---
title: Preprocessing
contributors:
- DrSlony
- XavAL
tags:
- 'Tool Description'
- 'Raw Tab'
---
There are several preprocessing settings and they are split into two
parts in the user interface, both in the "Raw" tab: one part appears as
the main "Preprocessing" tool, and the other part appears within the
"Sensor with Bayer Matrix" tool. The preprocessing settings in the main
"Preprocessing" group affect raw files of all types, while the ones in
the "Sensor with Bayer Matrix" tool affect only raw files from cameras
which use a bayer filter.
## PDAF Lines Filter
Pdaf_lines_filter_sony.png
Cameras with Phase Detection Auto Focus (PDAF) are susceptible to
producing line striping artifacts when photographing backlit scenes with
visible flare. Some mirrorless interchangeable lens cameras from Sony
are known to suffer from this.
Enable the PDAF lines filter to fix these PDAF line striping artifacts.
RawTherapee 5.5 can fix PDAF line artifacts for the following cameras:
- Sony DSC-RX1RM2
- Sony ILCE-6000
- Sony ILCE-6300
- Sony ILCE-6500
- Sony ILCE-7M3
- Sony ILCE-7RM2
- Sony ILCE-7RM3
- Sony ILCE-9
## Line Noise Filter
Line noise appears as horizontal or vertical banding most visible in
noisy images. It is caused by noise in the sensor electronics which read
the value of each photosite row by row or column by column.
You can see examples of what line noise looks like at this MagicLantern
forum post:
### Nikon PDAF Banding
Pdaf_banding_nikon.png
Several mirrorless interchangeable lens cameras from Nikon use Phase
Detection Auto Focus (PDAF), and they perform in-camera filtering to
mitigate the PDAF striping artifacts. However, the filtering performed
seems excessive and leads to "PDAF banding" - darker lines in shadow
areas.
Use the line noise filter to fix PDAF banding artifacts with the
direction set to "Horizontal only on PDAF rows".
RawTherapee 5.5 can fix Nikon PDAF banding artifacts for the following
cameras:
- Nikon Z 6
- Nikon Z 7
The "PDAF lines filter" has no effect on Nikon PDAF banding.
## Green Equilibration
Some cameras (for example
Olympus, Panasonic, Canon 7D, and some medium format cameras) use
slightly different green filters in the two green channels of the
[color filter array](https://en.wikipedia.org/wiki/Color_filter_array) on the
camera sensor. This is generally not a designed feature of the sensor,
but rather a result of limitations in the manufacturing process when the
color filters are applied to the sensor surface. One green filter may
get a small pollution from the red filter and the other from the blue
for example. Green equilibration suppresses interpolation artifacts that
can result from using demosaic algorithms which assume identical
response of the two green channels. The threshold sets the percentage
difference below which neighboring green values are equilibrated.
Set the value high enough for the mazing to disappear but no higher. The
DCB demosaicing algorithm is very sensitive to green split so it is good
to use while trying to find the best value.
Green equiliberation can also be used to equalize green split caused by
crosstalk. If you, for example via an adapter, use an analog ultra-wide
angle lens with your digital camera the incoming light may arrive at
such a low angle that some light passes through one color filter and
gets registered in a neighboring pixel belonging to a different color
channel - this is crosstalk. As one green channel has blue neighbors and
the other red, the first will get crosstalk from blue and the other from
red, hence they will separate which can cause mazing. Also red and blue
channels will in such a situation suffer from crosstalk, but as they
only receive from green there's no separation in those channels. Mild
crosstalk will not have any visible effect if green is equalized, while
heavy crosstalk will show as desaturated dull colors (as the color
channels have been mixed). Note that crosstalk generally doesn't occur
before heavy color cast, so in this case you will be using flatfield
correction too.
## Hot/Dead Pixel Filter
This tool suppresses [hot and dead pixels](https://en.wikipedia.org/wiki/Defective_pixel) by replacing them
by a neighborhood average.
"Hot pixels" appear as bright and saturated
tiny dots. Each one is the result of a photosite on the sensor
outputting a higher current than it should. Whether a single photosite
on the sensor corresponds to a single pixel in the processed photo
depends on the chosen demosaicing method (and other tools); most
methods, such as the default AMaZE, do not have a direct photosite:pixel
relationship, and so hot pixels may appear not only as single-pixel dots
but also as tiny 3x3 pixel crosses or slightly larger blobs. Hot pixels
are a completely normal phenomenon present in all cameras, however in
typical daylight photography you will not encounter them. The longer the
exposure, the higher the chance of hot pixels appearing and the larger
their number, with exposures longer than two seconds generally being
regarded as prone to this problem. Heat is also a factor, hot sensors
being more prone.
"Dead pixels" on the other hand appear as black dots (or crosses or
blobs). They are the result of dead photosites on the sensor, and as
such, exposure time does not have any influence on whether they appear
or not - if a photosite is dead, every photo will have the dead pixel in
the same spot. Because their position is static and always present when
using the same camera body, you can fix them not only using the
automatic "Dead pixel filter" but also by adding their coordinates to a
*.badpixels* file; see [Bad Pixels](dark-frame#bad-pixels).
It is impossible to detect hot and
dead pixels with absolute certainty by analyzing only one photo (as
opposed to analyzing a whole series of photos), and as such one must
find the balance between adequate removal and
[false positives](http://en.wikipedia.org/wiki/False_positives_and_false_negatives).
The threshold slider allows you to set the sensitivity of the automatic
detection of hot and dead pixels. Lower values make hot/dead pixel
detection more aggressive, but false positives may lead to artifacts. If
you notice any artifacts appearing when enabling the Hot/Dead Pixel
Filters, gradually increase the threshold value until they disappear.