Convert an image
Again, this is a vocabulary specific to color management. Conversion is essential because it serves to keep the same colors (i. e. L*a*b*) from one device to another. An image is converted when its RGB values are modified for slightly different ones to take into account the defects of the destination device. If you are a beginner in color management, the most important thing is to know that Photoshop can take care of everything. But what satisfaction if you take the time to know how to do it yourself: your prints, your images on your site will only be more beautiful !
Since there are more or less large color spaces, ICC profiles for each device, it is now necessary to know how to communicate the right color, the "same color" as much as possible, from one device to another, taking into account their characteristics (their defects). The communication of the "correct color" and the change of the corresponding RGB values is called conversion. Let's see why and how now... knowing that it's a bit like in our exchange office : you have 100 dollars in your possession (as if you were in a given color space, the one of the image) and you want the equivalent in euros (you want to print your file correctly with a given printer and paper) and it's the exchange office and the daily rate (the equivalent of the L*a*b* space therefore our standard) that will give you the equivalent through a conversion.... as if by chance we are also talking about conversion when we talk about money.
How to communicate the "right" color ?
Let's start with a little history. More than twenty 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 - or ICM for Microsoft ® - and the color conversion tools that must necessarily accompany it were invented. However, in 1993, the Colorsync tool was only a conversion tool and not a tool for creating ICC or ICM profiles. So they had yet to be invented !
In fact, all these ICC profile creation and conversion tools have only been effective for the general public since 2000, i.e. since Photoshop is now in version 6 and since companies such as MonacoSystems or X-Rite have created great affordable profile creation software and quality sensors. So it's quite recent.
What is the "right" color ?
We now know that the "right" color is actually an L*a*b* color and therefore the "true" color perceived by a camera for example. However, this "true" color is transcribed into RGB value for obvious practical reasons, but since no camera is perfect, it is unable to see them correctly directly. So each RGB value that must transcribe these L*a*b* colors is tainted with a small error. This error is measured during the calibration of the device in question and held available on a small post-it that now accompanies this photo and is called its ICC profile. To display a photo correctly, we saw on the previous page that we had to assign this ICC profile to this photo. Its RGB values are then correctly interpreted and displayed with the "right" L*a*b* colors.
However, we also saw on the previous page that we could then face an embarrassing technical problem. The RGB values that can be measured with a eyedropper on an image no longer correspond to the displayed colors. We had taken as an example a neutral grey (128, 128, 128, 128) but transcribed in the image in the RGB values 128, 138, 126. Thanks to the assignment of the ICC profile, we saw that the image was displayed correctly because Photoshop, thanks to the ICC profile, knew that for this camera, a value 128, 138, 126 should actually be displayed in 128, 128, 128, 128 so in a neutral grey. But this did not only remove the RGB values by eyedropper, they also contained a dominant value. So at this stage of color management there is a problem of "logic" between what the eyedropper measures and what the eye sees. The eyedropper measures a grey with a dominant tinge and our eye sees a neutral grey. It is therefore necessary to carry out another operation to make everything "logical" again. This operation, as you will have guessed, is called a conversion.
What is a conversion and how is it performed ?
Converting an image is therefore used this time to change the RGB values - and no longer the displayed colors - into other R'V'B' values WITHOUT changing - or as little as possible - the L*a*b* colors of the image so that the colors of a photo are printed as accurately as possible, this time taking into account the printer's defects.
Let's take again our example from the previous page - our IT8 test chart - dedicated to the assignment of a profile to understand. We had opened the image file of this test pattern (figure a. below) in Photoshop. The image displayed in Photoshop on the left is an image displayed without a profile to fully understand the process. Finally without a profile, not quite because Photoshop necessarily needs to assign him a profile (ideally his own). However, in the absence of it, he will force the image to use one at all costs and will therefore attribute to it what is called his RGB working space. If we look "stupidly" at the RGB values of the grey parts in figure a (left below), we read 98, 91, 87. However, in the Adobe RGB color space, this corresponds well to a dark reddish gray since the R value is predominant. So at this point, our scanner file has been assigned a force ICC profile but as it is not the right one, the image is displayed strangely. It's time to give him his! And when you assign it the right ICC profile - my-scanner.icc - everything goes back in order at the display level (figure b. on the right below).