"Assigning a profile" is a notion both fundamental for color management and not so easy to vulgarize. Nevertheless, I am going to try through this page to give you the basics, because that's where everything starts...
Key points if you are a beginner ...
Here are the key points to remember on assigning an ICC profile. The rest of this page will be dedicated to those of you who want to dig into more details.
First of all, it is important to understand that assigning an ICC profile to an image is only done once !
When you open a picture in Photoshop without any embedded ICC profile, it cannot be displayed correctly. You have then to assign an ICC profile and if possible the right one.
That means you absolutely need to know from where it comes from : internet, scanner, camera in sRGB mode or Adobe RGB 1998 mode ...
It is as if you were finding a bank note on the floor. You see 100 written on it but you do not know the country it comes from. Before using it in France, you will need to go to an exchange office, so that they can tell you from which country that money is and change it to Euro.
The action of knowing from which country that money is like assigning a profile to the picture.
Remember that assigning a profile is only done once. Once you know the country of the bank note, you do not need to question yourself a second time ! Then, you can go through multiple profile conversion (see next page) to go from one device to another (as if you were travelling from one country to another).
Since there are wider or smaller color spaces, ICC profiles for each external device, what we now need to know is how to communicate the right color, the "same" color as much as possible from one device to another, while taking their characteristics into account. Communicating the "right color" starts with assigning the right ICC profile to an image. Let's now see why and how...
If I were to sum it up! It is quite like in a bureau de change: you find a 100-something bill on the floor (a currency you've never seen before) and you want the equivalent in euros so you go to the bureau de change, which will enable you to identify this famous bill (and accessorily make you rich... or not!). Before giving you the equivalent in euros, it is first necessary to know where it comes from... because that's what will give its absolute value (relatively to a global standard that could be gold). It is the same for images...
Why is it necessary to assign an ICC profile or a color space to an image?
This paragraph is one of the most important of this site. A corner stone!
Assigning an ICC profile or a color space to an image enables it to be displayed correctly, to display the "right" color hence the real La*b* colors.Let's take the history of an image file back from the start...
Important note! I'm going to take the shooting of a photo as an example. Whether it is taken with a smartphone, an "amateur" camera or a professional camera body, it is always shot as a RAW file.
All cameras, even smartphones, don't know how to shoot a photo in anything other than RAW. The RAW file is thus internally developed by a sort of mini-Photoshop in order to provide you with a nice JPEG file. Color management and the "profile assignment" then "conversion to a color space" session is thus done transparently for the user but is automatic. Without color management, no colors, even in a smartphone!
My RAW file is thus a digital negative. With pro cameras or some amateur ones, you can get this RAW file to develop it yourself in a demosaicing program like Camera Raw or Lightroom. We'll see how it works a little later. But let's start with a smartphone or an amateur camera because they deliver a JPEG file directly, without other possibilities by the way, allowing exception. But they are necessarily shot as RAW files that will be internally developed by a sort of mini-Photoshop in order to give you your image under the form of this famous JPEG file.
So how does it work? In two steps, invariably... following this analogy: you find a 100-something bill on the floor. You don't know which currency it is. Of course you can't use it in your favorite stores! So first you need to figure out what it is. What currency is it? In color management that would be the "assignment of a profile". OK, so it's 100-something hence completely useless in France! You need to change them into euros following the change rate of the day hence in reference to a universal value - let's say gold. In color management this standard is called the La*b* color space. It is called "conversion". In photo, assigning an ICC profile doesn't change the RGB values of the file. It's just that the RGB values are displayed "correctly" and not in a matter-of-fact reading. Then it is converted to another currency, for instance in euros, and it becomes 10 euros - hence still with the same absolute value - but 100 became 10. La*b* colors are identical but the RGB values of the file have been changed.
1 - Case of a JPEG photo: assigning the camera's ICC profile to the image directly inside the camera
The camera's mini-Photoshop must read the RGB values contained in the photo and translate them into the "right color" hence its "ideal" La*b* colors. A 255, 0, 0 RGB signal must be displayed with a red but what La*b* red? It is thus done reading the ICC profile's instructions.
