Image conversion : why, how ?
Again, there is a specific vocabulary to manage colors. The conversion is needed if you want to keep the same colors from one device to another. 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, ICC profiles for each device, we must now know how to communicate the right color, I mean the "same color" as much as 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 a bit like in an exchange office : you have 100€ in your possession (as if you were in a given color space, the one of the image) and you want the equivalent in dollars (to print correctly your file with a given printer and paper). It is the exchange office and daily exchange rate (the equivalent of the Lab space) that will provide you the equivalence through a conversion ... coincidentally we also talk about conversion when it comes to money.
How to communicate the "right" color ?
Over 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 1995, ICC profiles were therefore invented - or ICM for Microsoft ® - and color conversion tools that must necessarily accompany it. But in 1995, Colorsync tool was only a conversion tool and not a tool for ICC or ICM profiles creation. They remained to be invented !
In fact, all of these tools of profile creation and color conversion are really performant 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 have created great software for creating profiles and affordable quality probes. This is brand new !
In fact, each camera, 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 color, it is said that it convert an image from one device to another. The devices can then communicate through the CMM and their ICC profile. In Photoshop since version 6, it is called : ACE engine. There was none before this version in Photoshop.
To work, the CMM - Color Management Module - needs to know what ICC profile is assigned to an image - ICC profile of origin - to find out what Lab colors he has to deal with, for a given RGB signal and to which device it is sent - ICC profile of destination - in order to convert it into R'G'B' signal. CMM is the core of this process and is based on the Lab color and not CMYK or RGB signals, because, as we have seen repeatedly, the Lab colors are absolute colors. The CMM knows, it must transfer information for a Lab color with the use of ICC profiles in CMYK or RGB signal for instance. It will therefore translate this Lab color, an RGB value into another R'G'B' signal or C'M'Y'K' so that the destination device reproduces correctly the same Lab color. If it can not do it directly, it will replace this color by another one without betraying the visual sensation perceived. This is especially its greater strength ! This translation is called a conversion and there are four different ones existing depending on the desired rendering of the image. We will investigate this later on.
Profile conversion, what is it for and how to do it ?
Convert an image serves to change the RGB values into R'G'B' values WITHOUT changing - or the least possible - the Lab colors of a photo to be displayed on a monitor or printed. We need to convert an image, for instance, when you want to print a picture you have scanned. Not only does each device "distorts" the colors but also because of their physical limitations, they can not reproduce the same set of colors. Traditionally, printers are known to have a smaller color space than monitors or scanners, in 2011, espacially on matte paper . This does not stop them from being able, on certain colors, to be better than these devices ! In all cases, color space change and thus the profile does, in order to :
Maintain the same perceived color, a Lab 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 Lab color in a certain color space (your camera) are changed to another R'G'B' value corresponding to the same color Lab - or if that is not possible, the closest matching - for another device, such as a printer.
Match the RGB values of the info palette with the colors displayed. As seen on the previous page about choosing a neutral color as workspace area, once we have assigned the ICC profile of your scanner to a photo you have scanned, this one 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 DonRGB or 1998, there will be a good correspondence between what is seen and the info palette.
In order to convert an image in Photoshop, go to Menu Edit / Convert to profile ...
Let's keep on going with our example :
the image we opened in Photoshop on my calibrated monitor from the previous page ...
I open an image - IT 8 target - coming from my scanner (figure a) in Photoshop and assigned it the ICC profile of my scanner, that we have just created (no generic ICC profile). The image displayed on the left, without any profile, in Photoshop whose workspace is Adobe 1998, looks reddish. As soon as you assign the right ICC profile - the one of this scanner - everything turn back to normal (we have seen that in détail on the previous page).
My image, that is now displayed correctly, has a big handicap. The monitor colors are not all matching with the RGB values that I can legitimately expect. I see the grey values correspond to a reddish grey. So I will proceed to a conversion so as to match the colors displayed with "realistic" RGB values. For this I need 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 one 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 ICC profile. Using Image/Mode/Convert to Profile in Photoshop up to version CS2 or Edit/Convert to profile in the Photoshop menu, I will convert this image in my workspace, Adobe 1998 in this case (Figure d). This time, the displayed L*a*b* color does not change BUT the RGB values associated change to become "almost" neutral 109, 110, 110. In the Adobe 1998 color space, 109, 110, 110 results in a neutral grey because it is a neutral color space.
Once converted into a neutral color space, I have a visual correspondence between my image and the RGB values.
Now that my image is displayed correctly and in a neutral and wide workspace, I can retouch my image the way I want before converting it into sRGB if I want to publish it on internet or CMYK to match my printer need, if I want to print with Photoshop.
Conversion, what is being done ?
We have seen above the different roles of conversion. When we choose to perform a conversion, what is really happening at the RGB values of my image ? How the conversion tool, CMM, offers the best La*b* colors, that is to say, the best visual sensation of the picture, if the target device does not allow it as it is unfortunately often the case with printers ? The conversion is a kind of translation which aims not to distort the original ...
Large or smal color spaces
As seen on the page dedicated to color spaces, color spaces of devices are more or less wide. Some are so large that they cover all the others (ProPhotoRGB or DonRGB). Some are so small that they are included in all the others. But sometimes, some are a little bit larger for a specific color and vice versa for another. This is clearly seen in the illustration on the right. Here, the color space contained in the ICC profile of this printer/paper set is smaller than the gamut of the monitor except in the blue-green colors. These colors are printable but not visible on the monitor. This is more often the reverse case you will face.
In other words, some colors may be contained in the original file, but be not printable. So two scenarios are observed :
In the first case, if the colors in the photo are printable, conversion serves only 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?
What should be done of non printable or out of gamut colors ?
Suppose the profile of your image is the profile 1 (in red) and that of your printer the profile 2 (in white). Your image has greens and yellows than the printer can not reproduce because they are said to be out of gamut. No combination of CMYK can exactly reproduce the color Lab of my image. What should I do then ? The conversion is to force, as with a shoehorn, the greens into the printer color space so that they will be printed, even if normally the printer would not know how to do so ! But the visual impression must remain as close as possible to the visual sensations of the original image. To achieve this task, the color management and conversion tools use four conversion rules - only two of them are used for photographers :
They are discussed next page : relative colorimetric and perceptual conversion modes There will inevitably be losses, but a good rendering engine will reduce them, without altering the visual sensations of the original image. The Photoshop one is particularly powerful if you choose the correct conversion mode. Obviously some image editing softwares, much cheaper, ara also having edition tools, such as clone tool and others of high quality, but none has such a good color management. Unfortunately, this comes at a price ...
Notes on color spaces
We could very well imagine a single neutral color space such as Adobe 1998 may be sufficient to perform the ICC profile conversion of a device to another neutral color space but then why create so many more or less large as ProPhotoRGB or sRGB ... Well, the answer stands in two main and simple reasons :
- digital devices, not only do not reproduce all Lab colors but additionally do it in various 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.
- the larger the color space is the more number of colors it contains, therefore the bigger 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 ...
- What is a right color ? It is a La*b* color corresponding to a single color.
- This right color can be "translated" into a large variety of RGB values. So, an RGB color is not a right color but only a color for a given device in a given color space.
- In order to communicate, two devices have to exchange RGB values after conversion : RGB values of a color are converted, so to say "translated" into R'G'B' values for another device.
- This conversion tries to maintain the original aspect and colors of a photo even if some colors are unprintable for instance.
There is now a last point to talk about when dealing with image conversion and that I just mentioned : perceptual and relative colorimetric conversion modes
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