## spectro/dispcal

### Summary

Given calibration target information [white point, maximum brightness, and response curve ("gamma")], display a series of test patches on the display, and using the colorimetric values read, create a calibration lookup tables that make the display meet the desired target. The type of instrument is determined by the communication port selected. Emission and display measurement instruments are supported.

### Usage

dispcal [-options] inoutfile
-v [n] Verbose mode
-display displayname [X11 only] Choose X11 display name
-d n[,m] [X11 only]Choose the display from the following list (default 1),
and optionally choose a different display m for VideoLUT access.
-d n Choose the display from the following list (default 1)
-dweb[:port] Display via a web server at port (default 8080)
-c listno Set communication port from the following list (default 1)
-r Report on the calibrated display then exit
-R Report on the uncalibrated display then exit
-m Skip adjustment of the monitor controls
-o [profile.icm] Create fast matrix/shaper profile [different filename to outfile.icm]
-O description Fast ICC Profile Description string (Default "outfile")
-u Update previous calibration and (if -o used) ICC profile VideoLUTs
-q [lmh] Quality - Low, Medium (def), High
-p Use projector mode (if available)
-y X Display type - instrument specific list to choose from.
-t [temp] White Daylight locus target, optional target temperaturee in deg. K (deflt.)
-T [temp] White Black Body locus target, optional target temperaturee in deg. K
-w x,y Set the target white point as chromaticity coordinates
-b bright Set the target white brightness in cd/m^2
-g gamma Set the target response curve gamma (Def. 2.4)
Use "-gl" for L*a*b* curve
Use "-gs" for sRGB curve
Use "-g709" for REC 709 curve (should use -a as well!)
Use "-g240" for SMPTE 240M curve (should use -a as well!)
-G gamma Set the target response curve actual technical gamma
-f [degree] Amount of black level accounted for with output offset (default all output offset)
-a ambient Use viewing condition adjustment for ambient in Lux
-k factor Amount to try and correct black point hue. Default 1.0, LCD default 0.0
-A rate Rate of blending from neutral to black point. Default 4.0
-B bkbright Set the target black brightness in cd/m^2
-e [n] Run n verify passes on final curves
-E Run only verify pass on installed calibration curves
-P ho,vo,ss Position test window and scale it
ho,vi: 0.0 = left/top, 0.5 = center, 1.0 = right/bottom etc.
ss: 0.5 = half, 1.0 = normal, 2.0 = double etc.
-F Fill whole screen with black background
-n [X11 only] Don't set override redirect on test window
-J Run instrument calibration first
-N Disable auto calibration of instrument
-H Use high resolution spectrum mode (if available)
-V Use adaptive measurement mode (if available)
-X file.ccmx Apply Colorimeter Correction Matrix
-X file.ccss Use Colorimeter Calibration Spectral Samples for calibration
-X N 0: Technology (Display description)
1: etc.
-Q observ Choose CIE Observer for spectrometer or CCSS colorimeter data:
1931_2 (def.), 1964_10, S&B 1955_2, shaw, J&V 1978_2, 1964_10c
-I b|w Drift compensation, Black: -Ib, White: -Iw, Both: -Ibw
-C "command" Invoke shell "command" each time a color is set
-M "command" Invoke shell "command" each time a color is measured
-W n|h|x Override serial port flow control: n = none, h = HW, x = Xon/Xoff
-D [level] Print debug diagnostics to stderr
inoutfile Base name for created or updated .cal and .icm output files

### Comments

This is the tool is used for adjusting and calibrating a display to reach specified target behaviour, and optionally profiling it. For best results on a CRT, you should run this against a neutral grey desktop background, and avoid having any bright images or windows on the screen at the time you run dispcal. You could also use the -B option to black the whole screen out, although this will make it impossible to control dispcal unless you have more than one display.
The -v flag reports progress information, as well as other statistics about the progress of calibration. A numerical argument greater than 1 gives greater verbosity.
When running on a UNIX based system that used the X11 Windowing System, dispcal will by default use the $DISPLAY environment variable to determine which local or remote display and screen to read from. This can be overridden by supplying an X11 display name to the -display option. Note that if Xinerama is active, you can't select the screen using $DISPLAY or -display, you have to select it using the -d parameter.
By default the main display will be the location of the test window. If the system has more than one display or screen, an alternate display/screen can be selected with the -d parameter. If you invoke dispcal so as to display the usage information (i.e. "dispcal -?" or "dispcal --"), then the discovered displays/screens will be listed. Multiple displays may not be listed, if they appear as a single display to the operating system (ie. the multi-display support is hidden in the video card driver). On UNIX based system that used the X11 Windowing System, the -d parameter will override the screen specified by the $DISPLAY or parameter.
Note that if VideoLUTs for a display are not accessible (i.e. no hardware calibration capability), dispcal will will issue a warning, but continue creating a calibration based on the display "as-is" rather than its native response. See the -o flag for an explanation of the implications of having no access to the VideoLUTs.
