Smooth Transition

As mentioned, the granular selection is the domain of the parametric mask whose control is at the very bottom in the pallet. You need to click the h (hue) toggle. Drag the two right wedges in the color bar for input to the left up to the border with the red hues (Figure 10, bottom). Then, you only need to adjust the color contrast: If you drag the green vs magenta slider to the right and the blue vs yellow slider to the left, the tomato appears bright purple.

Figure 10: The parametric selection filters the red hues from the preselection using the brush.

Parametric and drawn masks are available for almost all filters in Darktable. The filled-in wedge controller in the color bar controls the value at which the mask completely finishes. You can use the hollow wedge to create a smooth transition. The circled plus icons to the right of the color bars switch between inclusive and exclusive selection modes.

The parametric mask selects image areas by lightness (L toggle, luminance) and by saturation (C, chrominance) as well as by color. The a and b toggles represent Green/Magenta, and Blue/Yellow from the LAB color model [5], which maps human color perception better than the RGB model.

The two differently colored tomatoes in Figure 8 were created using the already known color zones tool. You need several instances of the tool to create different colors; Darktable only introduced this feature a few versions back. To accomplish this, first click on the clipboard icon in the color zones palette header and then select the new instance option.

A rough selection with the mask brush again follows. Here, you can leave out the parametric mask, which separated the red of the tomato from the surrounding green in the first color example. The color zones tool itself is color-selective. Thus, it's sufficient to select the drawn mask point in the blend list box and make a rough selection with the mask brush in the normal way.

Drawing Rather than Painting

Often you can accurately cut an object out of an image using a rough preselection in combination with selecting the color. However, this principle does not work in the next example with the chili pepper, which I want to paint red. It directly borders other objects of the same color.

Selection by Bézier curve is also possible in Darktable for such cases, as used in most professionally knocked out images: After selecting drawn mask, do not click on the pencil icon as before, instead click on the second icon from the right (add path). Figure  11 shows the mask created with the vector tool for selecting the red pepper including its control points. You can also see the colorize tool settings, which provide the red color.

Figure 11: The Bézier curves known from vector graphics are also available for area selection. They are painstaking to draw but can accurately map even complex shapes.

A closed path, to which you can add new control points, appears after clicking on the image. If you have experience with Inkscape, then you will be familiar with the principle of Bézier curves, although the details of the procedure are slightly different in Darktable.

A right-click exits drawing mode and releases the mouse from the curve. Existing control points can then be moved. As in Inkscape, an active control point also has a control tangent that lets you manage the progression and degree of the curve. Ctrl+mouse-click creates a new control point, a right-click deletes a control point. Ctrl+left-click on an existing point converts this into a corner without its own curve.

The mouse wheel scales the whole mask as long as the pointer remains within the path. A thinner dashed line then appears, and the mask gradually fades out toward this line in Darktable. If you mouse over this line, then the mouse wheel changes the width of this transition area.

Darktable also converts the brush selections into vector curves after you release the mouse button. The vector curves' control points can be moved just as for curves created with the vector tool, which allows for minor, retroactive corrections. The vector tool, which you can use specifically to set the control points manually, provides more control.

Humble Beginnings

In this article, I described just five of the more than 45 filter modules in Darktable. In part, the filters affected the entire image, although I used the program's powerful masking functions for other edits. An additional version of the color-manipulated vegetable still life (Figure 12) gives you another impression, with this example showing blurred edges. An artificial film grain accentuates the outline of the white table; the Velvia filter, which is named after a type of color film, also slightly exaggerates the hues of those parts of the image that you did not colorize manually.

Figure 12: Even if the colors of the tomato are purely imaginary, a few more alienating effects (blurring, Velvia colors, artificial film grain) can't hurt.

The color mapping feature, which transfers an image's dominant colors to another photograph was particularly impressive in our lab. Designs with multiple images represent an obvious field of application: The color mapping filter then ensures a uniform color scheme, but maybe you just want to apply the romantic shades of a sunset from another picture.

The Darktable manual [6] updated for the current version 1.6 explains all other filters in enough detail that you should understand the program with your own experiments. You can set the value for ui_last/gui_language to C in the ~/.config/darktable/darktablerc file; there should be no spaces before and after the C.

I worked exclusively with JPEG originals with 8-bit color depth, which barely exploits Darktable's huge internal color depth of four 32-bit floating points. You only leverage this if you work with RAW originals. The program relies on the LibRaw [7] library for RAW support but also provides optimized lightness curves and color tables for a number of popular SLR cameras [8].

I only briefly touched on the image viewer (lighttable) here, although – with indexing; geotagging, including map projection; and a search function – it makes standalone management software such as digiKam unnecessary. The current version of Darktable 1.6 also comes with a slideshow function. The program works better on the high-resolution monitors popular with photographers; it can read and write TIFFs, even in 32-bit floating point color depth, and should work faster overall.

A chromatic aberrations filter that tries to iron out lens aberrations, an automatic mode for exposure compensation (exposure filter), and improved highlight reconstruction are new additions