I know that assign is just for the monitor whereas convert actually changes the profile and hence, the numbers.
Assign is not just for monitors. It is a metadata label/tag for colour managed software. The use of the tag could be for "raw" monitor previews, softproofing or for actual colour conversion at a later point in time. As you say, the current file numbers do not change as a conversion has not taken place, however future numbers in later conversions will change based on the input label/tag.
But what I've found is that if I have a file in 3 channels and I have to convert it into another profile without losing my 3 channels file, it is easy to go to assign profile and save it with the profile embedded.
What is the colour mode of the three channel file? Is it an RGB file? Is it a CMYK file with one channel empty? Is it a file with spot channels?
You know that assigning a profile does not convert it, it is simply adding a different description/tag/label of the colour numbers in the file (the values in the file only being half the story, with a colour/tone description being the other half).
So all you are doing is putting on a different label and saving with a different label. Colour managed software will usually honour this label, even if the contents of the label are not what the label indicates! You could put white vinegar in your water bottle and label it as "water" (assign profile).
If putting a "fake/false" profile onto the image creates the desired or correct values at a later conversion, then all is good in a practical sense, even if this is incorrect by the textbook. If putting a "fake/false" profile onto the image creates an incorrect result, then there would obviously be a problem with incorrectly using the assign profile command.
Now, I know that I did not change the numbers but my file appears with the "right" profile embedded. Why is Photoshop allowing me to do these things?
Assign is simply a tool. It is up to the end user on how a tool is used.
Later versions of Photoshop have a warning dialog box when one uses the assign profile command on a tagged image (the warning can be disabled though).
There are valid reasons for using the assign profile command. A colour managed workflow requires flexible tools and the assign command provides required flexibility.
What happens if I receive a file with a profile that I think is the expected but it was generated by this way?
This has been the question ever since Photoshop 5 (not CS5).
There are only two real options:
i) Explicity trust that the source file is colour manged correctly and that the tagged profile is true and accurate. One simply honours all profiles, if the colour/tone is then wrong, the problem is with the person who supplied the file tagged with the incorrect profile.
ii) Honour incoming profiles, but don't put 100% faith in them...if the colour looks wrong with the incoming profile, question this with the supplier of the file, tell them that the tone or saturation or hue appears to be too incorrect and would they like you to honour their file or to use your best judgement on what their intent was.
And last question, if I already have my 3 channel file, perfect and ready to print, how do I convert it into another profile without losing my work?
This is where I am getting lost...is this an RGB file? Unless the file is in final device space, then it will need a conversion to device RGB. To see what would happen if no conversion takes place, use the assign command. You either keep the current RGB numbers and accept the colour they produce with no conversion (likely very different to the original colour) - or you convert and change the numbers so that you get the "same" visual appearance as the original image.
If you are working in a 3 channel device space, then the file is ready and perfect for that device without further conversion.
If you are working in an abstract "working/editing" space that is not a device space - then the file will require conversion.
If the file can simply be (mis)tagged with the device space when it is not a device space file, and if the colour is still "correct" - then don't convert, the colour tag is now device and the file should be considered in device colour space (even if you know that this is a white lie). Simply pass the data to print without colour management/conversion to device profile - as the file is currently in the device space (if only as a white lie).
Stephen Marsh