On X11 the inability to access VideoLUTs could be because you are trying to access a remote display, and the remote display doesn't support the XF86VidMode extension, or perhaps you are running multiple monitors using NVidia TwinView, or MergedFB, and trying to access anything other than the primary monitor. TwinView and MergedFB don't properly support the XF86VidMode extension for multiple displays. You can use dispwin -r to test whether the VideoLUTs are accessible for a particular display. See also below, on how to select a different display for VideoLUT access. Also note that dispcal will fail if the Visual depth doesn't match the VideoLUT depth. Typically the VideoLUTs have 256 entries per color component, so the Visual generally needs to be 24 bits, 8 bits per color component.
Because of the difficulty cause by TwinView and MergedFB in X11 based systems, you can optionally specify a separate display number after the display that is going to be used to present test patches, for accessing the VideoLUT hardware. This must be specified as a single string, e.g. -d 1,2 . Some experimentation may be needed using dispwin on such systems, to discover what screen has access to the VideoLUT hardware, and which screens the test patches appear on. You may be able to calibrate one screen, and then share the calibration with another screen. Profiling can be done independently to calibration on each screen.
-dweb or -dweb:port starts a standalone web server on your machine, which then allows a local or remote web browser to display the the color test patches. By default port 8080 is used, but this can be overridden by appending a : and the port number i.e. -dweb:8001. The URL will be http:// then name of the machine or its I.P. address followed by a colon and the port number - e.g something like http://192.168.0.1:8080. If you use the verbose option (-v) then a likely URL will be printed once the server is started, or you could run ipconfig (MSWin) or /sbin/ifconfig (Linux or OS X) and identify an internet address for your machine that way.
Note that if you use this method of displaying test patches, that there is no access to the display VideoLUTs and that the colors will be displayed with 8 bit per component precision, and any screen-saver or power-saver will not be disabled. You will also be at the mercy of any color management applied by the web browser, and may have to carefully review and configure such color management. See the -o flag for an explanation of the implications of having no access to the VideoLUTs.
-c The instrument is assumed to communicate through a USB or serial communication port, and the port can be selected with the -c option, if the instrument is not connected to the first port. If you invoke dispcal so as to display the usage information (i.e. "dispcal -?" or "dispcal --"), then the discovered USB and serial ports will be listed. On UNIX/Linux, a list of all possible serial ports are shown, but not all of them may actually be present on your system.
The -r and -R flags perform a quick measurement of current display behaviour, reports and then exits. If the -r flag is used the measurement are taken using the currently loaded calibration (Video LUT) curves. If -R is use, then the uncalibrated ("raw" or "native") behaviour is measured. Reported are:
Black Brightness in cd/m^2
White Brightness in cd/m^2
The approximate Gamma
The white point x,y chromaticity co-ordinates
The correlated color temperature in Kelvin, and the CIEDE200 to the Black Body locus.
The correlated Daylight temperature in Kelvin, and the CIEDE200 to the Daylight locus.
The visual color temperature in Kelvin, and the CIEDE200 to the Black Body locus.
The visual Daylight temperature in Kelvin, and the CIEDE200 to the Daylight locus.
The visual color temperature in Kelvin
(for -R "raw":)
The apparent VideoLUT entry number of significant bits.
Note that the correlated color temperature is the temperature of a black body radiator that has the closest color to the white point measured using the traditional CIE 1960 UCS space color difference formula. The correlated daylight temperature is a similar thing, except the CIE daylight locus is used. The visual color temperature values are calculated similarly to the correlated color temperatures, but using the modern CIEDE2000 color difference formula to calculate a better visual approximation to the closest temperature to the displays white point. There will be no difference between the UCS and CIEDE2000 temperatures if the display white point actually lies on the particular locus.
The -m option skips the usual process of adjusting the display monitor contrast, brightness and white point controls, and skips straight to calibration.
-o [profile.icm] Normally dispcal creates just a calibration file, which can then be used for subsequent characterization using dispread and profiling using colprof. If the -o flag is used, dispcal will also create a shaper/matrix profile. By default it will create a profile named inoutfile.icm, but a differently named file can be created or updated by specifying the name after the -o flag. If the -u flag is used with -o, then the ICC profile vcgt calibration curves will be updated.
Note that if VideoLUT access is not possible for the display, that hardware calibration is not possible. dispcal will create calibration curves anyway with a warning, and if a profile is created, it will not contain a 'vcgt' tag, but instead will have the calibration curves incorporated into the profile itself. If calibration parameters are chosen that change the displays white point or brightness, then this will result in a slightly unusual profile that has a white point that does not correspond with R=G=B=1.0. Some systems may not cope properly with this type of profile. See the tutorial for a further explanation.
