Catching up with RawTherapee 5.x

Free-software raw photo editor RawTherapee released a major new revision earlier this year, followed by a string of incremental updates. The 5.x series, released at a rapid pace, marks a significant improvement in the RawTherapee's development tempo — the project's preceding update had landed in 2014. Regardless of the speed of the releases themselves, however, the improved RawTherapee offers users a lot of added functionality and may shake up the raw-photo-processing workflow for many photographers.

It has been quite some time since we last examined the program during the run-up to the 3.0 series in 2010. In the intervening years, the scope of the project has grown considerably: macOS is now supported in addition to Windows and various flavors of Linux, and the application has seen substantial additions to the tool set it provides.

The competitive landscape that RawTherapee inhabits has also changed; 2010-era competitors Rawstudio and UFRaw are not seeing much active development these days (not to mention the death of proprietary competitors like Apple's Aperture), while darktable has amassed a significant following — particularly among photographers interested in a rich set of effects and retouching tools. At the other end of the spectrum, raw-file support improved in the "consumer" desktop photo-management tools (such as Shotwell) in the same time period, thus offering casual users some options with a less intimidating learning curve than darktable's. Where RawTherapee sits amid all of the current offerings can be a bit hard to define.

The 5.0 release landed on January 22, 5.1 then arrived on May 15, and 5.2 was unleashed (in the words of the announcement) on July 23. The project also migrated its source-code repository and issue tracking to GitHub, launched a new discussion forum, and has assembled a wiki-style documentation site called RawPedia.

Core functionality

There are several new features that serious photographers might consider "infrastructural" (which is to say that they were sorely missed in the 4.x series). Chief among these is color management; this ensures that pixel data is correctly transformed for display and output devices. A properly calibrated setup is required, but RawTherapee now supports monitor profiles, rendering intent, and even "soft proofing" to simulate print output on screen.

In the same vein are some user-interface and workflow improvements. For instance, image-sample points in the navigator window can show pixel values in absolute terms, as percentages, or in a [0,1] range. Which option is best will vary from situation to situation; having the ability to choose is the real benefit. Another nicety is that users must now hold down the shift key in order to change any settings with mouse wheel. This prevents accidentally trashing one's fine-tuned slider settings when trying to simply scroll through the list of available image operations. Accidents of that nature are all too common with some of RawTherapee's competitors and among free-software graphics applications in general. Dragging points in the curves editor now has modifier-key support: holding down Shift lets the user snap curve points to useful positions like the 45-degree diagonal, while holding down Control enters fine-granularity mode, letting the user slowly adjust curve values.

Also of note are improvements to the noise-reduction tool set. Chroma noise (that is, color noise) can now be reduced automatically, while there are new controls available to manually reduce luminance (that is, brightness) noise. The accepted wisdom is that the human eye is more sensitive to small differences in luminance, so it is arguably better to leave that feature in the hands of the user. Chroma noise can be erased more safely, with less chance that the result will offend the eye.

New tools and features

There are two brand-new image-editing tools worth discussing as well: the Retinex and Wavelet tools.

Retinex attempts to restore color accuracy in situations where digital sensors perform poorly, using an algorithm that mimics how the human brain interprets color in similar situations. For example, in dim lighting, the brain uses what it has already observed about the colors of objects in a scene to "fill in the gaps" — hence, your eye still interprets a banana as yellow, even when seen it is in the strong blue tones of moonlight. This neurological behavior is called color constancy, and it also allows the brain to "see through" haze and fog. The downside is that it can also confuse the brain, as observable in the optical illusions on Wikipedia's color-constancy page (and as was made famous by the dress in 2015).

RawTherapee's Retinex tool is, at this stage, a complex beast. There are four basic presets: "Low" to improve the appearance of darker image areas, "High" to improve the appearance of brighter areas, "Uniform" to attempt to balance the image for the midtones, and "Highlight," which provides an alternate take on the "High" preset that is tailored for improving ultra-bright image areas like reflections. But the tool also has eleven other sliders, four other optional parameters, and up to four transformation maps to worry about. In my experiments, I found it easy to brighten up some overly dim images but next to impossible to remove any haze from foggy images. It did not help that the only documentation of the tool in English is a rough translation from a French wiki page. Regardless, one hopes that this tool will see some more refinement in future releases to make it into something genuinely usable.

