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 brightness] [-c file] [-g gamma] [-m method[:options]] [-l latitude:longitude | -l provider[:options]] [-O temperature | -o | -p | -t temperature | -x] [-P | +P] [-r | +r] [-dv] redshift -h redshift -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 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 Screen 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 file Load settings from specified configuration file. -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. -g gamma Synonym for "-g gamma:gamma". -g day:night Synonym for "-g day:day:day:night:night:night". -g r:g:b Synonym for "-g r:g:b:r:g:b". -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 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 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". -p Print parameter and exit. -P Reset exiting gamma ramps before applying new colour effects. +P Preserve preexisting gamma adjustments. (Default) -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. -v Enable verbose output. -V Show program implementation and verison. -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. 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", , . 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", , . The list is terminated by an empty line. Additional information for human users is printed after the empty line. 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", , . 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", , . 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 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", , , , , , , , , , . 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(), signbit() ? 'S' : 'N', fabs(), signbit() ? '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", where is "None", "Daytime", or "Night", or in the format "Period: Transition (%f%% day)", . 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", ; "Brightness: %f\n", ; "Gamma: %f, %f, %f\n", , , . 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", . 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 Default. 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. 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. 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)