The -O parameter allows setting of the shaper/matrix profile description tag. The parameter should be a string that describes the device and profile. With most command line shells, it will be necessary to enclose the parameter with double quotes, so that spaces and other special characters are included in the parameter, and not mistaken for the start of another flag, or as a final command line parameter. Many programs that deal with ICC profiles use the description tag to identify a profile, rather than the profile filename, so using a descriptive string is important in being able to find a profile. By default, the profile file name will be used as the description.
-u Normally dispcal creates a new calibration file and optional profile, based on the requested targets and the response of the display. This can take a fair amount of time, particularly if a high quality level has been selected, so to speed up the process of keeping a display in calibration the -u flag can be used. This uses the same calibration targets as the previous calibration but does a smaller number of refinement passes, enough to improve the accuracy of the calibration to account for drift in the device. If the -o flag is used as well, then the ICC profile will have its vcgt tag updated with the new calibration. This keeps the profile up to date with the display. Normally dispcal -u will use the same quality level that was specified in the previous calibration, but this can be overridden using the -q flag. Any options that attempt to change the calibration target (ie. white point, brightness, gamma etc.) will be ignored. Adjustment of the display monitor controls is skipped. A profile cannot be updated if the display does not support hardware calibration (no VideoLUT access).
Quality - Low, Medium (def), High. The -q flag determines how much time and effort to go to in calibrating the display. The higher the quality, the more test readings will be done, the more refinement passes will be done, the tighter will be the accuracy tolerance, and the more detailed will be the calibration of the display. The result will ultimately be limited by the accuracy of the instrument, the repeatability of the display and instrument, and the resolution of the Video Lookup table entries and Digital or Analogue output (RAMDAC).
The -p flag allows measuring in projector mode, using instruments that support this.
The -y flag allows setting the Display Type. Most colorimeters need this parameter set to operate correctly. The selection typically determines two aspects of of the instrument operation: 1) It may set the measuring mode to suite refresh or non-refresh displays. Typically only LCD (Liquid Crystal) displays have a non-refresh nature. 2) It may select an instrument calibration matrix suitable for a particular display type. The selections available depends on the type and model of instrument, and a list of the options for the discovered instruments will be shown in the usage information. For more details on what particular instruments support and how this works, see Operation of particular instruments.
-t Set the target white point locus to the equivalent of a Daylight spectrum of the given temperature in degrees Kelvin. By default the white point target will be the native white of the display, and it's color temperature and delta E to the daylight spectrum locus will be shown during monitor adjustment, and adjustments will be recommended to put the display white point directly on the Daylight locus. If a Daylight color temperature is given as an argument to -t, then this will become the target of the adjustment, and the recommended adjustments will be those needed to make the monitor white point meet the target. Typical values might be 5000 for matching printed output, or 6500, which gives a brighter, bluer look. A white point temperature different to that native to the display may limit the maximum brightness possible.
-T Same functionality as the -t option, except the white point locus will be the Black Body, or Planckian locus, rather than the Daylight locus. While these two white point loci are quite close, they are subtly different. If a temperature is given as an argument, this will become the Black Body target temperature during adjustment.
-w An alternative to specifying a white point target in Daylight or Black Body degrees Kevin, is to specify it in chromaticity co-ordinates. This allows the white point to be a color other than one on the Daylight or Black Body. Note that the x,y numbers must be specified as a single string (no space between the numbers and the comma).
-b Set the target brightness of white in cd/m^2. If this number cannot be reached, the brightest output possible is chosen, consistent with matching the white point target. Note that many of the instruments are not particularly accurate when assessing the absolute display brightness in cd/m^2. NOTE that some LCD screens behave a little strangely near their absolute white point, and may therefore exhibit odd behavior at values just below white. It may be advisable in such cases to set a brightness slightly less than the maximum such a display is capable of.
-g gamma Set the target response curve gamma. This is normally an exponential curve (output = input ^gamma), and defaults to 2.4 on MSWindows and Macintosh OS X 10.6 or latter and Linux/Unix (which is typical of a CRT type displays real response), and 1.8 on a Macintosh (prior to OS X 10.6). Four pre-defined curves can be used as well: the sRGB colorspace response curve, which is an exponent curve with a straight segment at the dark end and an overall response of approximately gamma 2.2 (-gs), the L* curve, which is the response of the CIE L*a*b* perceptual colorspace (-gl). the REC 709 video standard response curve (-g709) and the SMPTE 240M video standard response curve (-g240)
Note that a real display can't reproduce any of these ideal curves, since it will have a non-zero black point, whereas all the ideal curves assume zero light at zero input. In the case of a gamma curve target, dispcal uses an actual technical power curve shape that aims for the same relative output at 50% input as the ideal gamma power curve. To allow for the non-zero black level of a real display, by default dispcal will offset the target curve values so that zero input gives the actual black level of the display (output offset). This ensures that the target curve better corresponds to the typical natural behavior of displays, but it may not be the most visually even progression from display minimum, but this behavior can be changed using the -f option (see below).
