Last week I mentioned how I had started running F12 with a colour managed desktop wherever possible with the application. Today I had need to capture some screenshots of a new (& as yet unannounced ) application I’m working on. There are two ways I normally capture screenshots. The first is to just press the ‘PrintScreen’ button, let GNOME save the PNG image and then crop it in GIMP or something like that. The second way is to use GIMP’s own screen capture function (File -> Create -> Screenshot), useful if you want an capture of a specific window instead of the whole desktop.
Today I acquired the screenshot using GIMP since I already had it open. And the colours in the screenshot looked like complete garbage. It shouldn’t be hard to understand what went wrong here. The default ICC profile for new images created in GIMP is the
sRGB colourspace. In image windows, GIMP applies a transformation to the image colours, going from the sRGB profile to the monitor’s calibrated profile. Except that since this image was created from a screenshot of a colour managed display, the colours have already been transformed according to the monitor’s profile. GIMP is in essence applying a duplicate conversion. It is no wonder the result looks awful.
Having realized that a duplicate conversion was taking place, the solution is easy. Tell GIMP that the image is in the monitor’s colourspace, rather than the default sRGB. This is done using the menu
Image -> Mode -> Assign Color Profile. With the ‘Assign colour profile’ operation, you are not changing the pixel values in the source image, merely telling GIMP how to interpret them. Since it now knows the image is already in the monitor’s colourspace, the transformation becomes a no-op, and the image displays in sensible colours again.
It is possible to leave it at that, save the image and do whatever you were going todo with it. This is sub-optimal if you intend to distribute the image to other people. The sRGB colourspace is intended as a generic colourspace which has reasonable display characteristics even on monitors which are not calibrated / colour managed. If uploading the web, most people viewing the image are not going to have colour managed displays. Thus, if you want the image to look reasonable for them, it is wise to now convert it to the sRGB colourspace. This is done using the menu
Image -> Mode -> Convert to Color Profile. In contrast to the ‘Assign’ operation, the ‘Convert’ operation does the change the actual pixel values in the source image. Depending on the overlap between the monitor’s colourspace and the sRGB colourspace, and the rendering intent chosen, this will be a slightly lossy process. The image colours won’t display in quite same way as before, but it will display better on other people’s monitors.
In summary, if you are taking screenshots of a colour management aware application on a colour managed display, you need to first assign the monitor profile to the captured image, and then convert it to the sRGB profile. Oh and remember that, depending on the source of the data, this assign+convert step may also be required when pasting image data from the clipboard.