Convert an image: why, how?
Here again, it is a vocabulary that is specific to color management. The conversion is necessary if you want to keep the same colors (L*a*b*) from one device to another. An image is converted when its RGB values are changed to others slightly different in order to take into account the defects of the destination device. If you are new to color management, the most important is to know that Photoshop can take care of everything. But what a satisfaction, if you take the time to learn to do it yourself: your prints, your images on your website will be even more beautiful!
Key points if you are a beginner ...
Here are the key points to remember on image conversion. The rest of this page will be dedicated to those who want to go into more details.
Since you want to keep the same color on your monitor and your print for example, the RGB values displayed on your monitor should be changed to take into account the "defects" of the printer. This modification of values to other RGB values is called a conversion.
This is exactly the same as in an exchange office : you have 100€ in your pocket and you change it for 80$ during the day. You have 100 something then only 80 of something else, but it is the same (the same color in the end !).
This conversion is very complex and fortunately done automatically in Photoshop, if you use the color management tools of the software.
To achieve this, you need two things : a picture WITH its ICC profile and a destination device with its ICC profile (the one of the printer for instance). If one of the two ICC profiles is missing, it is impossible to achieve it. And you will also have to choose the conversion mode which I explain next page...
Next key point : conversion modes
Since there are larger or smaller color spaces, and since there are also ICC profiles for each device, the problem is now to know how to communicate the right color, the "same color" when it is possible, from one device to another, taking into account their characteristics. The communication of the "right color" and change of the corresponding RGB values is called conversion. Let's see how and why now... knowing that it is kind of similar to a foreign currency exchange office: you have 100€ in your possession (as if you were in a given color space, the image's) and you want the equivalent in dollars (to print your file correctly with a given printer and paper) and it is the exchange office and daily exchange rate (the equivalent of the L*a*b* space) that will provide you the equivalence through a conversion... coincidentally, it is also called a conversion when it comes to money.
How to communicate the "right" color?
More than fifteen years ago, an international organization, the International Color Consortium - ICC - founded by Adobe, Microsoft, Apple, Agfa, Kodak, Silicon Graphics and Sun invented and installed, first on an Apple computer, a fabulous tool: Colorsync. In 1993, ICC profiles were therefore invented - or ICM for Microsoft ® - and color conversion tools that must necessarily accompany it. But in 1993, the Colorsync tool was just a conversion tool and not a tool for ICC or ICM profiles creation. They were still to be invented!
In fact, all these tools of profile creation and color conversion are really efficient since the year 2000 for the general public, that is to say, since Photoshop reached its version 6, and since companies like MonacoSystems or X-Rite created great and affordable software for creating profiles and quality colorimeters. This is thus very recent!
In fact, each camera, each picture etc... has its own ICC profile and a program like Photoshop knows how to interpret it. It can assign a profile to an image or "translate" the colors, the right term being convert an image, from one device to another. The devices can thus communicate through the CMM- color conversion engine - and their ICC profile. In Photoshop since version 6, it is called: ACE color engine. It didn't exist before this version of Photoshop (so a long time ago!).
To work, the CMM - Color Management Module - needs to know what ICC profile is assigned to an image - source ICC profile - to find out what L*a*b* colors correspond to given RGB signals and towards which device it has to be sent - destination ICC profile - hence converting them into R'G'B' signals.
CMM is the core of this process and is based on L*a*b* colors and not on CMYK or RGB signals, because, as we have seen repeatedly, L*a*b* colors are absolute colors. The CMM knows what information it must transmit for an L*a*b* color and thanks to the ICC profile, it knows to what specific CMYK or RGB signal it corresponds. It will therefore translate this L*a*b* color, an RGB signal, into another R'G'B' signal or C'M'Y'K' so that the destination device reproduces correctly the same L*a*b* color. And the best part is that if it cannot do it directly, it will replace this color by another one without betraying the visual sensation perceived. This is where its main interest lies! This "translation" is called a conversion and there are four different ways to do it depending on the desired rendering. We will investigate this later on in the next page.
What is a conversion for and how is it done?
The aim of converting an image is thus to change RGB values into different R'G'B' values WITHOUT changing - or as slightly as possible - the L*a*b* colors of a photo for its colors to be printed as accurately as possible, while taking the defects of the printer into account.
On a donc besoin de convertir une image quand on veut, par exemple, imprimer une photo que l'on vient de scanner. Or non seulement chaque appareil "déforme" les couleurs mais surtout, à cause de leurs limites physiques, ils ne peuvent pas reproduire le même ensemble de couleurs. Un écran peut très bien afficher certaines couleurs qui pour autant "ne sont pas imprimables" et inversement ! Quoiqu'il arrive, on change d'espace colorimétrique et donc de profil ICC - source vers destination - pour :
So an image conversion is necessary, for instance, when you want to print a picture you just scanned. Not only does each device "distort" the colors but also, because of its physical limits, it can't reproduce the same set of colors as another device. A monitor can display colors that can't be printed and the contrary is also true! However, you change color space and thus ICC profile -source to destination - to:
Keep the same perceived color, an L*a*b* color, from one device to another, from one space to another, even if in theory it can not be displayed or printed. RGB values that correspond to a given L*a*b* color in a certain color space (your camera's) are changed into other R'G'B' values corresponding to the same L*a*b* color - or if it is impossible, to the closest match.