Also note that many color spaces are encoded with, and labelled as having a gamma of approximately 2.2 (ie. sRGB, REC 709, SMPTE 240M, Macintosh OS X 10.6), but are actually intended to be displayed on a display with a typical CRT gamma of 2.4 viewed in a darkened environment. This is because this 2.2 gamma is a source gamma encoding in bright viewing conditions such as a television studio, while typical display viewing conditions are quite dark by comparison, and a contrast expansion of (approx.) gamma 1.1 is desirable to make the images look as intended. So if you are displaying images encoded to the sRGB standard, or displaying video through the calibration, just setting the gamma curve to sRGB or REC 709 (respectively) is probably not what you want! What you probably want to do, is to set the gamma curve to about gamma 2.4, so that the contrast range is expanded appropriately, or alternatively use sRGB or REC 709 or a gamm of 2.2 but also use the -a parameter to specify the actual ambient viewing conditions, so that dispcal can make an appropriate contrast enhancement. If your instrument is capable of measuring ambient light levels, then you can do so during the interactive display control adjustment. See <http://www.color.org/sRGB.xalter> for details of how sRGB is intended to be used.
It is hard to know whether Apple Macintosh computers prior to OS X 10.6 should also have such an adjustment, since it is not really possible to know whether colors labelled as being in such a colorspace are actually encoded in that gamma with the expectation that they will be displayed on a display with that actual response, or whether they are intended to be displayed on a display that contrast expands by a power 1.1. Both situations might be the case, depending on how source material is created!
-G gamma As explained above, the gamma value provided to the -g option is used to set and actual response curve that makes an allowance for the non-zero black of the actual display, and will have the same relative output at 50% input as the ideal gamma power curve, and so best matches typical expectations. The -G option is an alternative that allows the actual power to be specified instead, meaning that when combined with the displays non-zero black value, the response at 50% input will probably not match that of the ideal power curve with that gamma value.
-f [degree]: As explained in for the -g and -G options, real displays do not have a zero black response, while all the target response curves do, so this has to be allowed for in some way. The default way of handling this (equivalent to -f 1.0) is to allow for this at the output of the ideal response curve, by offsetting and scaling the output values. This defined a curve that will match the responses that many other systems provide and may be a better match to the natural response of the display, but will give a less visually even response from black. The other alternative is to offset and scale the input values into the ideal response curve so that zero input gives the actual non-zero display response. This ensures the most visually even progression from display minimum, but might be hard to achieve since it is different to the naturally response of a display. A subtlety is to provide a split between how much of the offset is accounted for as input to the ideal response curve, and how much is accounted for at the output, and this can be done by providing a parameter -f degree, where the degree is 0.0 accounts for it all as input offset, and 1.0 accounts for all of it as output offset. If -f is used without a specified degree, a degree of 0.0 is assumed, the opposite of the default.
-a ambient: As explained for the -g parameter, often colors are encoded in a situation with viewing conditions that are quite different to the viewing conditions of a typical display, with the expectation that this difference in viewing conditions will be allowed for in the way the display is calibrated. The -a option is a way of doing this. By default dispcal will not make any allowances for viewing conditions, but will calibrate to the specified response curve, but if the -a option is used, or the ambient level is measured during the interactive display controls portion of the calibration, an appropriate viewing conditions adjustment will be performed. For a gamma value or sRGB, the original viewing conditions will be assumed to be that of the sRGB standard viewing conditions, while for REC 709 and SMPTE 240M they will be assumed to be television studio viewing conditions. By specifying or measuring the ambient lighting for your display, a viewing conditions adjustment based on the CIECAM02 color appearance model will be made for the brightness of your display and the contrast it makes with your ambient light levels.
-k factor: Normally this will be set automatically, based on the measured black level of the display. A -k factor of 1.0 will make all colors down the neutral axis (R=G=B) have the same hue as the chosen white point. Near the black point, red, green or blue can only be added, not subtracted from zero, so the process of making the near black colors have the desired hue, will lighten them to some extent. For a device with a good contrast ratio or a black point that has nearly the same hue as the white, this should not affect the contrast ration too severely. If the device contrast ratio is not so good, and the native black hue is noticeably different to that of the chosen white point (which is often the case for LCD type displays, or CRT type displays with one channel which has a poor level of black), this could have a noticeably detrimental effect on an already limited contrast ratio, and result in a black that is not as good as it can be, and a lower -k factor should be used. -k values can range between 0.0 (no correction of black) to 1.0 (full correction of black). If less than full correction is chosen, then the resulting calibration curves will have the target white point down most of the curve, but will then blend over to the native or compromise black point that is blacker, but not of the right hue. The rate of this blend can be controlled with the -A parameter (see below).
