the pixelpipe & module order
The ordered sequence of processing modules operating on an input file to generate an output image is known as the “pixelpipe”.
The order of the pixelpipe is represented graphically by the order in which modules are presented in the user interface – the pixelpipe starts with a RAW image at the bottom of the list, and applies the processing modules one by one, piling up layer upon layer of processing from the bottom up, until it reaches the top of the list, where it outputs the fully processed image.
Note: The order in which processing modules are executed exactly matches the order in which the modules appear in darktable’s user interface. Changing the order of the modules in the user interface will change how your image is processed.
🔗module order and workflows
The order in which modules are executed within the pixelpipe has been carefully chosen to give the best output quality. In previous versions of darktable it was not possible to change the module order. However, there are a number of very specific use cases where the movement of some modules within the pixelpipe is advised.
One of the main reasons to change the module order came about with darktable version 3.0, which introduced the new scene-referred way of working. Version 3.2 formalised this by introducing the display-referred and scene-referred workflows, which are controlled by the preferences > processing > auto-apply pixel workflow defaults setting.
The main difference between these two workflows is that display-referred operates primarily in the low-dynamic-range of the user’s screen, whereas scene-referred operates in the high-dynamic range of the original RAW file. The transition between scene-referred and display-referred is usually performed by a tone mapping module which takes the high-dynamic-range input from the camera and compresses it to fit the dynamic range of the output medium.
Prior to version 3.0 darktable’s workflow was display-referred (auto-apply pixel workflow defaults = “display-referred”) and this is still provided as the default workflow in the current version. In this workflow, the base curve or filmic rgb module performs tone mapping early in the pixelpipe and most other darktable modules operate on image data in the compressed display-referred space.
This worfklow enables the legacy (pre-darktable-3.0) module order and automatically switches on the base curve module for new images.
Pixel data within the display-referred space is non-linear and is not a physically realistic representation of the original scene. This can lead to various artifacts with some modules, hence the creation of the (now recommended) scene-referred workflow.
The new scene-referred workflow (auto-apply pixel workflow defaults = “scene-referred”) was introduced as part of darktable 3.0. The module order was entirely rearranged to place the filmic rgb and base curve tone mapping much later in the pixelpipe. This means that most pixel operations now operate in linear rgb space with only a few modules remaining within the non-linear display-referred space. Within this workflow it is now recommended that the majority of image processing takes place using the modules modules up to and including filmic rgb. Operations in this section of the pixelpipe, being truly linear, are much more physically realistic and produce fewer artifacts.
This workflow enables the new (darktable 3.0+) module order and automatically enables the exposure and filmic rgb modules with some presets designed to act as a reasonable starting point for scene-referred image editing.
🔗changing module order
It remains highly recommended that users not change the order within the pixelpipe for a number of reasons:
- The sequence of modules has been selected with great care in order to give highest output quality. Changes to the sequence often worsen the result rather than improving it.
- Some processing modules simply don’t make sense if they are shifted in the pixelpipe. For example, highlight reconstruction needs to be performed on raw data before demosaic, which itself needs to be performed before any input color profile can be applied. For this reason it is still not possible to move some of the modules that are placed early in the pixelpipe.
- Most of darktable’s modules are designed to work within a specific color space (see the color management section for more details). Full flexibility would require modules to support different parallel algorithms depending on the color space they are working in, which would drastically increase complexity.
Despite the general recommendation to leave the pixelpipe order alone, it is possible to move modules within the pixelpipe by holding Ctrl+Shift and dragging and dropping the desired module to a new location. This should only be done by experienced users who understand the impact this will have on the image.
The module order can be manually changed back to either the 3.0 or legacy versions using the module order module, which can also be used to define your own custom module order presets.