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PROLOGUE
redshift-ng is a fork Redshift. redshift-ng strives to keep backwards
compatibility with Redshift and be usable as a drop-in replacement.
Therefore, redshift-ng implements the command "redshift" just like the
original Redshift implementation.
NAME
redshift - Automatically adjust display colour temperature according the Sun
SYNOPSIS
redshift [-b day:night] [-c file] [-g r:g:b] [-m method[:options]]
[-l latitude:longitude | -l provider[:options]]
[-o | -O temperature | -t day:night | -x] [-pPrv] | -h | -V
DESCRIPTION
redshift adjusts the colour temperature of your screen according to your
surroundings. This may help your eyes hurt less or reduce the risk for
delayed sleep phase syndrome if you are working in front of the screen
at night.
The colour temperature is set according the the position of the Sun.
A different colour temperature is set during the night and during the
day. During dawn and early morning, the colour temperature transitions
smoothly from night- to day-time temperature to allow your eyes to
slowly adapt over a period of about an hour. At night, the colour
temperature should be set to match the maps in your room. This is
typically a low temperature at around 3000K–4000K (default is 4500K).
During the day, the colour temperature should match the light from
outside. Typically around 5500K–6500K (default is 6500K). The light has
a higher temperature on an overcast day.
In addition to the command-line tool redshift, the GUI redshift-gtk
provides an alternative interface that shows up as a notification icon
in the desktop environment.
OPTIONS
The following options are supported:
-b day:night
Screen brightness to apply at daytime and at nighttime.
(Default: 1:1)
The value most be between 0.1 and 1.0.
-c file
Load settings from specified configuration file.
-g r:g:b
Additional gamma correction to apply. (Default: 1:1:1)
-h
Display help message.
-l latitude:longitude
Your current location, in degrees. Shall be formatted a single
real number, rather than split into integer degrees, minutes
and seconds. The location should be specified using the GPS
coordinate system.
-l provider[:options]
Select provider for automatic location updates.
Use "-l list" to see available providers.
Use "-l provider:help" to see available options.
-m method[:options]
Method to use to set colour temperature.
Use "-m list" to see available methods.
Use "-m method:help" to see available options.
-o
One-shot mode (do not continuously adjust colour temperature).
Use this with the -P option to clear the existing gamma ramps
before applying the new color temperature.
-O temperature
One-shot manual mode (set colour temperature).
Use this with the -P option to clear the existing gamma ramps
before applying the new color temperature.
-p
Print parameter and exit.
-P
Reset exiting gamma ramps before applying new scolour effects.
-r
Disable fading between colour temperatures.
-t day:night
Colour temperature to set at daytime and at nighttime.
-v
Enable verbose output.
-V
Show program implementation and verison.
-x
Remove adjustments from screen.
OPERANDS
None.
STDIN
Not used.
INPUT FILES
None.
ASYNCHRONOUS EVENTS
redshift takes the standard action for all signals except:
SIGINT, SIGTERM, SIGQUIT
Smoothly disable the effects of redshift and terminate the
process. If already sent, immediately disable the effects
and terminate the process.
SIGUSR1
Disable the effects of redshift, or if already disabled,
reenable them.
STDOUT
TODO
STDERR
Default.
OUTPUT FILES
None.
FILES
TODO
EXTENDED DESCRIPTION
Gamma ramps
redshift applies a redness effect to the graphical display. The
intensity of the redness can be customised and scheduled to only be
applied at night or to be applied with more intensity at night.
redshift uses colour correction lookup tables (CLUTs), usually called
gamma ramps or gamma correction ramps, to apply this effect.
Colour temperature
The redness effect applies by redshift is modelled after black-body
radiation, specifically with a 10 degree observer. Although black-body
radiation starts at 0, redshift's model start at the conventional 1000K
(1000 Kelvin). For this reason, no colour temperature below 1000K can be
specified. However, as there is a limit can be determined for the colour
when the colour temperature appreciates infinity, the upper limit for
allow colour temperature is instead determined by the data type it is
stored in. However, it also means that it is meaningless to use colour
temperatures above 40000K.
The sRGB colour space, and modern monitors, use the standard illuminant
D65 as the reference for pure white, modelling ideal day light. The
correlated colour temperature of D65 is called 6500K, however it's
actually 6504K, but redshift's defines this illuminant has having the
colour temperature 6500K. This means that 6500K is the neutral (no
effect) colour temperature.
The current version redshift assumes the monitor uses sRGB. However
this is usually only true for CRT monitors. HDR-capable monitors
particular diverges significant for sRGB. This means that the display
colour does not perfectly correlated to the specified colour temperate.
Lower (more red) colour temperatures, about 1900K and below, are out of
gamut, and thus incorrect even on sRGB monitors.
EXIT STATUS
TODO
EXAMPLES
TODO
KNOWN ISSUES
No or incorrect effect on cursor
Some graphics drivers apply the effect (colour corrects) twice or not at
all on hardware cursors. It is often possible to reconfigure the display
server to use software cursors, to avoid this problem, however at mouse
pointer performance cost that may be noticeable on very low-end computer.
D65-flashes
For some versions of some graphics drivers, there will be an occasional
flash where gamma ramps are not applied to the output.
Limited hardware support
Low-end hardware, especially embedded devices, often lack colour
correction features redshift abuse to apply it's affect. redshift is not
always able to tell if support is missing.
Limited software support
redshift does not yet support Waylaid. If your environment contains the
variable WAYLAND_DISPLAY, you are using a Wayland compositor and cannot
currently expect redshift to work. Even with Wayland support, it would
be up to each individual Wayland compositor to opt in to support
applications like redshift.
Backlight control
redshift uses gamma ramps rather than backlight control to adjust
brightness. This actually intentional and for your best. Most
contemporary monitors require Pulse-Width Modulation, which causes
flicker than can cause eye-strain and headaches, to adjust backlight.
Using gamma ramps is a safe option, it's also considerably less work
basically no extra code and posses no additional limitations. It's often
not possible to adjust backlight on desktop monitors from software, for
devices for which it is possible (mostly telephones and laptops, however
not all have fine-grained enough configurability to be usable) it's not
possible from software to determine well enough how changing the
backlight settings changes the backlight physically. If you still want
backlight to be controlled, you can hook in a tool such as
adjbacklight(1).
Flickering and temporary suspension
redshift uses the gamma ramps for the monitor to apply it's effect. The
gamma ramps where originally intended for colour correction. Therefore
there is no standardised why have multiple applications applying
different effects without overriding each other. This can cause
continuous flicker if multiple instance are running or effects
temporarily disappearing. By default, redshift uses coopgammad, which is
a daemon applications can opt to use instead of directly setting the
gamma ramps themselves, coopgammad can then calculate the result of all
of the effects and apply them as one, allowing the user to use multiple
applications that apply different effects. However coopgammad still has
to compete with applications that does not use it.
RATIONALE
To prevent the user from accidental making the screen black, brightness
level below 0.1 are forbidden.
To prevent colour distortion and making the screen too white, brightness
level above 1.0 are forbidden.
NOTES
"Colour temperature", or just "temperature", is actually short for
"correlated colour temperature". (Your monitor is not a black-body
radiator.) And specifically the correlated colour temperature of the
monitor's whitepoint.
It's common for users to miss to specify a coordinate as negative,
which, if missed on the longitude can swap day and night. The latitude
is negative on the southern hemisphere and the longitude is negative on
the western hemisphere.
SEE ALSO
cg-tools(7), coopgammad(1), radharc(1)
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