Let's imagine you shot a perfectly grey wall. Your JPEG photo should be perfectly grey at this place and your file should thus have RGB values like 128, 128, 128. But your camera has a few defects and the RGB values saved in the file are rather like 128, 138, 126. If you display these values in Photoshop, you'll have a greenish/yellowish grey instead. How disappointing! But since your camera's manufacturer knows it has defects, he calibrated it so he inserted its defects in its ICC profile meaning in a table (a matricial table) where he inscribed: I give you a neutral grey 128, 128,128 and you save it as 128, 138, 126 in the RAW file hence with an ugly color predominance! So the table is here to tell you that if you have an RGB value of 128, 138, 126 you should in fact display a 54, 0, 0 La*b* color and not a weird grey.
The RGB value in the file is still128, 138, 126 BUT the camera acted like it's 128, 128, 128 to display it. Assigning an ICC profile only has one use: make sure that the RGB values are interpreted correctly and thus displayed without trying to get a match between the RGB values and their "logical" display. An RGB value of 128, 138, 126 doesn't have any reason to be displayed as a medium neutral grey without an ICC profile. An ICC profile doesn't change the file. The RGB values of the RAW file remain identical but are then interpreted correctly. It is just a sort of user manual, a post-it stuck to the RAW file to tell it how display/see its RGB values correctly.
All of this results in the fact that an ICC profile only needs to be assigned once in the life of a RAW file. Many conversions can then be made for this or that printer, this or that screen but the assignment only happens once. In the history of a JPEG photo, it is thus directly done into the camera body or the smartphone.
Then the RGB values of the file and its display are going to have to be matched. Indeed, the eye sees a nice neutral grey but if you place the pipette on this grey in Photoshop, it indicates 128, 138, 126!!! It doesn't match! This alignment is called conversion and we'll see this in details in the next page .
2 - Case of a RAW file you develop yourself
As I'll explain a little later in this page and in my pages dedicated to color management in Camera Raw or Lightroom, the assignment of an ICC profile is also done automatically when opening your RAW file in the demosaicing program. To display your photo "correctly", it read the file's Exif data, saw with what camera it was shot and thus assigned this camera's ICC profile stored in its database - Adobe get their hands on all cameras on the market and calibrate them themselves in order to establish each one's ICC profile - to display the "right" colors hence the ones the camera body is supposed to have shot and not the ones that are "logically" inscribed in its RGB values (a 128, 138, 126 is displayed "as if" it had read 128, 128, 128 hence with a nice grey without any predominance while the RGB value does have one).
3 - Case of an image from the internet without an ICC profile...
When you open it in Photoshop - this program can manage colors so it will detect that there's no ICC profile assigned to this photo, but it needs one to display it correctly - it will offer you to assign a profile to this image thanks to a dedicated menu. You'll only have to let it know that this image's ICC profile is most probably sRGB. We'll see that in details in the pages dedicated to color management in Photoshop.
4 - Last precision - the "true" color: RGB values and La*b* color?
In order to see clearly why a same RGB value corresponds to a different real color - hence a L*a*b* one - différente, I projected four different color spaces or ICC profiles on the L*a*b* color space. You can see here the top right part (towards red values) of this projection. The angle of each color space or ICC profile corresponds to the most saturated red value since its RGB values are: 255, 0, 0. But you can see that this same color definition isn't projected at the same place in the L*a*b* space hence really corresponds to "real" different red colors.
Note on use - If you have the feeling that these 4 red points are displayed in the same shade of red on your screen, it is only due to your screen's gamut that doesn't enable to distinguish distinct shades for such saturated colors.
One same RGB value (255, 0, 0) and yet four different real hence L*a*b* colors in this example. To know which one's the right one, we only need the associated ICC profile or color space of an image file for instance and to assign it. The RGB value is then interpreted correctly and is displayed with the "right" La*b* red.
5 - And by the way, what happens if you don't assign an ICC profile to an image?