-A rate: If the black point is not being set completely to the same hue as the white point (ie. because the -k factor is less than 1.0), then the resulting calibration curves will have the target white point down most of the curve, but will then blend over to the native or compromise black point that is blacker, but not of the right hue. The rate of this blend can be controlled with the -A parameter. The default value 4.0, which results in a target that switches from the white point target to the black, moderately close to the black point. While this typically gives a good visual result with the target neutral hue being maintained to the point where the crossover to the black hue is not visible, it may be asking too much of some displays (typically LCD type displays), and there may be some visual effects due to inconsistent color with viewing angle. For this situation a smaller value may give a better visual result (e.g. try values of 3.0 or 2.0. A value of 1.0 will set a pure linear blend from white point to black point). If there is too much coloration near black, try a larger value, e.g. 6.0 or 8.0.
-B Set the target brightness of black in cd/m^2. Setting too high a value may give strange results as it interacts with trying to achieve the target "advertised" gamma curve shape. You could try using -f 1 if this causes a problem.
-e [n] Run n verify passes on the final curves. This is an extra set of instrument readings, that can be used to estimate how well the device will match the targets with the computed calibration curves. Note that the usefulness of the verification is sometimes limited by the repeatability of the device & instrument readings. This is often evident for CRT displays, which (due to their refresh rate) flicker. More than one verification pass can be done by providing the parameter n, and by then comparing the successive verifications, some idea of the repeatability can be ascertained. The verification uses a fixed number of semi-random test values to test the calibration.
-E Run verify pass on the display as it is currently setup (currently installed LUT curves). This will use the usual input parameters to establish the expected (target) characteristic. Note that if the initial calibration was modified due to it being out of gamut of the display, verify will show the resulting discrepancy. You can use dispwin to load a .cal file into the display before running dispcal -E. Note that if you set an Ambient light level interactively during the calibration, you need to enter the same number that was measured and set using the -a parameter for verify.
The -P parameter allows you to position and size the test patch window. By default it is places in the center of the screen, and sized appropriately for the type of instrument. The ho and vo values govern the horizontal and vertical offset respectively. A value of 0.0 positions the window to the far left or top of the screen, a value of 0.5 positions it in the center of the screen (the default), and 1.0 positions it to the far right or bottom of the screen. The ss parameter is a scale factor for the test window size. A value of 0.5 for instance, would produce a half sized window. A value of 2.0 will produce a double size window. Note that the ho,vo,ss numbers must be specified as a single string (no space between the numbers and the comma).
For example, to create a double sized test window at the top right of the screen, use -P 1,0,2 .
The -F flag causes the while screen behind the test window to be masked with black. This can aid black accuracy when measuring CRT displays or projectors.
-n When running on a UNIX based system that used the X11 Windowing System, dispcal normally selects the override redirect so that the test window will appear above any other windows on the display. On some systems this can interfere with window manager operation, and the -n option turns this behaviour off.
The -J option runs through the black and sensor relative calibration routines for the Xrite DTP92 and DTP94 instruments, the black level calibration for the Eye-One Display 1, and a CRT frequency calibration for the Eye-One Display 2. For the black calibration the instrument should be placed on an opaque, black surface, and any stray light should be avoided by placing something opaque over the instrument. If a Spectrolino is being used, then a white and black calibration will always be performed before the instrument can be placed on the display, unless the -N flag is used. Generally it is not necessary to do a calibration every time an instrument is used, just now and again. There is also no point in doing a CRT frequency calibration, as this will be done automatically at the commencement of patch reading, and will be lost between runs.
The -N option disables the automatic calibration of an instrument if it is possible to do so. Sometimes it is awkward to have to re-calibrate and instrument every time a tool starts, and unnecessary if the instrument has recently been calibration, and this switch prevents it happening.
The -H option turns on high resolution spectral mode, if the instrument supports it, such as the Eye-One Pro. See Operation of particular instruments for more details. This may give better accuracy for display measurements.
The -V option uses adaptive emission measurement mode, if the instrument supports it, such as the Eye-One Pro. This may give better accuracy for low level measurements, but may be more inconsistent overall due to the varying integration times. Overall measurement time will probably be longer.
The -X file.ccmx option reads a Colorimeter Correction Matrix from the given file, and applies it to the colorimeter instruments readings. This can improve a colorimeters accuracy for a particular type of display. A list of contributed ccmx files is here.
The -X file.ccss option reads a Colorimeter Calibration Spectral Sample from the given file, and uses it to set the colorimeter instruments calibration. This will only work with colorimeters that rely on sensor spectral sensitivity calibration information (ie. the X-Rite i1d3, or the DataColor Spyder4).This can improve a colorimeters accuracy for a particular type of display.
The -X N option selects a Colorimeter Calibration Spectral Sample from the list of installed ccss files, rather than specifying a specific ccss files as in the above usage. The CCSS file that came with the i1d3 colorimeters can be installed using i1d3ccss, or one can be created and installed using ccxxmake.
The -Q flag allows specifying a tristimulus observer, and is used to compute PCS (Profile Connection Space) tristimulus values from spectral readings or using a colorimeter that has CCSS capability. The following choices are available:
1931_2 selects the standard CIE 1931 2 degree observer. The default.
