren't the only one confused by this...I was too, even when reading Michael Kieran's mostly excellent "Color Correction in Photoshop" book. It only became really clear to me when I attended one of Andrew's color management course in Santa Fe. Seeing him cover the differences real-time, on-screen, and talking about what each of the two commands do was what really made it sink in for me.

The way I think about it is that Assign Profile shows how the unchanged numbers will look in a given color space...this is why the colors may shift. Convert to Profile changes the actual numbers (and a digital image is nothing but numbers) to the color space or profile you are converting to. When we created custom profiles for our printers at Andrew's workshop, we used "Convert to Profile" to print our test targets using our new custom profiles.

Stephen Scharf


5.) It helps me to think in terms of languages. Someone hands you a book without telling you what language it is written in - an "untagged" book. You look in it and, for the sake of argument, assume it is written in English. It makes no sense to you - obviously the language is not English. Photoshop goes through a similar process, only it is not as bright. Any untagged files (no embedded color space or EXIF tag) are assumed to be in your default color space. If this assumption is incorrect, the image will look wrong. This is the same as with our book: no matter how slowly and loudly you holler out the syllables, it still won't make sense interpreted as English.

Assigning a profile is the same as the person telling you what language the book was written in. This does not change any of the words in the book - it just provides context for understanding them.

If the assignment is correct, you can translate the book into something you understand. In color terms, this is converting between profiles. Luckily, Adobe's Color Engine is better at translating color data than google or babelfish are at translating web pages. The color values in the file are changed by the profile conversion, with the goal of keeping the visual appearance the same.

The quick summary: Assigning a profile to a file leaves the data unchanged but does change the interpretation of the color. The look of the image will change as you assign different profiles.

Converting to a profile changes the image data - translating from one color language to another. Ideally, the image will not change visually after the profile conversion. In the real world, there will often be some change, particularly when converting to a printer profile. If the destination color space is smaller than the source, something has to give. Choosing different rendering intents gives some level of control over how this conversion takes place.

/Ethan Hansen
http://www.drycreekphoto.com/


6.) The analogy I like (for photographers) is that of having a box of 4x5 film with a single sheet in it with NO notches. If the file is untagged, you've got the box unlabeled so it's anyone's guess what kind of film (C41 or E6) or ISO the film is at. Shooting and processing this is almost impossible to get right. If I simply write "VPS III" on the box, I'm doing the same as assigning a profile. The content of the box doesn't change one bit but know I know what type of film, ISO and processing is needed. Profiles are no different. They only tell us what's in the box, they don't alter the contents.

Convert to profile is like processing that exposed film. We are changing the contents.
Andrew Rodney


7.) Assign profile is only to be used if you open a image that does not have a tag to let you know where it came from.

Yes but if the image is unknown I usually assume (assign) sRGB, then convert to aRGB (alpaRGB?) if desired.

There's also cases where an image is improperly tagged. eg. a CM-4 shot from a Canon 1D is tagged as sRGB but is actually aRGB and needs to have this profile assigned when opened in PS.

--------------------
Don Lashier
www.lashier.com


8.)

-->Yes but if the image is unknown I usually assume (assign) sRGB, then convert to aRGB if desired?

I would not go that far! If you know the file came from a PC user, sRGB is about as good a guess as anything. If you know the file came from a Mac user, I'd assign ColorMatch or maybe Apple RGB. You're playing a big guessing game anyway. First you need to have the person who sent you the untagged file punished big time. Then you play the game of assigning different spaces on a calibrated display and hope something looks decent.

Once you Assign say sRGB, you're done. There is no "Convert to sRGB" as the numbers have meaning. Assigning sRGB then doing a conversion to the same spaces doesn't do a thing but waste a lot of time.

What we all need to realize is that there are tons and tons of different RGB and CMYK colorspaces. The notion that they are sRGB is like taking that box of unlabeled sheet film and assuming it's the best selling 4x5 since statically you'll have a better guess here. But if you're wrong, you're going to hose that piece of film big time. The big problem with all the manufacturers telling you they shot/scan/print into sRGB is it's easy for them but it's kind of bogus. It might be close, it might be a mile off. There really isn't a camera/scanner or printer that produces sRGB. If just hitting the side of the barn is OK, this might work. Frankly everything would be vastly easier if people would simply tag the files to accuractly describe what the numbers really are. That's one reason we have boxes of film with labels and notches!

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Andrew Rodney
11/14/03 04:27 PM


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