Without an ICC profile assigned to the image, it will be displayed arbitrarily following two possible rules - because it can't be otherwise:
1 - In a program that doesn't manage colors - It will be displayed in the screen's color space, quite simply, hence differently on a screen displaying sRGB or Adobe RGB 98 like it is more and more common to see since the RGB values of the file are directly interpreted by the screen. Example: If the image comes from a camera that was set in JPEG and Adobe RGB 1998 but the ICC profile has been lost and the image opens in Windows or Mac OS explorer (that don't manage colors), the image will be displayed by default according to the screen's color space (hence still nowadays most of the time in sRGB).
2 - In Photoshop if you don't activate color management - which is what happens by default even if it can seem weird! - Here, the image will be assigned Photoshop's RGB workspace as ICC profile(sRGB by default).
How to assign an ICC profile to an image in Photoshop?
Opening a "classic" image file in Jpeg, Tiff, PSD, etc.
When you open a JPEG, TIFF or PSD image without an ICC profile (so more accurately without a color space) in Photoshop (unlike a RAW file as we'll see later) Photoshop warns you and asks you to choose one if you chose to activate color management in this program (see color preferences in Photoshop for more information and examples in images). It is either done directly at the opening of the image, either once the image is opened Photoshop via the menu Edition / Assign a profile...
Let's take an example with an image coming from my old scanner that I open in Photoshop with the right settings on my calibrated screen:
So I open this image - an IT 8 test card - coming from my scanner (figure. a below). To understand well, I chose to open it without an ICC profile (figure a below on the left) - I didn't assign my scanner's ICC profile at the opening of the image - but since Photoshop necessarily needs to assign an ICC profile to an image to display it, it will force its assignment and in this case assign what is called its RGB workspace. Here the image opens in Adobe RGB 1998 since I'd chosen this color space as RGB workspace back in time. So the image is displayed with arbitrary default colors, as if it were an Adobe RGB 1998 image. Each pixel contains RGB values that are interpreted in this color space. In the image on the left below, you can see that the greys on the test card are reddish. Indeed, in the color space Adobe RGB 98, the RGB value 98, 91, 87 is reddish. For my image to recover its "true" colors of display, I need to assign it the "right" profile, its own one! In the photo below on the right, I assigned my scanner's ICC profile to my image and my greys are neutral again! It's magic. And you'll notice that the RGB values are still identical. The RGB values in the info palette don't change BUT the colors displayed, the L*a*b* colors, do change! The assignment of an ICC profile only changes the display of a photo for given RGB values.
The image that was assigned the right ICC profile displays correctly, quite simply, here on the right!
To do so, I used the menu Image/Mode/Assign a profile in Photoshop until PS CS2 and the Menu Editions/Assign a profile for PS CS3 and next. Taking into account the characteristics (defects) of my scanner by reading its ICC profile, Photoshop now knows that for this device, a signal of 98, 91,87 must be displayed as a neutral color, a neutral grey and not a reddish one. The image has recovered a normal contrast in the operation, by the way.
Now my image has an ICC profile that depends on my scanner and that enables to display it correctly taking into account my scanner's colorimetric characteristics. However, there's a big inconvenience! There's no match between what I see and the "logical" RGB values anymore. That's what we saw in the page - Dependent/independent profile -.
To get a match again, I need to change color space hence ICC profile or more accurately here color space since only them are neutral. It is called a conversion and we'll see in the next page what happens then...
Opening of a RAW file
The case of RAW files stands a bit apart and I would add that there are also two subcategories:
when RAW files are directly developed in the camera or the smartphone (as we saw before),
and when they are demosaiced in a dedicated program like Photoshop's famous Camera Raw or its equivalent in Lightroom.
Let's re-affirm that all cameras, even compact ones or smarthpones, can only shoot photos in RAW format. They never shoot Jpeg directly even if the files that are saved on our memory cards are Jpeg files. In fact, the RAW file is demosaiced internally, with a "mini Photoshop" hence a "mini" Camera Raw to deliver a Jpeg, more convenient for the vast majority of users. The process is as follows: the RAW file is "opened" by the mini-Photoshop installed in your camera. For the colors to be correct, the camera assigns the right ICC profile to the file, of course the one that the engineers incorporated in the camera. Logical! Then the same micro-program converts the file in a neutral color space, in most cases the famous sRGB (as we'll see more into details in the next page and as shown in the figure below).
