PROLOGUE
redshift-ng is a fork of 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 brightness] [-c config-file] [-D | +D]
[-E | +E | -e elevations] [-g gamma] [-H hook-file]
[-l latitude:longitude | -l provider[:options]]
[-m method[:options]] [-P | +P] [-r | +r] [-dv]
[-O temperature | -o | -p | -t temperature | -x] | -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 lower temperature on an overcast day.
In addition to the command-line tool redshift, the GUI redshift-gtk(1)
provides an alternative interface that shows up as a notification icon
in the desktop environment.
OPTIONS
The following options are supported:
-b brightness
Synonym for "-b brightness:brightness".
-b day:night
Monitor whitepoint brightness to apply at daytime and at
nighttime. (Default: 1:1)
The values most be between 0.1 and 1.0.
day or night may be omitted, to keep unmodified, however
at least one must be specified.
-c config-file
Load settings from specified configuration file.
/dev/null can be used to tell redshift not to load the
configuration file.
If "-", the standard input will be used.
-D
Start in enabled state. (Default)
+D
Start in disabled state.
Ignored in one-shot mode.
-d
Keep the process alive and remove the colour effects
when killed.
Ignored for -p and -x; always active for -t and the
"quartz" adjustment method.
-E
Use wall-clock based schedule.
+E
Use solar elevation based schedule.
-e elevations
Synonym for "-e elevations:elevations".
-e elevation-high:elevation-low
Sets the lowest solar elevation during daytime to elevation-high
and the higest solar elevation during nighttime to elevation-low.
(Default: 3.0:-6.0, may change between versions.)
The value should be formatted as real, decimal values measured
in degrees. Each value shall be formatted as one complete value,
without unit suffix, and not split into degrees, minutes, and
seconds. Positive values are above the horizon and negative
values are below the horizon.
elevation-high or elevation-low may be omitted, to keep
unmodified, however at least one must be specified.
Implies +E.
-g gamma
Synonym for "-g gamma:gamma".
-g day:night
Synonym for "-g day:day:day:night:night:night".
However, if day is omitted, it is a synonym for
"-g :night:night:night", or if night is omitted, it is a
synonym for "-g day:day:day:".
-g red:green:blue
Synonym for "-g red:green:blue:red:green:blue".
-g day-r:day-g:day-b:night-r:night-g:night-b
Additional gamma correction to apply at daytime and
at nighttime. (Default: 1:1:1:1:1:1)
The values most be between 0.1 and 10.0.
day-r:day-g:day-b or night-r:night-g:night-b may be omitted,
to keep unmodified, however at least one set must be specified.
Individual components of one set cannot be omitted, either
nothing is omitted or an entire set, including its two colons
(:) are omitted.
-H hook-file
Select hook file or directory.
/dev/null or /var/empty can be used to tell redshift not to run
hook files.
-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.
options is a colon- (:) and semicolon-separated (;) list. Each
option an option name and value separated by an equals sign (=).
Use "-l list" to see available providers.
Use "-l provider:help" to see available options, or refer to the
EXTENDED DESCRIPTION section.
-m method[:options]
Method to use to set colour temperature.
options is a colon- (:) and semicolon-separated (;) list. Each
option an option name and value separated by an equals sign (=).
Use "-m list" to see available methods.
Use "-m method:help" to see available options, or refer to the
EXTENDED DESCRIPTION section.
-O temperature
This is a synonym for "-O temperature:temperature".
-O day:night
One-shot manual mode (set colour temperature). The colour set
is interpolated between day and night depending on the Sun's
elevation or the clock time (depending on which redshift is
configured to use).
Values must be at least 1000 and integral.
Use this with the -P option to clear the existing gamma ramps
before applying the new color temperature.
This is a synonym for "-t day:night -o".
-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.
-P
Reset exiting gamma ramps before applying new colour effects.
+P
Preserve preexisting gamma adjustments. (Default)
-p
Print parameters and exit.
-r
Disable fading between colour temperatures.
+r
Enable fading between colour temperatures. (Default)
-t temperature
This is a synonym for "-t temperature:temperature".
-t day:night
Colour temperature to set at daytime and at nighttime.
Values must be at least 1000 and integral.
The value 6500 is equivalent to no colour temperature
adjustment.
Default mode, but default values may change between
versions.
-V
Show program implementation and version.
-v
Enable verbose output.
-x
Remove adjustments from screen.
