Photoshop Color Management Policies in Detail
Photoshop's color management support is very complete, but because of this it can sometimes also seem very complicated and perhaps a bit confusing. Central to all of this are the color management policy settings and how they interact with any profiles that may be embedded in images that you open to work on.
Under Edit >> Color Settings on Photoshop's menu you will find what we are after for this week's PhotoTip. Photoshop has color management policies for RGB, CMYK and Grayscale, but since photographers tend to work in RGB, I'm going to skip over the other two here. They work relatively the same as does RGB if you do need them. The setting we are going to address here are the RGB dropdown at the top of the Color Management Policies section..
There are three possible choices for the RGB color management policy: "Off, " "Preserve Embedded Profiles," and "Convert to Working RGB." Generally speaking, you want to pick one of the latter two choices. Don't pick "Off." Read on for why.
With "Preserve Embedded Profiles," your working space profile only gets used for new documents and those that lack profiles of their own. Any document that already has an embedded profile will stay as it is. It will be rendered using its embedded profile and will look the way it is supposed to (assuming of course that your monitor is profiled). In fact, you can open multiple documents, each with a different embedded profile, and each will stay in its own color space and all will be rendered correctly. Nothing forces you to have all open images in the same profile.
On the other hand, "Convert to Working RGB" means that images you open will always be converted to your working space profile. Since Photoshop does a convert rather than an assign profile operation, each RGB value in the file will be changed to what that color is represented by in the working space profile so the image will retain its correct appearance, within the limits of gamut differences. If you click on the "More Options" button in the Color Settings dialog you can see the rendering Intent that will be used for this conversion.
Rendering intent is the method by which Photoshop forces the gamut of a larger color space to fit within the gamut of a smaller one. Basically, the problem can be restated as how one gets a large peg to fit in a small hole. There are really only two practical solutions: either you squeeze the peg smaller until it fits or you chop off the sides of the peg until what's left fits. If the peg is the gamut of your source profile and the hole is the gamut of your target profile, compressing the peg would be Perceptual rendering intent while chopping off the sides would be Relative Colorimetric intent. Some images work better with one of these and some with the other and it can be hard to generalize as to which is more often preferable. Absolute Colorimetric and Saturation intent aren't used for photographic images. If you are converting from a small space to a larger one, rendering intent is mostly irrelevant so if you work in Adobe RGB or ProPhoto RGB you won't need to worry too much about Intent.
While you might expect that "Off" means that you have turned color management off, such is not entirely the case. When opening most documents with this setting, any profile embedded will be discarded and the image will be rendered as if it were in your current working space. Change your working space after opening and the document appearance will change along with it. But oddly enough if the embedded profile when opening was the same as your working space profile at the time, the embedded profile will honored as if color management weren't off. Change your working space now and this image won't change appearance. To me, this seems more like a bug than a feature, but that's the way it works. Images with any profile other than your current working space profile will be displayed incorrectly while those that are lucky enough to have the same profile as your working space behave as if you had selected "Preserve Embedded Profile" (or "Convert to Working RGB" for that matter since the profiles are the same). As I said, don't pick "Off." Photoshop CS3 continues to behave this way, at least in the current beta release.
Adobe RGB, Untagged RGB and sRGB images (left to right) opened with Adobe RGB working space but color management Off. Note that the sRGB image is slightly over-saturated from having been treated as Adobe RGB due to the choice of working space.
The same three images after changing the working space. Note that the Adobe RGB image on the left still looks the same since Photoshop honored its embedded profile, even with color management off while the other two now look distinctly different. Strange, but true.
If you know that any profiles embedded in your files are correct and you want to always convert to your working space using your default rendering intent, you can save yourself some time and select "Convert to Working RGB" so that Photoshop will handle things for you automatically. This will work well for most users and make it easy to ensure that all images get saved in the same color space regardless or what they started as. If you are less sure of what profiles your images have before you open them or want to exercise control over the choice of rendering intent for any needed conversions, set Photoshop to "Preserve Embedded Profiles." This will keep your images unaltered until you have a chance to see them open in Photoshop where they will get rendered with whatever profile they may have embedded. You can then use Edit >> Convert to Profile as desired, checking the Preview option so can see the effect of any choices you make before you commit them. And of course "Preserve Embedded Profiles" is the only viable choice if you do want to keep each file in its original profile regardless of what profile that may be.
We'll look at the implications of the Warning checkboxes at the bottom of the Color Management Policies section in more detail next week.