3.4. Modules

Modules are organized into five functional groups: basic, tone, color, correction and effect. You either view all modules in one long list or instead click on a group to just display modules belonging to that group.

3.4.1. Basic group

The basic group of modules contains the modules for basic development. These are ones you probably will use most often, such as exposure, white balance etc.

3.4.1.1. Crop and rotate

Overview
This module is used to crop, rotate and correct perspective distortions of your image. You can overlay your image with various helpful guidelines that assist you using the tools.

Some of the tools of this module, namely adjustment of angle and corrections of perspective distortion, will require the original image data to be interpolated. For best sharpness results set lanczos3 as pixel interpolator in core options (see Section 6.2, “Core options”).

Usage

Whenever the user interface of this module is in focus, you will see the full uncropped image overlayed with handles and guiding lines.

First off, select what aspect ratio you want and size the crop boundaries by dragging border and corner handles. Use the button right of the aspect box, to swap between portrait and landscape mode. You can move around the crop rectangle by holding down left mouse button and move around. When you are done and want to execute the crop, just give focus to another module. You can at any time change your crop area by just revisiting this module.

flip

This tool is used to flip the image on the horizontal,vertical or both axis.

angle

This tool corrects the rotation angle helping you level an image. You can either set a numerical value or use your mouse directly on the image. To use your mouse, right-click, hold it down and draw a line along the horizon; as soon as you release the mouse button the image is rotated so the line you drew matches the horizontal axis.

keystone

This tool is used to correct perspective distortions in your image. Useful for example when you shoot a high building from ground with a short focal length, aiming upwards with your camera. The combobox lets you select the type of correction you want to use :

verticalif you want to limit the correction to vertical lines
horizontallimit the correction to horizontal lines
freeif you want to correct horizontal and vertical lines

Depending on the selected correction type you will see two or four straight adjustment lines overlaid to your image. Two red circles on every line let you modify the line positions with your mouse. Each line additionally carries a symmetry button. If activated (and highlighted in red) all movements of the affected line will be mirrored by the opposite line.

In order to correct perspective distortions, you need to find suitable horizontal and/or vertical features in your image and align the adjustment lines with them. When done, press the OK button, which is located close to the center of your image. The image will be corrected immediately. You can at any time come back and refine your corrections by selecting correction applied in combobox keystone.

automatic cropping

Use this options to avoid black edges on the image borders. Useful when you rotate the image.

aspect

Here you can change what aspect ratio you want to have on the result, thus constraining the ability to drag and crop rectangle out of the aspect ratio of your choice. Many common numerical ratios are pre-defined. You can also select any other ratio after opening the combobox and typing it in the form of x:y. A few special aspect ratios deserve explanation:

freefree forming the rectangle without any ratio restrictions
imagethis option constrains the ratio to be equal to image ratio
golden cutthis option constrains the ratio to be equal the golden number
squarethis option constrains the ratio to be 1

guides

Many self-explaining guides are available to help you compose your image.

Examples

A cropped image in center view when module crop and rotate is in focus. The cropped area is visible as well as some guiding lines.

3.4.1.2. Shadows and Highlights

Overview
The shadows and highlights module allows adjustment to the tonal range of darker parts of an image (shadows) and lighter parts (highlights); it can bring back details in shadows and highlights by enhancing local contrast.
Usage
shadows

This slider controls the effect on shadows; positive values will lighten up shadows while negative values will darken them.

highlights

This slider controls the effect on highlights; negative values will darken highlights while positive values will lighten them up.

soften with

This combobox chooses the underlying blurring filter, gaussian or bilateral. Try bilateral filter if you experience halos with gaussian blur.

radius

This slider controls the radius of the involved blurring filter. Higher values give softer transitions between shadows and highlights but might introduce halos. Lower values will reduce the size of halos but may lead to an artificial look. As said, bilateral filter is much less prone to halo artifacts.

compress

This slider controls how strong the effect extends to midtones; high values reduce the effect to the extreme shadows and highlights; low values cause strong adjustments also to midtones. You normally only need to touch this parameter if you want to limit the effects to the extreme shadows and highlights; increase the value in this case. At 100% this module has no visible effect any longer as only absolute black and absolute white are affected.

shadows color adjustment

This slider controls the color saturation adjustment made to shadows; high values cause saturation enhancements on lightened shadows; low values cause desaturation on lightened shadows. It is normally safe to leave this at its default of 100%. This gives a natural saturation boost on shadows - similar to the one you would also expect in nature if shadows would receive more light.

highlights color adjustment

This slider controls the color saturation adjustment made to highlights; high values cause saturation enhancements on darkened highlights; low values cause desaturation on darkened highlights. Often highlights do not contain enough color information to give convincing colors when darkened. You might need to play a bit with this parameter in order to find the best fitting value depending on your specific image; but be aware that sometimes results still might not be fully satisfying.