Match the RGB values of the info palette with the colors displayed on screen. As seen in the previous page, once the ICC profile of your scanner is assigned to a photo you just scanned, it will be displayed correctly, but if the eyedropper tool is placed in an area that is normally a neutral grey, it will be marked by a dominant color (green or magenta in most cases). Instead of RGB values like 115, 114, 115, there will be 119, 114, 109. It is therefore necessary to compensate the colors displayed and the corresponding RGB values. If you choose to convert the RGB values of the image to a neutral color space as Adobe 98 or DonRGB, there will be a good correspondence between what is seen and the info palette.
To convert an image in Photoshop, you can use the Edit menu / Convert to profile... For further information, I invite you to read my page "Color management with Photoshop".
Let's stick to our example of the previous page with an image opened in Photoshop on my calibrated monitor...
We'd opened an image - an IT 8 chart - coming from my scanner (figure a. below) in Photoshop and assigned it the ICC profile of my scanner that we'd created (no generic ICC profile). The image displayed in Photoshop on the left, hence without an ICC profile, which workspace is Adobe 1998, looks reddish. As soon as it is assigned the right ICC profile - the one of this scanner - everything goes back to normal (as we saw in details in the previous page).
But my image, that is now displayed correctly, has a big handicap. The colors displayed (with very neutral greys) don't match at all the RGB values that I could legitimately expect to read in the info palette. I see grey, but the values correspond to a reddish grey. I am thus going to make a conversion in order to match the colors displayed with "realistic" RGB values. For that, I need to convert these RGB values from the space hence the profile of my scanner towards a neutral color space as sRGB, Adobe 1998 or Prophoto.
Important! When you convert to a neutral color space, you must choose one that will be a little bit larger than the ICC profile of the device you used. That is why there are so many, from the smallest sRGB to the largest ProPhotoRGB.
In Figure c, similar to Figure b above, I see my image correctly displayed but still with my scanner's ICC profile. Using Image/Mode/Convert to profile in Photoshop up to version CS2 or Edit/Convert to profile in most recent Photoshop versions, I am going to convert this image in a wide color space, Adobe 1998 in this case (Figure d). This time, the display of L*a*b* colors does not change BUT the associated RGB values do change to become "almost" neutral 109, 110, 110. I can say that I balanced my image. In the Adobe 1998 color space, the RGB values 109, 110, 110 result in a neutral grey because it is a neutral color space.
Once converted into a neutral color space, I have a visual match between my image and its RGB values.
Now that my image is displayed correctly and in a neutral and wide workspace, I can edit it the way I want before converting it into sRGB if I want to publish it on the internet or CMYK to match my printer's needs, if I want to print it with Photoshop.
What happens during a conversion?
We just saw in the paragraph above the different roles of conversion. When you choose to perform a conversion, what does really happen to the RGB values of my image? How does the conversion tool, the CMM, keep L*a*b* colors as much as possible, that is to say, the visual sensation of the picture, if the target device does not allow it as it is unfortunately often the case with printers? A conversion is a kind of translation which aim is not to alter the original...
Large or smal color spaces
As we already saw in the page dedicated to color spaces, devices color spaces or workspaces are more or less wide. Some are so large that they cover all the others (ProPhotoRGB or certain digital cameras). Some are so small that they are included in all the others as for certain printers on matt paper. But sometimes, some are a little bit larger towards a specific color and vice versa for another. This can clearly be seen in the illustration on the right. Globally, the color space contained in the ICC profile of this printer/paper pair is smaller than the gamut of the monitor except towards blue-green colors. These colors are printable but not visible on the monitor (not displayable originally hence displayed with less saturation). This is more often the reverse case you will face, by the way.
In other words, some colors can be contained in the original file, but not be printed. So two scenarios can be observed:
In the first case, if the colors in the photo are printable, conversion is only used to change the RGB values of the image to get the same colors on the print. So far, so good! But what to do in the second case or if the gamut of the image is much larger than the gamut of the printer or... the opposite?
Of course not! In fact, they're "replaced" by the closest color(s) - see the next page about conversion modes - hence the most saturated color(s) of this screen. On a print, colors don't disappear either! They're just printed with the closest and most saturated colors that this printer and its inks can manage to produce. So where there was originally a color gradient on screen (for instance), you might end up with a color block, and a less saturated one moreover, on the print. In fact, with glossy papers, it is more often the opposite!