For mutually exclusive options or options specified multiple times,
the last specified takes effect, except the first specified option
that outputs text (except -p) is used. However, if the daytime
value or nighttime value is omitted for an option, the last previously
specified value will be used; that is, for example, "-t 5000:" and
"-t :3000" do not override each other, but "-t 5000:" overrides,
if specified later, "6000" but not "3000" in "-t 6000:3000".
Options in the command line override settings from the configuration
file.
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.
SIGUSR2 with value 0
Normally signals may be processed out of order, however
when this signal is received, SIGUSR2 will be blocked until
all pending SIGUSR2 signals has been processed, creating
signal processing order barrier. This is useful when mixing
SIGUSR2 value 3 (reloading configuration file) with other
configuration changing SIGUSR2 values.
SIGUSR2 with value 1
Disable the effects of redshift.
SIGUSR2 with value 2
Enable the effects of redshift.
SIGUSR2 with value 3
Reload the configuration file.
Settings from the command line will be overriden.
SIGUSR2 with value 4
Execute into the currently installed version of the program.
Only available on Linux.
SIGUSR2 with value 5
Set the "fade" setting to off.
SIGUSR2 with value 6
Set the "fade" setting to on.
SIGUSR2 with value 7
Set the "preserve-gamma" setting to off.
SIGUSR2 with value 8
Set the "preserve-gamma" setting to on.
SIGUSR2 with value 9
Exit the process without removing the its effects.
If the used adjustment method does not support leaving
the effects, they will be removed.
SIGUSR2 with value 10
Do not terminate redshift the standard output and
standard error are closed.
SIGUSR2 with value 11
Enable verbose mode. (The -v option will be treated as
specified.)
SIGUSR2 with value 12
Disable verbose mode.
Ignore if started in verbose mode (-v option).
SIGUSR2 with other values or no value
Ignored.
STDOUT
The standard output is used to print state information and requested
help information. The output is subject to localisation, and the
following formats apply for the "C" locale. Applications taking use
of this information must make sure to set the message locale to "C".
For floating-point values ("%f") the precision is not documented as
it may change between versions and applications should not expect any
particular precision to be used.
When "-m list" is specified, the available gamma ramp adjustment methods
are printed with the header "Available adjustment methods:\n" followed
by the list in the format
"%s%s\n", <arbitrary whitespace>, <method name>.
The list is terminated by an empty line. Additional information for
human users is printed after the empty line.
When "-l list" is specified, the available location providers are
printed with the header "Available location providers:\n" followed by
the list in the format
"%s%s\n", <arbitrary whitespace>, <provider name>.
The list is terminated by an empty line. Additional information for
human users is printed after the empty line.
When "list" is specified for the "edid" suboption to "-m", a list
of available monitors will be printed to the standard output,
with the header "Available outputs:\n", in the format
"%s%s\n", <arbitrary whitespace>, <monitor identifier>.
When "-m method:help", "-l provider:help", or "-h" is specified help
information is printed on in unspecified format, intended only for
human users.
When "-V" is specified, the used version of the program is printed to
the standard output in the format
"%s %s\n", <implementation name>, <version number>.
If "-v" is specified and the colour settings depend on the Sun's
elevation, the elevation thresholds are printed to the standard
output in the format
"Solar elevations: day above %f, night below %f\n",
<minimum solar elevation at daytime>,
<maximum solar elevation at nighttime>.
This line may be printed, if "-v" is specified, if redshift is
configured.
If "-v" is specified and the colour settings depend on the clock time,
the time schedule is printed to the standard output, with the header
"Schedule:\n" and the footer "(End of schedule)\n", in the format
"%s%f%% day at %02u:%02u:%02u\n", <arbitrary whitespace>,
<dayness level (0–100)>, <start hour (0–23)>,
<start minute (0–59)>, <start second (0–59)>.
These lines may be printed, if "-v" is specified, if redshift is
configured.
If "-v" is specified, the colour settings is printed to the standard
output in the format
"Temperatures: %luK at day, %luK at night\n"
"Brightness: %f:%f\n"
"Gamma (Daytime): %f, %f, %f\n"
"Gamma (Night): %f, %f, %f\n",
<daytime colour temperature>, <nighttime temperature>,
<daytime whitepoint brightness>, <nighttime brightness>,
<daytime red gamma>, <daytime green gamma>,
<daytime blue gamma>, <nighttime red gamma>,
<nighttime green gamma>, <nighttime blue gamma>.
