Photo editors differ from image manipulation software like Photoshop or GIMP. Sometimes editors only open RAW images created by a digital camera instead of the usual JPEG or PNG files seen by users. These apps don't manipulate anything within images; they simply tweak things like color settings.
UFRaw (v. 0.16)
This application gets right to the point. The second UFRaw is opened, a file selection must be made. We were able to quickly correct a washed-out 37 megabyte RAW test file. The previewer is quick to reflect any changes to the various sliders housed in the application.
The user interface is unconventional and completely lacks a menu bar. Settings most commonly associated with the menu bar can be accessed from an Options button near the lower-right corner of the window. Zoom controls and file menu options are also located on the bottom. The left pane is a multi-tabbed toolbar containing all of the editing options.
UFRaw is mostly speedy enough. Just don't click zoom too many times at once or it may hang. Overall, UFRaw is a decent lightweight RAW photo editor.
RAWTherapee (v. 3.0 alpha1)
This up-and-coming RAW photo editing application is probably the most feature-rich, free RAW photo app out there.
However, using RAWTherapee is most definitely not the most straightforward experience. We found this out several months ago when we added it to our Linux benchmarks. The user interface is the epitome of clutter; tabs within tabs and multiple panes fill the UI. Part of the problem is the total lack of a menu bar, placing every single option on-screen. If you've never used a RAW photo app before, you will need a manual to figure out how to get this application to do its job. At the same time, after perusing the manual, you'll find RAWTherapee offers more adjustable settings than any other FOSS app of its kind.
If you're into photography, RAWTherapee is one of the best options there is, FOSS or not. If you need to tweak photographs only occasionally, RawTherapee is overkill.
Darkroom (v. 1.5.0)
Darkroom is a RAW photo processor. This application lets you tweak settings, such as color, light, white balance, tint, temperature, and more.
Darkroom produces a preview of the RAW image with user adjustments applied right in the center of the UI. While this lets the user see the effect of any changes, the lack of an undo function makes unwanted tweaks a complete pain in the neck to discard. Images can be saved in either JPEG or PNG format with a slider to control compression/quality.
showFoto (v. 1.2.0)
showFoto is the image editing component of digiKam, the professional-grade photo management suite for KDE. Users who simply want the editing capabilities without the photo manager can install showFoto by itself.
While this app can be used independently from digiKam, the focus here is definitely still on photographs and not images. Most of the tools are geared toward improving image quality or applying basic effects like film grain or oil paint. One nice feature in showFoto is the ability to try different changes in a preview instead of having to wait for effects to be applied to the actual file, or waiting to undo them if they're not what you expected.
The user interface is standard KDE fare, complete with multiple panes that can be rearranged within the single window or torn off to float independently. Nearly every corner of the screen is filled with toolbars housing various functions--if you like KDE you'll be right at home.
As with many modular KDE applications, showFoto is a perfect compliment to the larger digiKam suite. But it lacks a compelling reason to choose it over other standalone apps in the absence of its parent package.
RawStudio (v. 1.2)
The focus of RawStudio is on processing RAW photos straight from the digital camera, before editing in a separate image editor. Features include tools to adjust sharpness, hue, tint, warmth, and saturation. Also included is a real-time histogram.
The inclusion of split view is great for seeing before and after an image is edited, without relying too heavily on undo. Images can be sorted into groups, or Priorities, in RawStudio. This allows the user to create separate collections of images, which can be batch processed using the same adjustments. There are three tabs in the Tools pane for dialing in adjustment settings. You can apply just one or a combination of alterations to each image or Priority set. Obviously, this comes in handy if you have several different types of photos on your camera. You wouldn't want to apply the same adjustments to pictures of your dog that you would apply to a scenic landscape.
fotoxx (v. 10.3.1)
fotoxx is a photo viewing, organization, and editing application. Images can be organized into galleries and edited. Editing features include sharpen, blur, red eye removal, trim, rotate, flip, and color correction. Batch options include rename and tagging.
This title also has HDR and panorama options. However, Hugin is much better at automatically creating panoramas than the panorama function in fotoxx.
Overall, this application is intended for photos and not images in general; most of the editing options are geared toward photo enhancement.
Phatch--Photo BATCH Processor (v. 0.2.7)
Phatch is a great little batch image editor. This application can change color and contrast, add a background or border, copy, crop, rotate, round corners or add text, shadows, and watermarks. But that's just the tip of the iceberg. There are simply too many options to fit in one paragraph. Phatch even allows you to chain these events together, potentially saving an immense amount of time in front of a standard image editor like GIMP or Photoshop.
The addition of the Geek action actually allows Phatch to run external commands via the terminal, essentially multiplying the potential of what this application can do. This is another title I wish I knew about before researching for this series.