What to do with non printable or out of gamut colors?
Suppose the profile of your image is profile 1 (in red) and your printer's is 2 (in white). Your image has greens and yellows that the printer won't be able to reproduce because they are said to be out of its gamut. No combination of CMYK can exactly reproduce this L*a*b* color in my image. What should I do then? The conversion consists into forceing, these greens into the printer's color space so that they're printed anyway, even if normally the printer wouldn't be able to, as with a shoehorn! But the visual impression must remain as close as possible to the visual sensations of the original image. To complete this shoehorn task, color management and conversion tools use four conversion rules - among which only two are used by photographers:
They are discussed in the next page: relative mode and perceptual mode There will inevitably be losses, but a good rendering engine will reduce them to a minimum (the losses!), without altering the visual sensations of the original image. Photoshop's is particularly powerful if you choose the correct conversion mode. Obviously some image editing programs, much cheaper, also feature edition tools like the clone tool and others of high quality, but none has such a good color management. Unfortunately, this comes at a price...
Notes about color spaces
It is perfectly imaginable to think that a one and only neutral color space such as Adobe 1998 could be sufficient to make conversion from the ICC profile of a device towards a neutral color space, but then why have so many been created, more or less broad, like ProPhotoRGB, sRGB? Well, the answer stands in two main and simple reasons :
On the one hand, it is only justified by the fact that digital devices not only do not reproduce all L*a*b* colors but additionally do it in very different ways. However, when a color of a photo - so one seen by your camera - is unprintable, conversion should find a way to recreate the color on your print so that it looks like your original. But, the more differences there will be between the original and the capabilities of the printer, the bigger the risks of having the original colors changed. Color management tools do not deliver miracles! A printer on matte paper does not have the same gamut than on glossy paper.
Secondly, the larger the color space, the more colors it contains, therefore the more the computer work is important. But computers did not use to have the computing power they have today. Do not forget that Colorsync was created in 1995 ...
... And concretely?
Conversion can be performed in three different ways:
Directly in your camera body (in JPEG),
In your demosaicing program (Camera Raw, Lightroom, DXO, Capture One, etc.) in RAW,
In Photoshop (all types of image files).
1 - In your camera body (in JPEG)
The assignment of the ICC profile of your body must have happened automatically in your camera body. Conversion towards a neutral color space like the famous sRGB will also be performed directly in your body, to the confition that you're shooting in JPEG. For that, you'll have to go into the menus of your camera and choose your color space from two options (very seldom, three):
- sRGB (by default),
- Adobe RGB (or Adobe 1998 but it's the same).
Caution! This choice only applies to JPEG files, even if you're shooting in RAW + JPEG.
2 - In your demosaicing program (Camera Raw, DXO, etc.) for RAW files
The assignment of the ICC profile of your body applies automatically at the opening of your RAW file in your program. You have nothing to do and you can't do anything at this stage! If your body isn't recognized by your software because it is very recent or you haven't updated the database of your program, you just won't be able to open your RAW file.
Once opened with the right ICC profile, you need to choose your destination color space, just like on your body (see above), but with an important difference: you can choose from more options among which the very interesting and useful ProPhoto, in certain situations. Example with Photoshop's Camera Raw below:
Right at the bottom of the window, you can choose the destination color space. In Camera Raw, you can choose between the classic sRGB, the bigger Adobe 1998 and Colormatch RGB and the very broad ProPhoto.
Important update! Since the update of Photoshop CC to 14.1, it is now possible to choose your destination color space from all these profiles! If you installed them on your computer, you can thus choose DonRGB, Best RGB, Melissa RGB, etc. and not just between 4 possibilities and moreover, you can now choose the conversion mode. (Until then, the default choice was relative, which is quite logical). The situation is thus perfect now and finally leaves us the complete choice of color management in Camera Raw.
When you'll click the button "Open an image", your photo will necessarily have this color space.
3 - In Photoshop...
Conversion with Photoshop is treated in a dedicated page: color management with Photoshop
To be remembered!
A right color (L*a*b*) can be "translated" by a great variety of RGB values, depending on the device. An RGB color is thus not a "right" color but only a color for a given device hence in a given color space.
To be sure to communicate the same color (meaning the same L*a*b* color), two devices must exchange RGB values thanks to a conversion: the RGB values of a color are converted hence translated into R'G'B' values for another device hence for this same L*a*b* color.
So this conversion tries to keep the look and colors of a photo even if some of the colors are unprintable, for instance.
There are two "ways" to solve these issues of unprintable colors: relative and perceptual conversion modes, studied in the next page.
This conversion can also be performed first in your camera if you're working in JPEG, in your demosaicing program if you're working in RAW, and then in Photoshop or your photo editing program.
There is now a last point to talk about when dealing with image conversion, and one that I just mentioned: perceptual and relative colorimetric conversion modes