RAW converters generate normal bitmap files from digital negatives. One of the most powerful representatives of this genus is Darktable [1]. The free program (see the "Installing Darktable" box), which is strongly oriented on the commercial Adobe Lightroom relies on a fixed workflow that guarantees reproducible results. It implements non-destructive image editing; although it loads the original files ("negatives"), it always creates separate output files ("positives" or "prints") with the settings you have chosen. Darktable writes these settings – as well as keywords, geospatial information, and other data for the image – to so-called "sidecar files" with a suffix of .xmp.

The program interface takes some getting used to and consists of four different parts in the current version 1.2 [2]. Besides a light table for file management, there is a darkroom for photo editing, tethering for triggering of cameras, and a map for geotagging images.

Darktable basically divides the window into three vertical panes with two broad sidebars that frame the image window. In the bar on the left-hand side, you usually find management information, whereas the bar on the right displays information and actions for the selected shots viewed in the image window. Clicking the small outward-pointing triangles in the panels hides them temporarily and enlarges the image window.

Light Table

On the light table (Figure 1), you can import and sort images, assign key words, and export images to bitmap formats. The mouse wheel lets you adjust the size of the thumbnails infinitely; pressing Z quickly generates a full-screen preview of the image under the mouse. At the bottom of the screen, you can toggle the light table mode. Besides the classic image view, you can use a simple File manager located at the bottom of the screen.

Figure 1: On the light table, you can select images for editing, adding keywords, and much more.

Darktable is designed for processing multiple images at a time. For example, to rate a group of images with up to five stars, select some photos – use the usual Ctrl+click or Shift+click mouse combinations – and Darktable shows the ratings at the bottom of the screen. To evaluate the selected pictures, it is best to use the keyboard and press 1 for one star, 2 for two stars, and so on. Parallel to and independent of the star rating, you can use color tags that appear above the images (Figure 2). You can use function keys F1-F5 to assign and remove color tags.

Figure 2: Using the switches at the bottom, you can tag your images with up to five colors; then, sort by at the top of the window shows the tagged images sorted accordingly.

The left sidebar has options to load images; Darktable will load single images or the contents of an entire directory. The new Darktable 1.2 also draws on Gphoto2 to load images directly from your camera if it is not mounted as a mass storage device. Another new feature is that in the right panel below tagging (Figure 3), the program adds the keywords it finds when loading the images to the internal database and makes them available as tags for other images.

Figure 3: The left sidebar is used to import images in light table mode. The right sidebar includes additional features to manage files and styles.

The right sidebar groups many of the basic functions, and the current version adds to these functions. As before, you can automatically select a large number of images in the select section; then, you can decide what to do with them under selected image[s]. For example, remove removes images from the database but leaves them on the hard drive. In contrast, delete physically removes the images. The duplicate option creates a new entry in the database but does not occupy any additional storage space. A new feature, group (Ctrl+G), groups the selected photos in a preview marked with a G.

The developers updated the HDR function, which is also present in the selected image[s] panel, so it is now capable of creating an HDR image in DNG format from a normal shot, and it often works surprisingly well (Figure 4).

Figure 4: Developed for combining images from an series of exposures, the HDR function now also creates single-shot HDRs (left).

Stacked High

One of the main functions of the light table hides behind history stack. The history stack summarizes the proposed actions for the current image and describes all the steps necessary to create the "print." The XMP file for the image stores this information. If you assign a name to a history stack, it is known as a "style." The good thing about history stacks: You only need to generate a stack once, and then you can apply it to any number of images. Choosing append or overwrite determines how history information that already exists is handled.

Normally, you will want to keep a collection of stacks well-suited to your camera as styles and apply them to newly loaded images with paste or paste all (below history stack). The styles tab shows you all the stacks and lets you use them to edit your images. Incidentally, applying a style does not mean that you need to commit to something: You can still make arbitrary changes to the images.

Export

The export selected section in the right sidebar lets you export images. This is also where you can specify how Darktable should process your prints. Under target storage, you can select the export method – from storing on the local disk, to emailing, to sending to the cloud. The latex book template option generates three files: the image as a PNG file, a document class photobook.cls, and the main.tex file, which imports the image.

Whichever export method you choose, you always have the option of choosing a special output format in file format – now also including JPEG2000 – with format-dependent settings. Additionally, you can adjust the image size, define an offset, and do more under global options.