Choose sRGB or Adobe RGB 1998 in the camera?
First, this choice is made through the menus of the camera when this one is already quite an evolved one, and this ONLY APPLIES TO JPEG FILES! If you choose RAW + JPEG on your camera, the sRGB or Adobe RGB 1998 color space will only be applied to Jpeg files. The choice of a color space for RAW files is made in the demosaicing program and you're choosing it. This has two assets:
You get to choose,
and above all, you'll choose from more options. In Camera Raw, it is possible to choose the broad ProPhoto for instance, sometimes useful in certain cases!
And there's still the case that can be interesting when you're trying to optimize the quality of the files shot by your camera: demosaicing RAW files yourself in a demosaicing program like the famous Photoshop's Camera Raw, Lightroom or also DXO and Capture One.
In this case, the program MUST assign the camera's ICC profile at the opening of the file and this automatically in order for the colors to be correctly displayed. To that extent, these programs all have a complete database of all cameras on the market that are able to deliver RAW files with their ICC profile. When they read the file's EXIF data, they know with which camera it was shot and thus how this camera "sees" colors. Here's the reason why, when your program isn't up-to-date, you can't open files from most recent cameras. Since they can't assign them an ICC profile because it hasn't been created yet (the camera hasn't been calibrated by Adobe yet), the program can't open them. The big asset of demosaicing programs is that it is possible at the next step, which is conversion to a neutral color space, to have other options than the classic sRGB or Adobe RGB 1998.
Calibrate your camera or not?
As we saw for printer calibration, generic ICC profiles, meaning created by the brand for a given printer model, can't work when you're looking for the best quality for your equipment. The reason is very simple:
It is very complex and above all, expensive, to make all printers of a same model rigorously equal from a colorimetric point of view while it is so simple and cheap to calibrate them one by one later for an optimal result.
It should be the same for cameras! But Camera Raw or also DXO apply the same ICC profile to all the bodies of a same model... Here's why more and more photographers calibrate their camera themselves... even if it's not as easy as for a printer...
What is the right color? It is an L*a*b* color hence an absolute one. A L*a*b* corresponds to one color only.
Assigning a profileis used to give a colorimetric meaning - a "true" color - to a photo's RGB values. When assigning an ICC profile to an image, each RGB value (of each pixel for instance) becomes the "right" L*a*b* color. If a photo is assigned the right ICC profile, it will display the "right" colors, the ones that should have been seen if the camera had been perfect! Imagine you find a 100-something bill on the floor. You don't know this currency. You go to the bureau de change and the first thing the agent is going to do is identify this bill, give it some sense. Once you know what it is, you'll be able to ask for a conversion of this currency into yours in order to be able to use it in shops. For a value of 100 at the start, you might only get 40 in your currency but the absolute value will be the same. For colors, it represents the same L*a*b* colors but different RGB values!
Assigning a profile is thus only used ONCE, at the very beginning of the history of this photo, for instance in the camera or in Camera Raw if it's a RAW file. A 100-euros bill is worth 100 euros. It isn't worth 100 dollars. Then it will undergo numerous conversions in order to keep the same absolute value but this time in a different currency...
The assignment of the right ICC profile for cameras is done automatically:
. Either directly in the camera body if you're shooting in Jpeg,
either in your demosaicing program like DXO or Camera Raw.
If your camera's recent and you didn't update your program hence its cameras database, you just won't be able to open its RAW files because it won't be able to assign it the right ICC profile because it won't have it! No profile = no "right" colors!
The assignment of the right ICC profile to an image downloaded on the web is done manually at its opening in Photoshop by assigning it the likeliest color space, sRGB.
What's coming next, logically, is the conversion of a photo or more exactly the conversion of the RGB values of a photo in different R'G'B' values, corresponding to the same L*a*b* color: Convert an image
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