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 !
What is the difference between conversion and rendering intents ? The conversion takes care of changing the RGB values - origin to destination - and the conversion mode also called rendering mode takes care of knowing what to do with the out-of-the-box values - keep, delete, replace or transform -.
Why is there a need for two rendering intents ?
As we have seen on the page dedicated to colour spaces, the colorimetric spaces called peripheral or work spaces, are more or less large. Some are so big that they include all the others (ProPhoto RGB or DonRGB). Some are so small that they are encompassed by all the others. But sometimes, however, some are a little larger in one color or another and vice versa for another. This can be seen in the illustration on the right. Overall, the color space contained in the ICC profile of this printer/paper pair is smaller than the gamut of the screen except towards the blue-green ones. These colors are printable but invisible on the screen or rather, "replaced" by less saturated colors. More often than not, the opposite is true.
In other words, some colors may be contained in the original file but not printable. So we observe two scenarios :
In the first case, if the colors of the photo are printable, the conversion is only used to change the RGB values of the photo to obtain the same printed color on the printout, taking into account the defects of the printer. So far, so good ! But what if 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 with non-printable or out-of-gamut colours ?
Let's assume that the profile of your image is the ICC profile 1 (red line) - and that of your printer the 2 (white line) -. Your image has green/yellow colors that the printer will not be able to reproduce because they are said to be out of range for this one. No combination of CMYK can reproduce exactly this L*a*b* color belonging to my image. So how do we do that ? The conversion then consists in "bringing", as with a shoehorn, these greens into the printer's space so that they can still be printed when normally the printer does not know how to do it. However, the visual impression must remain as close as possible to the visual sensations of the original image. To do this "shoehorn work", the color management and conversion tools use four conversion rules - only two of which are used for photographers :
They are discussed below. In both cases there will inevitably be losses but a good engine will be able to reduce them to the maximum (the losses!) without distorting the visual sensations of the original image. Photoshop's is particularly powerful if you choose the right conversion mode. Of course, some image editing software, which is much cheaper, also has editing tools, such as stamps and other high quality tools, but none have such good color management. Unfortunately, this has a price...