Examples

Original image exposed for the outer sunlit wall to avoid clipped highlights. As a consequence the interior of the barn has pitch black shadows.

Shadows get lightened; highlights are untouched; overall effect toned down a bit by blend mode normal and an opacity of 65%.

Resulting image.

3.4.1.3. Base curve

Overview
Camera sensors provide data in linear RGB format, the original image appears flat and dull. That's the reason why camera manufacturers apply their characteristic base curves to the RAW data when they generate in-camera JPEG images with better colors and contrast. darktable comes with base curve presets that mimic the curves of various manufacturers. These are automatically applied to RAW images according to the manufacturer ID found in EXIF data.
Usage

You can adjust an existing base curve or create a new one. The base curve is defined by two or more nodes. You can click on any node and drag to modify it. You can also create additional nodes by clicking on a curve segment between two nodes. In order to remove a node drag it outside of the widget area.

scale

This combobox toggles between linear and logarithmic view. In the double logarithmic view more space is given to the lower values allowing a more fine-grained adjustment of the shadows.

Tip: If you intend to take full manual control of the tonal values with the tone curve module or the zone system module (see Section 3.4.2.3, “Tone curve” and Section 3.4.2.4, “Zone system”) it may be easier to leave the image in linear RGB. Disable the base curve module in this case.

3.4.1.4. Exposure

Overview
This module is used to tweak the exposure. It is directly linked to the histogram panel. Indeed, if you correct exposure graphically, using the histogram (see Section 3.3.6, “Histogram”), you automatically activate the exposure module. The histogram simply acts as a view for the exposure module.

You can activate multiple instances of this module each with different parameters acting on different parts of the image which you select by a drawn mask (see Section 3.2.3, “Multiple instances” and Section 3.2.6, “Drawn mask”). The histogram is always linked to the lowest instance in pixelpipe.

Usage

This module is responsible for one of the most basic steps in each raw image development. An exposure adjustment value allows you - within certain limits - to correct for under- or overexposure. A shift by 1EV is equivalent to a change of exposure time by a factor of 2.

Positive exposure corrections will make the image brighter. As a side effect noise level gets higher. Depending on the basic noise level of your camera and the ISO value of your image, positive exposure compensations with up to 1EV or 2EV still give reasonable results.

Negative exposure corrections will make the image darker. Given the nature of digital images this can not correct for blown out highlights (see also Section 3.4.1.7, “Highlight reconstruction”).

A black level adjustment is a basic tool to increase contrast and pop of an image. The value defines at what threshold dark gray values are cut off to pure black. Use with care as the clipped values can not be recovered in other modules further down the pixelpipe. Please also have a look at the tone curve module (see Section 3.4.2.3, “Tone curve”) and the levels module (see Section 3.4.2.2, “Levels”) which can produce similar results with less side effects as they come later in pixelpipe.

black

Adjust the black level.

exposure

Adjust the exposure correction [EV].

auto

Calculate a correct exposition for the rectangular view appearing in the centre of the image. You can draw your own selection using your mouse. An adjustment slider right to the auto exposure checkbox lets you define what percentage of bright values are to be clipped out in the calculation.

3.4.1.5. Contrast Brightness Saturation

Overview
This module offers a very basic tool for adjusting an image's contrast, brightness and saturation.
Usage

The module has sliders for each of the three affected attributes. In their neutral position (zero) the image remains unchanged. Shifting sliders left to negative values reduces contrast, brightness and saturation, respectively. Shifting right to positive values leads to an increase.

Much more versatility for contrast and brightness adjustment is offered by the tone curve, levels, and zone system modules (see Section 3.4.2.3, “Tone curve”, Section 3.4.2.2, “Levels”, and Section 3.4.2.4, “Zone system”). Likewise you may adjust color saturation in a more detailed way with the tone curve, color contrast, and color zones modules (see Section 3.4.2.3, “Tone curve”, Section 3.4.3.4, “Color contrast”, and Section 3.4.3.7, “Color zones”).

contrast

This slider adjusts the image's contrast.

brightness

This slider adjusts the image's brightness.

saturation

This slider adjusts the color saturation.