Each line may be printed, if "-v" is specified, if redshift is
configured.
If the colour effects depend on the Sun's elevation, the user's
geographical location will printed to the standard output in the
format
"Location: %f %c, %f %c\n",
fabs(<GPS latitude>), signbit(<GPS latitude>) ? 'S' : 'N',
fabs(<GPS longitude>), signbit(<GPS longitude>) ? 'W' : 'E'.
This message is printed when the program starts and any time the
location is updated.
If the colour effects are non-static, the current period of the day
(which determine the colour effects) is printed to standard output,
if "-v" or "-p" is specified, in the format
"Period: %s\n", <period>
where <period> is "None", "Daytime", or "Night", or in the format
"Period: Transition (%f%% day)\n", <dayness level> * 100.
<dayness level> is exclusively between 0 (night) and 1 (daytime).
This message is printed when the program starts and any time it
changes (if "-v" is specified).
If "-v" or "-p" is specified, the colour settings are printed to the
standard output when the program standard and any time it changes
(fade effect is ignored). These are printed in three different
messages and, on chagne, only the settings that changed are printed:
"Color temperature: %luK\n", <colour temperature>;
"Brightness: %f\n", <whitepoint brightness level (0–1)>;
"Gamma: %f, %f, %f\n", <red gamma>, <green gamma>, <blue gamma>.
If "-v" is specified, and if running in continual mode, the program
will print "Status: Enabled\n" if starting in or when entering enabled
mode, and "Status: Disabled\n" if starting in or when entering disabled
mode.
If "-v" is specified, and if running in continual mode, the program
will print "Fade: Enabled\n" or "Fade: Disabled\n" to indicate whether
the "fade" setting is enabled, when the program starts and when the
setting is modified.
If "-v" is specified, and if running in continual mode, the program
will print "Preserve gamma: Enabled\n" or "Preserve gamma: Disabled\n"
to indicate whether the "preserve-gamma" setting is enabled, when the
program starts and when the setting is modified.
If the "dummy" gamma ramp adjustment method is used, any time a colour
change is applied (including each fade step), the colour temperature
is output, for debugging purposes (brightness and gamma are not printed),
to the standard output in the format
"Temperature: %lu\n", <colour temperature>.
STDERR
Default.
OUTPUT FILES
None.
FILES
Unless the -c option is used, redshift will look for its configuration
file, and if found, load it. When searching for the configuration file,
redshift will load the first found file. It will primary look for
"redshift-ng/redshift.conf", secondarily for "redshift/redshift.conf",
and tertiarily for "redshift.conf", in each directory it searches. It
will search the following directories in order: the directory set in
the environment variable XDG_CONFIG_HOME, the directory set in the
environment variable localappdata (Windows only), the ".config"
directory inside directory set in the environment variable HOME, and
the ".config" directory inside the user's home directory. For the two
latter, it will quaternarily look for the file ".redshift.conf". If not
found, it will also look for the file in each directory listed in the
environment variable XDG_CONFIG_DIRS (delimited by colon (:) on
Unix-like systems and by semicolon (;) on Windows), however it try each
directory before moving on to then next filename option. Lastly, on
Unix-like systems, it will look for the file in /etc. This means that
the preferred location for the configuration file is
${XDG_CONFIG_HOME}/redshift-ng/redshift.conf.
redshift will use the same pattern to find the hook directory, and the
tested subdirectories for each search directory are "/redshift-ng/hooks"
and secondarily "/redshift/hooks". All executable files, in the found
directory, that are neither prefixed with a period (.) or suffixed with
a tilde (~) will be used as hooks scripts, and will be executed in
arbitrary order. Subdirectories are not search for executable files.
This means that the preferred location for the hook scripts is (directly
inside) ${XDG_CONFIG_HOME}/redshift-ng/hooks/.
EXTENDED DESCRIPTION
Configuration file
The configuration file uses the standard INI format. General program
options are placed under the redshift header ("[redshift]"), while
options for location providers are adjustment methods are placed under
a hader with the name of that proivder or method.
