Developing images with darktable's Linear RGB tools

---

Questions and comments

• [Mica] Are we trying to make a tutorial for a full edit, or just show how the linear tools are supposed to work? My instinct is to restrict it to just the linear tools, and point other uses to the manual, especially things like crop and rotate and lens correction [asn] That's what I tried to do, that's why it only has one sentence for lens correction more or less.
• s7habo has made a nice write up: https://discuss.pixls.us/t/editing-moments-with-darktable/11770/63
• Mica suggested to use: https://discuss.pixls.us/t/the-fog-and-the/14233
  as the picture to create a full example and then attach the XMP file for reproduction.

---

by Mica, Boris and Andreas

Introduction

This aricle is based on information provided by darktable developers, especially from Aurélien Pierre. Thank you very much!

What is darktable?

darktable is a Free and Open-Source photography workflow application and raw image developer. It is an instrument to let you express your visual creativity, not a juke-box to replay old songs. This means you need to learn the craft. You can learn more about darktable here.

Why should I edit in linear space (TODO: phrase this question better)

darktable 3.0 puts more polish on the linear editing tools first introduced in 2.6 and also introduces some new linear tools. Tone equalizer and RGB curve join the improved filmic module. It is now possible to work in a scene-referred workflow, which means you can take advantage of outputting to radically different devices, such a regular, old paper print or the newest HDR screen, without having to redo the edit. Don't expect that darktable is like your previous used image developer like Likeroom. darktable does things differently in a precidctable way. You can find more details here.

What are the benefits of the scene-linear space ?

[proposal to rewrite the previous section - Aurélien]

Scene-linear RGB is a colour space where blurs and other physically-defined operations behave predictably, making alpha masks blending and edge softening easier to control and more natural looking. They also allow to handle HDR sceneries with very simple tools, keeping the High Dynmaic Range (HDR) data as long as possible in the pixelpipe, and remapping it to the Standard or Low Dynamic Range (SDR) output as late as possible. In this way, they enable output-agnostic workflows where the display output is taken care of at the file export time only.

Because they follow light transport models and respect radiometric relationships with the light emissions, they enable the software to behave like a virtual camera where it becomes easier to relight the scene a posteriori or apply visually-plausible optical effects (fake lens blurring or real lens deblurring, etc.).

[maybe include visual examples from https://discuss.pixls.us/t/survey-on-the-linear-workflow/13625/59?u=aurelienpierre ?]

darktable 3.0 provides more linear editing tools, like Tone equalizer, RGB curve and the improved filmic module, for a more consistent scene-referred workflow, while still allowing to use the legacy Lab modules from the version 2.

The basic group

We wont explain how the lighttable works lets jump directly to the darkroom. For the lighttable insecpt the fine manual here.

TL;DR:

1. Disable the basecurve
2. Enable the lens correction module if the camera and lens is supported
3. Use the exposure module if the image is not correctly exposed or is an HDR image
4. filmic rgb
5. tone eq
6. contrast equalizer
7. crop & rotate

1. Disable the basecurve

This is a module from the beginnings of darktable (ufraw). It does an OK job but with darktable 3.0 we want to do things differently and in a better way!

2. Lens correction

This module should be turned on, as it corrects errors introuced by different kinds of lenses. For more details take a look here. Important are are all corrections, but we are interested in vignetting which will increase the exposure in the edges especially for wide angle lenses.

3. Exposure

The historgram should be centered using the exposure module.

TIP

By default the histogram is shown for the output profile (default: sRGB). It should look like in Figure 1.

TODO: Figure 1

4. Filmic RGB

What is Filmic?

The filmic module tries to simulate what we know as real 'film'. Yes, the paper thing before we had Internet. The paper and the emulsion used to develop a film desaturated the extremities. It desaturated the luminance minimum (black) and maximum (white) and kept the full saturation for the midtones. This is what our eye and brain thinks is a pleasing look.

Overview

Modern cameras have double the dynamic range of modern monitors or different kind of displays like smartphones. Filmic does a three point mapping (white, grey, black) of the camera dynamic range (scene tab) to the display dynamic range (display tab). The critical settings are the scene ones. The look settings are designed to be portable and preset-able for styles and batch editing, provided your scene parameters have been adjusted correctly depending on the actual input picture.

The "tone curve" in the filmic module is just there to define a high priority zone for the remapping (that is the latitude), that can be enlarged (if contrast is bigger than 1) for better local contrast in midtones, and allow some slack to the extreme luminance, and blend everything smoothly.

For artistic contrast corrections, you get the colour balance and the tone equalizer modules, or the RGB (tone) curve.

Think of filmic as the companion of the output colour profile: one does the colour mapping to display, the other does the dynamic range mapping. Both ensure proper adaptation of the image for whatever display.

Using the module

To get the best out of filmic, edits should be split (divide and conquer):

1. Set up the luminance thresholds of the scene, e.g. the bounds of the dynamic range (white, grey, black). White and black depend only on the technical parameters of the scene (max and min RGB values in the picture), and grey depends on how much you want to decompress shadows (reminder: if you shot a colour checker on scene, you can directly use the colour picker on the grey patch).
2. Set up the the look, e.g. how much you are ready to compress extreme luminances to keep good looking midtones. Note that Filmic RGB needs a lot less contrast than the previous one. Keep the latitude as wide as possible while clipping should be avoided, and slide it up/down depending where your important details lie in the luminance range.
3. Don’t touch the display settings if you have a regular 8 bits screen SDR with gamma 2.2 following ICC specs.

Tip

You will find that for a specific camera, your look settings will consistently be the same from one picture to another, according to your taste. Therefore, put them in a preset, load them by default for new pictures, and all you need to care then are the scene parameters!

You need to wrap your head around what steps in your editings are driven by technical constraints (display gamut, display contrast, human vision, etc.) and what steps are driven by artistic choices (colour, brightness, etc.). If you get aware of that and carefully divide the technical job from the artistic one, you will fight a lot less with the software.

max RGB -> darken blues Mica: do you mean "saturate" not "darken"?
luminance -> darken reds
power norm RGB -> darken blues and reds

tone equalizer

The tone equalizer takes the functionalities of tone mapping and curves, but adapts them to a logarithmic representation of brightness (in Exposure Values (EV)) in accordance with Ansel Adams zone system. Its function is simply to define a parametric exposure compensation, which varies according to the original brightness of the pixels. It is possible to fully reproduce the operation of the tone equalizer by using several instances of the exposure module merged in parametric mode and set to isolate different exposure levels. The tone equalizer only speeds up the process, and automates parts of it.

contrast equalizer

This module can be used for a lot of stuff, the most important use cases are impoving contrast and sharpening. -> documentation

crop & rotate

Crop your image and straighten the horizon, this can ba done by right click and drawing a line … -> point to docs

The end

Do not mix

• base curve with filmic
• basic adjustments with filmic

Do not use

• tone mapping (use filmic + tone equalizer)
• shadow and highlights (use filmic + tone equalizer)
• monochrome (use the presets of channel mixer or lut 3D)