The Wavelets tool is simpler to understand (anyone who is still unconvinced that the Retinex tool is overly complicated in its current form should let that sink in for a moment). Wavelet transformations split an image up into multiple frequency levels; each level, in essence, isolates the image features of a given approximate size (as measured in pixels). This can make it easier to clean up artifacts that appear at one detail level without corrupting other detail levels. Pat David has written a detailed tutorial on using wavelets to touch up skin imperfections without smearing a portrait photograph — as methods like the traditional "clone" tool can do.

RawTherapee's Wavelet tool lets you activate a wavelet-decomposition of the current image and adjust the contrast, sharpness, denoising, and several other features of every wavelet detail level separately. Functionally, it works just as if the wavelet levels are built-in layers in the image. In contrast, an editor like the GIMP requires the user to split the image up into separate wavelet detail layers, then recombine them once editing is complete. This is a far more intuitive solution.

Although it is not an editing tool, a special mention also goes to RawTherapee's new dual-illuminant DCP (digital negative camera profile) support. This, too, is an entirely new feature that has only recently landed in any photo editor. A dual-illuminant camera profile is essentially just a pair of color-management profiles where each corresponds to one lighting source in a scene with two sources. For example, a room with a fluorescent light in one corner and sunlight beaming in from the other side has two considerably different colors of light. With one profile available for each light source, the editing application can interpolate the color values between the endpoints; it chooses how to interpolate by measuring the apparent color temperature of a white point in the image being edited. The technique can save a lot of time when working on large image sets because the same profile can be used for every shot. When the subject moves around the room, each shot can get automatically tuned depending on whether more light is coming from the window or the lamp.

Making use of this feature in RawTherapee requires the photographer to shoot a pair of reference images (one for each light source) and combine them into a dual-illuminant profile using external software, so it is not trivial. But it is still nice to see the feature available.

Other changes

As usual, a host of other updates and improvements are implemented in the new release. There is a new lockable color picker, with which you can select sample positions in the image and measure how various settings and adjustments affect their color as you edit. There are two new modes for the curve tool: luminance and perceptual. Luminance mode weights the red, green, and blue curves by their relative inherent luminances, so adjusting the image's curve keeps the RGB values more or less synchronized. Perceptual mode attempts to preserve hue and saturation, while letting the user freely skew the contrast and brightness.

Several existing tools gained nice new options. For example, the contrast by detail levels tool can now be set to before or after the image is converted to black and white. Obviously, that knob only makes a difference on an image where both the contrast-by-detail-levels tools and a convert-to-black-and-white operation are enabled, but changing the order of those operations can produce radically different output, and in prior releases, the user had no control. Along the same lines, the resize tool now has its own, built-in "sharpen" option, because in the standard raw-editing pipeline, sharpening the image after other processing steps might make unwelcome changes to their results.

It is certainly fair to say that this hardcoding of the order of operations causes raw-photo editing to be an uncomfortably rigid process at times. But the tradition stems from some inescapable realities, like the fact that demosaicing and white balancing have to occur before everything else. Essentially all raw editors are bound by this same model; RawTherapee is actually doing better than most by offering a bit of flexibility in situations where the effect is significant.

Also on the menu in 5.0 and 5.1 are support for new camera models, support for 32-bit pixel depth TIFF images, and support for grayscale JPEG and TIFF files. 5.2 added a GIMP plugin to open raw files, as well as a remote option (triggered by calling rawtherapee with the -R switch) that opens additional files in the same running instance of RawTherapee. Finally, developers and packagers may be happy to hear that the project is now actively working on its GTK+3 branch.

Raw photo editors tend to be designed to fully take over a user's workflow, handling every part of the image-processing pipeline from the raw demosaicing to the finishing touches. As such, it can be hard to persuade a regular user to jump ship and try an alternative. But it is possible to switch back and forth. At a personal level, I have always found darktable's user interface to be more awkward than it is worth, and that project's emphasis tends to be on special-effects processing, rather than straightforward correction and retouching.

For anyone of a similar bent, RawTherapee 5 is a significant improvement over what had been a capable but bare-bones editor in years past. For anyone who is committed to darktable or has never used RawTherapee before, the benefits may not be a sufficient sales pitch. But the project is worth tracking nonetheless for some of its newer features. For everyone who simply does not care, suffice it to say that it is an encouraging sign that the RawTherapee team is now pushing the bar forward on new technical features; all free-software image-editing projects benefit from that work, in the long term.