3.4.1.6. Demosaic

Overview
This module allows you to control how the demosaic is processed.
Usage

Demosaic is an essential step of any raw image development process.

A detailed description would be beyond the scope of this manual. In a nutshell, the sensor cells of a digital camera are only able to record different levels of lightness, not different color. In order to get a color image, each cell is covered by a color filter, either in red, green or blue. Due to the color sensitivity of the human vision, there are two times more green cells than red or blue. Filters are arranged in a certain mosaic, called Bayer pattern. Therefore each pixel of your image originally only has information about one color channel. Demosaic reconstructs the missing color channels by interpolation with data of the neighboring pixels. For further reading see the Wikipedia article on the Bayer filter.

As interpolation is prone to produce artifacts, various different demosaic algorithms have been developed in the past. Artifacts would typically be visible as moiré-like patterns when you strongly zoom into your image. Currently darktable supports PPG and AMAZE. Both algorithms produce high quality output with a low tendency to artifacts. AMAZE is reported to sometimes give slightly better results. However, as AMAZE is significantly slower, darktable uses PPG as a default.

Some further parameters of this module can activate additional averaging and smoothing steps. They might help to reduce remaining artifacts in special cases.

Demosaic is always applied when exporting images. Demosaic is done on monitor display only when zoom is greater than 50% or when the according preference setting demosaicing for zoomed out darkroom mode (see Section 6.2, “Core options”) is set accordingly. Else color channels are taken from neighboring pixels without an expensive interpolation.

method

Set the demosaic method. darktable currently supports PPG and AMAZE.

edge threshold

Set the threshold for an additional median pass. Defaults to 0 which disables median filtering.

color smoothing

Activates a number of additional color smoothing passes. Defaults to off.

match greens

In some cameras the green filters have slightly varying properties. This parameter adds an additional equalization step to suppress artifacts. Available options are disabled, local average, full average and full and local average.

3.4.1.7. Highlight reconstruction

Overview
This module tries to reconstruct color information that is usually clipped because of incomplete information in some of the channels. If you do nothing, your clipped areas are often toned to the not clipped channel. For example, if your green and blue channels are clipped, then your image will appear red in the clipped areas.
Usage
method

You can choose between two methods: clipping highlight or reconstructing in LCh. Clipping highlight analyses each pixel having at least one channel clipped. Then it sets all channels to the minimum value found among the channels. Reconstruct in LCh analyses each pixel having at least one channel clipped and transforms the information in LCh color space to linearly mix the channels.

clipping threshold

Manually adjust the clipping threshold against magenta highlights. The default is usually satisfactory without any need for additional adjustments.

3.4.1.8. White balance

Overview
This module is used to set the white balance. You have three ways to interact with it: (a) Set up tint and temperature, (b) define the value of each channel, or (c) choose from predefined white balances.
Usage
tint

Alter the colour tint of the image, from magenta (value < 1) to green (value > 1). The channel sliders will be updated when you adjust this parameter.

temperature

Set the color temperature (in Kelvin). The channel sliders will be updated when you adjust this parameter. darktable derives the color temperature from the EXIF data using some model assumptions. The value given is not meant to be authoritative. In the end only the updated channel values determine how the image is modified.

red, green and blue channels

Set the channel values on a scale from 0 to 8.

preset

Select a preset white balance.

camera white balance (default)White balance reported by the camera.
spot white balanceSelect a square area in your image containing mostly grey pixels. The white balance is calculated based on the selected area.
passthroughShow without adjusting for white balance.
camera presetsCamera specific white balance presets. Examples: direct sunlight, flash, cloudy, shade and a number of indoor lighting options.

finetune

Some cameras offer additional finetuning parameters if one of the camera presets is selected. Depending on camera white balance, can be adjusted in steps within a certain range. The adjustments are usually towards yellow (value < 1) or blue (value > 1).

3.4.1.9. Invert

Overview
The main purpose of this module is to invert scanned negatives.
Usage
color of film material

The only control element of this module is a color selector which allows to adjust for different colors of your film material. Clicking on the colored field will open a color selector dialog which allows to define a color in HSL or RGB color space. You can also activate a color picker by pressing and take a color probe from your image - preferably from the unexposed border of your negative.