General options are:
temp = temperature
temperature = temperature
Set temperature for daytime and nighttime. The value shall be
format in the same way as with the -O and -t options.
temp-day = integer
temperature-day = integer
Set temperature for daytime. That value shall be an integer no
less than 1000 (Kelvin).
temp-night = integer
temperature-night = integer
Set temperature for nighttime. That value shall be an integer
no less than 1000 (Kelvin).
brightness = brightness
Set whitepoint brightness for daytime and nighttime. The value
shall be format in the same way as with the -b option.
brightness-day = 0.1–1.0
Set whitepoint brightness for daytime. That value shall be an
within [0.1, 1.0].
brightness-night = 0.1–1.0
Set whitepoint brightness for nighttime. That value shall be an
within [0.1, 1.0].
gamma = gamma
Set gamma correction for daytime and nighttime. The value shall
be format in the same way as with the -g option.
gamma-day = 0.1–10.0
gamma-day = red:green:blue
Set gamma correction for daytime. Values must be within [0.1,
10.0], and are applied to each colour channel individually,
however if only one value is specified it is applied to all
each channels.
gamma-night = 0.1–10.0
gamma-night = red:green:blue
Set gamma correction for nighttime. Values must be within [0.1,
10.0], and are applied to each colour channel individually,
however if only one value is specified it is applied to all
each channels.
hook = file or directory
Set hook file or directory. If not specified, the default
paths are searched.
/dev/null and /var/empty can be used to prevent redshift from
executing hooks.
fade = 0 or 1
Disable (if 0) or enable (if 1) fading between colour settings
with large differences.
The -r and +r options can be used to override this setting.
preserve-gamma = 0 or 1
If 1, preapplied colour calibrations (all applied effects are
assumed to be colour calibrations) will be preserved, if 0,
colour calibrations will be reset while redshift is running.
The -P and +P options can be used to override this setting.
Note that if 0, colour calibrations will be reset even when
redshift is running but is disabled. This is necessary to
support the -o and -O options.
This setting is ignored when coopgamma is used as coopgamma
allows multiple programs to modify the gamma ramps at the
same time.
start-disabled = 0 or 1
Start redshift in disabled (if 1) or enabled (if 0) state.
The -D and +D options can be used to override this setting.
elevation-high = decimal
The lowest solar elevation, in degrees, during daytime.
elevation-low = decimal
The highest solar elevation, in degrees, during nighttime.
dawn-time = HH:MM[:SS][-HH:MM[:SS]]
dusk-time = HH:MM[:SS][-HH:MM[:SS]]
Custom time interval for the transition from night to day
(dawn-time) and for the transition from day to night
(dusk-time).
When specified, both settings must be specified and the
solar elevation will not be used to determine the current
daytime/nighttime period, nor will a location provider
used.
The left-hand hour must be within [0, 23], but the right-hand
hour may be within [0, 47], the timespan must not be greater
than 24 hours. The minutes and seconds must be within [0, 59],
and the default value for the seconds is 0.
adjustment-method = name
Select adjustment method. Options for the adjustment method can
be given under the configuration file heading of the same name.
Not used if the -p option is specified.
location-provider = name
Select location provider. Options for the location provider can
be given under the configuration file heading of the same name.
Not used if dawn-time and dusk-time are used or if the colours
settings are specified to be the same during the day and the
night.
Options for the location provider "manual" are:
lat = decimal
The GPS latitude of the user's geographical location.
Shall be specified in degrees and formatted a single
real number, rather than split into integer degrees,
minutes and seconds. Positive values used for the
northern hemisphere and negative values are ued for
the southern hemisphere.
lon = decimal
The GPS longitude of the user's geographical location.
Shall be specified in degrees and formatted a single
real number, rather than split into integer degrees,
minutes and seconds. Positive values used for the
eastern hemisphere and negative values are ued for
the western hemisphere.
There are no options for the location providers "geoclue2" (may be
available on Unix-like systems) and "corelocation" (available on
Mac OS X).
Options for the adjustment method "randr" (preferred method for X) are:
display = name
X display to apply adjustments to. Default is determined
by the environment variable DISPLAY.
The value is expected to contain a colon (:) and can only be
terminated with a semicolon (;).
screen = ordinal
Comma-separated (,) list of X screens to apply adjustments to.
All available X screens are used if the list is empty or if the
setting is omitted.
"all" may be specified as a synonym for an empty list.
crtc = number list or "all"
Comma-separated (,) list of CRTC numbers for monitors to
apply adjustments to. All available CRTCs are used if the
list is empty or if the setting is omitted.
A CRTC number may either be specified as an overall ordinal
for all selected X screens, or the index of the X screen
followed by a dot (.) and the index of the CRTC within the
specified X screen. The specified X screen is automatically
included in the screen selection.
"all" may be specified as a synonym for an empty list.