1964_10 selects the standard CIE 1964 10 degree observer.
1955_2 selects the Stiles and Birch 1955 2 degree observer
1978_2 selects the Judd and Voss 1978 2 degree observer
shaw selects the Shaw and Fairchild 1997 2 degree observer
1964_10c selects a version of the CIE 1964 10 degree observer that has been adjusted using a 3x3 matrix to better agree with the 1931 2 degree observer.
NOTE that if you select anything other than the default 1931 2 degree observer, that the Y values will not be cd/m^2, due to the Y curve not being the CIE 1924 photopic V(λ) luminosity function.
The -I b|w options invoke instrument black level, and display white level compensation (respectively). Instrument black level drift compensation attempts to combat instrument black calibration drift by using a display black test patch as a reference. If an instrument is not acclimatised sufficiently to the measurement conditions, changes in temperature can affect the black readings. Display white level drift compensation attempts to combat changes in display brightness as it warms up by measuring a white patch every so often, and using it to normalise all the other readings. If just instrument black drift compensation is needed, use -Ib. If just display white level compensation is needed, use -Iw. If both are needed, use -Ibw or -Iwb.
The -C "command" option allows a method of relaying each test value to some other display than that on the system running dispcal (for instance, a photo frame, PDA screen etc.), by causing the given command to be invoked to the shell, with six arguments. The first three arguments are the RGB test color as integers in the range 0 to 255, the second three parameters are the RGB test color as floating point numbers in the range 0.0 to 1.0. The script or tool should relay the given color to the screen in some manner (e.g. by generating a raster file of the given color and sending it to the display being profiled), before returning. Note that a test window will also be created on the system running dispread.
The -M "command" option allows a method of gathering each test value from some external source, such as an instrument that is not directly supported by Argyll. The given command is involked to the shell, with six arguments. The first three arguments are the RGB test color as integers in the range 0 to 255, the second three parameters are the RGB test color as floating point numbers in the range 0.0 to 1.0. The script or tool should create a file called "command.meas" that contains the XYZ values for the given RGB (or measured from the test window) in cd/m^2 as three numbers separated by spaces, before returning. If the command returns a non-zero return value, dispcal will abort. Note that a test window will also be created on the system running dispcal.
The -W n|h|x parameter overrides the default serial communications flow control setting. The value n turns all flow control off, h sets hardware handshaking, and x sets Xon/Xoff handshaking. This commend may be useful in workaround serial communications issues with some systems and cables.
The -D flag causes communications and other instrument diagnostics to be printed to stdout. A level can be set between 1 .. 9, that may give progressively more verbose information, depending on the instrument. This can be useful in tracking down why an instrument can't connect.
inoutfile The final parameter on the command line is the base filename for the .cal file and the optional ICC profile. Normally this will be created (or an existing file will be overwritten). If the -u flag is used, then these files will be updated. If a different ICC profile name needs to be specified, do so as an argument to the -o flag.
NOTE that on an X11 system, if the environment variable ARGYLL_IGNORE_XRANDR1_2 is set (ie. set it to "yes"), then the presence of the XRandR 1.2 extension will be ignored, and other extensions such as Xinerama and XF86VidMode extension will be used. This may be a way to work around buggy XRandR 1.2 implementations.

## Discussion and guide to display control adjustment:

The adjustment of the display controls (brightness, contrast, R, G & B channel controls etc.) is very dependent on the particular monitor. Different types and brands of monitors will have different controls, or controls that operate in different ways. Some displays have almost no user controls, and so you may well be best skipping display adjustment, and going straight to calibration.
Almost all LCD displays lack a real contrast control. Those that do present such a control generally fake it by adjusting the video signal. For this reason it is usually best to set an LCD's contrast control at its neutral setting (ie. the setting at which it doesn't change the video signal). Unfortunately, it can be hard to know what this neutral setting is. On some displays it is 50%, others 75%. If the LCD display has a "reset to factory defaults" mode, then try using this first, as a way of setting the contrast control to neutral. The LCD brightness control generally adjusts the level of backlighting the display gets, which affects the maximum brightness, and also tends to raise or lower the black level in proportion, without changing the displays response curve shape or overall contrast ratio. If your LCD display has a backlight control as well as a brightness control, then the brightness control is also probably being faked, and you are probably better off setting it to it's neutral setting, and using the backlight control in place of brightness in the following adjustments.
Some high end displays have the ability to mimic various standard colorspaces such as sRGB or AdobeRGB. You could choose to calibrate and profile the display in such an emulation mode, although you probably don't want to fight the emulations white point and gamma. To get the best out of such a display you really want to choose it's "Native Gamut" setting, whatever that is called. Note that some people have reported bad experiences in trying to use "6-axis custom controls" on displays such as the Dell U2410, so attempting to use such a mode should be approached with caution. Ideally such a mode should be used to give just the underlying native display response, but the settings to achieve this may be very difficult to determine, and/or it may not be possible, depending on how such a mode distorts the RGB signals.