The index of the first CRTC is 0.
edid = name list or "list"
Comma-separated (,) list of EDIDs of monitors to apply
adjustments to.
If "list" is specified, all available EDIDs will be
printed to the standard output and the program exits.
This list must not be empty; to select all monitors,
instead specify crtc=all.
Options for the adjustment method "vidmode" (fallback method for X) are:
display = name
X display to apply adjustments to. Default is determined
by the environment variable DISPLAY.
The value is expected to contain a colon (:) and can only be
terminated with a semicolon (;).
screen = ordinal list or "all"
Comma-separated (,) list of X screens to apply adjustments to.
All available X screens are used if the list is empty or if the
setting is omitted.
"all" may be specified as a synonym for an empty list.
Options for the adjustment method "drm" (method for Linux without
display server) are:
card = ordinal list or "all"
Comma-separated (,) list of indices of graphics card screens to
apply adjustments to. All available graphics cards are used if
the list is empty or if the setting is omitted.
"all" may be specified as a synonym for an empty list.
crtc = number list or "all"
Comma-separated (,) list of CRTC numbers for monitors to
apply adjustments to. All available CRTCs are used if the
list is empty or if the setting is omitted.
A CRTC number may either be specified as an overall ordinal
for all selected graphics card, or the index of the graphics
card followed by a dot (.) and the index of the CRTC within
the specified graphics card. The specified graphics card is
automatically included in the card selection.
"all" may be specified as a synonym for an empty list.
The index of the first CRTC is 0.
edid = name list or "list"
Comma-separated (,) list of EDIDs of monitors to apply
adjustments to.
If "list" is specified, all available EDIDs will be
printed to the standard output and the program exits.
This list must not be empty; to select all monitors,
instead specify crtc=all.
There are no options for the adjustment methods "wingdi" (available
on Windows), "quartz" (available on Mac OS X), and "dummy" (used for
debugging, does not apply any colour effects).
Hooks
Executable files (that are not dotfiles or tilde files) in the hook
directory (see the FILES section), are executed on certain events.
The file inherit the redshift process standard input and standard
error, however the standard output is redirected to the standard error.
Each file is executed with at least one argument. This first argument
indicate what event has taked place. Additional arguments may be
provided for additional event data.
redshift will be the parent process of the executed script.
Currently available events are:
period-changed
This indicate that the period of the day has changed (and at
start of continual mode). The script will be provided two
additional arguments (the second argument and the third
argument). The second argument will the previous period, and
the third argument will be the new period. The argument values
will be either of "night", "daytime", "transition" (transition
in either direction between night and daytime), or "none" (not
previously or no longer calculated). "none" appears as the
previous period at start up and can also appear when the when
redshift is reconfigured if the colour settings no longer
depending on the period of the day or has started depending
on the period of the day.
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 applied 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
Default.
EXAMPLES
Example for the superelliptical roundabout in Stockholm, Sweden:
redshift -l 59.333:18.065 -t 5700:3600 -b 1:0.8
Example configuration file equivalent to above command:
[redshift]
temperature-day=5700
temperature-night=3600
brightness-day=1
brightness-night=0.8
location-provider=manual
[manual]
lat=59.333
lon=18.065
Sample hook script:
#!/bin/sh
case "$1" in
period-changed)
notify-send "redshift-ng" "Period changed from $2 to $3";;
esac
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 its effect. redshift is not
always able to tell if support is missing.
Limited software support
redshift does not yet support Wayland. 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(1), which
is a daemon applications can opt to use instead of directly setting the
gamma ramps themselves, coopgammad(1) 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(1) still has to compete with applications that does not use
it.
DRM and display servers
Using the DRM gamma ramp adjustment method can block starting or
switching to and already started display server (like X). Users may
also find that trying to switch to and an already started display cases
the computer hang, or more precisely appear to hang, as the display
server is not beign presented, the screen freezes, and the keyboard
doesn't do anything. (Once upon a time, this wasn't as catastrophic,
and it probably depend on display server implementation details.) The
only solution, abort from restarting the computer, is to remote into
it and kill the display server.
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.
Gamma correction is preserved for backwards compatibility and is
deprecated (gamma parameters in particular).
":" was used as the option delimited for -l and -m in the original
redshift, this is preserved for backwards compatbility. However because
some new options are expected to have ":" in their value, ";" has added
as an additional delimiter. Despite this ":" is still the preferred
delimiter as it is more user-friendly and use of options that require
delimiting with ";" is uncommon.
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)