On CRT based displays, the brightness control generally adjusts the black level of the display (sometimes called the offset), and as a side effect, tends to change the maximum brightness too. A CRT contrast control generally adjusts the maximum brightness (sometimes called gain) without affecting the black level a great deal. On a CRT both the brightness and contrast controls will tend to affect the shape or gamma of the display response curve.
Many displays have some sort of color temperature adjustment. This may be in the form of some pre-set color temperatures, or in the form of individual Red, Green and Blue channel gain adjustments. Some CRT displays also have R, G & B channel offset adjustments that will affect the color temperatures near black, as well as affect the individual channels curve shape. The color temperature adjustment will generally affect the maximum brightness, and may also affect the black level and the shape of the display response curves.
Some special (expensive) LCD displays may have a white point adjustment that changes the color of the backlight. If you do not have one of these types of LCD displays, then attempting to change the white point of the display (even if it appears to have a "white point selection" or R/G/B "gain" controls") may not be a good idea, as once again these controls are probably being faked by manipulating the signal levels. Even if you do manage to change the white point significantly, it may do things like change the mid tone color too dramatically, or create a display response that is hard to correct with calibration, or results in side effects such as quantization (banding) or other undesirable effects. You may have to try out various controls (and your aim points for the display calibration), to decide what is reasonable to attempt on an LCD display.
Due to the variety of controls as well as the interaction between them, it can be an iterative process to arrive at a good monitor set-up, before proceeding on to calibrating and profiling a display. For this reason, dispcal offers a menu of adjustment modes, so that the user can interactively and iteratively adjust the display controls to meet the desired targets.
1) Black level (CRT: Brightness)
2) White point (Color temperature, R,G,B, Gain/Contrast)
3) White level (CRT: Gain/Contrast, LCD: Brightness/Backlight)
4) Black point (R,G,B, Offset/Brightness)
5) Check all
6) Measure and set ambient for viewing condition adjustment
7) Continue on to calibration
8) Exit
There are four basic adjustment modes. Normally one would proceed through them in the order above, then perhaps repeat the first adjustment, before checking the overall settings. The White point and White level modes operate slightly differently, depending on whether a white target point has been set using the -t -T or -w options, and on whether a brightness target has been set using the -b option.
The first mode lets you adjust the black level of a CRT display. Given the current white level, it calculates a value that should produce a 1% display brightness if the black level is set correctly. After doing some initial measurements, it will show the target brightness value (in cd/m^2) on one line, and then underneath it will show continuously updated readings from the display. The left most character will switch from '\' to '/' or back again each time a reading is updated. Some instruments can be quite slow in measuring dark colors, and it's best to wait for a reading update before changing the controls more than once. Underneath the target value is displayed the current reading, and to the right of this is a '+', '-' or '=' symbol, which gives a hint as to which way to adjust the brightness control to improve the match to the target.
Adjust CRT brightness to get target level. Press space when done.
Target 0.60
/ Current 0.68 -
Once happy with the adjustment, press space to go back to the menu.
The second mode lets you adjust the color of the white point of the display. If a target white point has been set, it will show the target brightness value (in cd/m^2) on one line, together with the target chromaticity co-ordinates for the white point, and then underneath it will show continuously updated readings from the display. The left most character will switch from '\' to '/' or back again each time a reading is updated. Underneath the target brightness value is displayed the current reading, and then the current chromaticity co-ordinate values. To the right of this is the current delta E of the white point from the target, and further to the right are hints '+', '-' or '=' as to which direction to adjust the individual Red, Green and Blue gain settings to move the white point in the direction of the target, and reduce the delta E. If the symbol is doubled, then this channel will have the greatest effect. If you do not have individual channel gain controls, then try choosing amongst color temperature pre-sets, to find one with the lowest delta E. Depending on the stability of the display, the coarseness of the controls, and the repeatability of the instrument, you may not be able to get a perfectly zero delta E.
Adjust R,G & B gain to get target x,y. Press space when done.
Target B 60.00, x 0.3451, y 0.3516
/ Current B 60.05, x 0.3426, y 0.3506 DE 1.4 R+ G+ B--
If you did not set a white point target, then the information shown is a little different - it will show the initial white point value, as well as the color temperature, and the CIEDE2000 of the white point to either the Daylight or Black Body locus (depending on whether the -T flag was set). The constantly updated values show the same thing, and the Red, Green and Blue control hints show the direction to adjust the controls to place the white point on the locus. The control that will have the most direct effect on the color temperature will be the Blue, while the Green will most directly move the white point towards or away from the locus, thereby reducing the delta E of the white point to the locus (but there is interaction).
Adjust R,G & B gain to desired white point. Press space when done.
Initial B 47.25, x 0.3417, y 0.3456, CDT 5113 DE 6.9
\ Current B 47.38, x 0.3420, y 0.3460 CDT 5104 DE 6.7 R-- G+ B-
The brightness value is just there as a guide to what effect the adjustment is having on the overall brightness. Usually the white level brightness is adjusted using the next adjustment mode. Once happy with the adjustment, press space to go back to the menu.
The third mode lets you adjust the brightness of white on the display. If you set a target brightness using the -b parameter, it will show the target brightness value (in cd/m^2) on one line, and then underneath it will show continuously updated readings from the display. The left most character will switch from '\' to '/' or back again each time a reading is updated. Underneath the target value is displayed the current reading, and to the right of this is a '+', '-' or '=' symbol, which gives a hint as to which way to adjust the CRT contrast or LCD brightness control to improve the match to the target.
Adjust CRT Contrast or LCD Brightness to get target level. Press space when done.
Target 60.00
/ Current 59.96 +
If you did not set a brightness target, it will show the initial brightness as the target, and the current brightness, which you can then set any way you want:
Adjust CRT Contrast or LCD Brightness to desired level. Press space when done.
Initial 47.32
/ Current 47.54
Once happy with the adjustment, press space to go back to the menu.
The fourth mode lets you adjust the color of the black point of the display, if the display has Red, Green and Blue channel offset controls. It will show the target 1% brightness value (in cd/m^2) on one line, together with the target chromaticity co-ordinates for the black point, and then underneath it will show continuously updated readings from the display. The left most character will switch from '\' to '/' or back again each time a reading is updated. Underneath the target brightness value is displayed the current reading, and then the current chromaticity co-ordinate values. To the right of this is the current delta E of the black point from the target, and further to the right are hints '+', '-' or '=' as to which direction to adjust the individual Red, Green and Blue offset settings to move the black point in the right direction. If the symbol is doubled, then this channel will have the greatest effect.
Adjust R,G & B offsets to get target x,y. Press space when done.
Target B 0.60, x 0.3451, y 0.3516
\ Current B 0.62, x 0.2782, y 0.2331 DE 10.3 R+ G++ B-
The 1% brightness value is just there as a guide to what effect the adjustment is having on the 1% brightness level. The combined channel offsets may have an effect on this in combination with the CRT brightness control. Press space to go back to the menu.
The fifth selection checks on the overall settings. If targets have been set, it will be like:
Target Brightness = 50.00, Current = 47.44, error = -5.1%
Target 50% Level = 10.32, Current = 8.10, error = -4.4%
Target Near Black = 0.47, Current = 0.68, error = 0.4%
Target white = x 0.3458, y 0.3586, Current = x 0.3420, y 0.3454, error = 7.55 DE
Target black = x 0.3458, y 0.3586, Current = x 0.2908, y 0.2270, error = 29.69 DE
or if no targets are set:
Current Brightness = 46.28
Target 50% Level = 10.07, Current = 7.52, error = -5.5%
Target Near Black = 0.46, Current = 0.46, error = -0.0%
Current white = x 0.3439, y 0.3466, VCT 5098K DE 3.0
Target black = x 0.3439, y 0.3466, Current = x 0.3093, y 0.2165, error = 30.30 DE
and will then go back to the menu.
The sixth selection 6) allows the reading of you ambient lighting conditions if your instrument supports such a mode. Doing so will enable the -a option to compensate for your viewing conditions in the subsequent calibration. See -a.
Once you're happy with the display set-up, you can either proceed on to the rest of the calibration by selecting 7), or exit and re-start by selecting 8). You might want to re-start if you want to change the calibration targets.

## Other caveats:

NOTE that some LCD screens behave a little strangely near their absolute white point, and may therefore exhibit odd behavior at values just below white. It may be advisable in such cases to set a brightness slightly less than the maximum such a display is capable of.
The program attempts to stop any screensaver or powersaver from interfering with the measurements, but this may not be effective on some systems, so it may be necessary to manually disable the screensaver and/or powersaver before commencing the calibration with a large number of patches.
The calibration tables produced maintain the maximum level of precision available on a system. If the display has VideoLUTs available (Video Lookup Tables that the frame buffer values pass through on their way to the display) and thier outputs are better than 8 bits per component, then the resulting curves can reflect this, although few current operating systems and/or display cards actually support better than 8 bit per component output.
If calibration curves are created for a display in which VideoLUTs are not available, then the resulting calibration file will be marked to indicate this, and a subsequent profile created with the calibration will not have the calibration converted to the 'vcgt' tag, since such a tag can't be loaded into the displays VideoLUTs.
If communications break down with a USB connected instrument, you may have to unplug it, and plug it in again to recover operation.
Some systems (Apple OSX in particular) have a special set of user interface controls ("Universal Access") that allows altering the display in ways designed to assist visually impaired users, by increasing contrast etc. This will interfere badly with any attempts to calibrate or profile such a system, and must be turned off in order to do so. Note that certain magic keyboard sequences can turn this on by accident.