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                        little cms Engine

           lprof - A profile construction set library

               How to use the the sample profilers

                         by Marti Maria

                            Ver 1.09


 Welcome to this small guided tour on littlecms sample profilers.
 The goal of this document is to show how these programs can be
 used to generate ICC profiles for your monitor, scanner, digital
 camera, film projector or input device. It has also some advice
 on color management in general, as well as some guidelines on
 how to handle several littlecms utilities.

 Since the best way to see how it works seems to be by example, we
 will step by step make a whole calibration procedure for monitor
 and scanner.

 In all this tour, we will make extensive use of the measurement
 and proof utility: qtMeasurementTool. You will also need a IT8
 target, available from several vendors. If you plan to do scanner
 profiling, make sure to get one of these.

 qtMeasurementTool is a "virtual colorimeter" that used in
 conjunction with a proper scanner profile can be used as a cheap
 replacement of expensive hardware. It can also be used to proof
 an Image across two profiles. The most simple way to check if
 all is going fine is by visual inspection. You can load a Image
 in the measurement tool, and apply a transform between your
 input profile (AKA, scanner or camera) and your monitor profile.
 Since right now we have no profiles to check, we should first
 create a profile for our monitor.

 This is done by qtMonitorProfiler. This program can work in two
 different modes. The "advanced" mode, that gives accurate
 profiles by using hardware devices like X-Rite DP92 or EyeOne,
 and the "simple" mode, that allow to build reasonably accurate
 profiles without any additional hardware.

 Building a monitor profile: a coarse approximation

 So, let's build our first monitor profile to check how all this
 stuff works. We don't need right now a big accuracy. We want to
 calibrate coarsely, just to visually check if is all going ok.

 After installing the package, type


 This will start the monitor profiler application. As said, we
 need only a coarse profile, so, we select the radio button

  "I want to build a coarse profile, giving approximate values"

 This is selected by default. We need not any measurement right
 now, only a few visual adjusts. Press the button labeled:

                     "Enter monitor values"

 You will see the monitor values page. In order to obtain maximum
 performance, we need to adjust first the physical monitor

 Set contrast to near maximum, (yes, near maximum. This is the
 gain of monitor and we need as many as monitor can deliver) and
 then adjust brightness until you can see all grays of a
 grayscale. Leave the monitor alone, and return to monitor
 profiler program.

 Now we will adjust gamma correction. This is by far the most
 important setting. On our first approach, we will try with only
 one gray channel (make sure "Link channels" is checked).

 Move the slide until the central frame seems to "fade" on background. 
 Don't  spend too much time adjust it, gamma varies with many factors.
 A coarse adjust will be enough right now.

 You should obtain a typical value of 2.1-2.4, in some cases it
 could be even lower.

 Now we go on the white point stuff. Some monitors refers white
 point as "temperature". Most users does have white point set to
 D93 in order to get more bright, unfortunately this  restricts
 the gamut and adds a big quantity of blue, so my advice would be
 to select something lower.

 If you can switch temperature of your monitor, D65 (near 6500K)
 could be a good choice for multiuse. For proofing devices, D50
 is almost a must, but it adds too yellow for a unadapted used.
 D50 requires a dim or dark room to give the best results. If you
 don't know which temperature your monitor has, select D65 or
 D93. On our fist approach both these will give reasonable

 If your monitor has any other white point, you can select "User
 defined", the last option. You can then select your particular
 white point as a temperature in Kelvin degree.

 Don't use CIE illuminants. They are intended for  workspace 
 construction, and will be discussed latter.

 Next adjust is primaries set. For the primaries, select the
 default ITU/R BT.709 if you are profiling a CRT monitor. There
 are primaries for Samsung TFT and LCD, but these devices are not
 fully supported by the profiler. The reason to not support them
 is the angular dependence, which makes them unusable for color
 management tasks.

 Other primaries are for building workspaces. If you don't know
 the primaries, don't worry. Use the default ITU/R BT.709. There
 are 6 or 7 CRT tube manufacturers in the world, and all them
 does use this set.

 Now you can fill the info about your monitor. Although this is
 not strictly required, it could prevent a lot of pain when you
 have more than a couple of profiles.  For filling the info field, 
 select the "profile identification" tab, and be creative on your 

 Now we are ready to generate our profile. We must specify the
 output filename, so, select the "Main" tab, and press the "..."
 button on right side of the "output profile file" edit.

 Select "profiles" directory. This is where other tools will look
 for monitor profile. There are a couple of profiles there, leave
 them alone. For the name, we could use "CoarseMonitor.icm" So,
 type in the name and then proceed to generate the profile by
 pressing the "GO!" button. If all is ok, the status console will
 show "Profile DONE!"

 Let's now check how well is working our profile by eye. For
 that, we need another tool. So, close the monitor profile and
 start the measurement tool by typing


 This utility has many usage. One would be to "proof" a color
 transform. Let's first configure the measurement tool. Select
 the "Options" tab. Adjust the default settings as following:

          On "Pick template"   --> "Empty Picker"

          On "Input profile"   --> "Tutorial Workspace"

 On "Monitor profile" should appear your profile. If you filled
 the info fields, here will be your description. If you didn't
 enter description it will be"(unknown) - Little cms construction
 set", which is the default string. Now you know why to fill
 information fields!

 We are going to check your profile with a highly tweaked sample
 Image This is a counterexample on what a good workspace should
 be. NEVER use this profile for practical purposes!

 Ok, let's see how a Image looks in this tweaked colorspace.
 Select the "Image" tab, and press "Load Image" button.

 Select "pics" directory. There is a "party_4s.png" Load it. You
 will see the raw, non color managed picture. Since the encoding
 space of this Image is quite different of any monitor, the
 images looks "flat" and "washed out" Now press the "Proof"
 button. The Image is shown using color management. Color
 changes should be noticeable, despite gamma of image is
 near gamma of monitor.

 Building scanner profile

 We have now our monitor calibrated, and will try to generate a
 scanner profile. For this, we will also need the measurement

 Scanner profiles are built by using a thing known as "IT8
 target". IT8 targets are just a photo with a set of color
 patches. There are known colorimetric measurements for each
 of these patches. There is also a standard way to specify 
 these measurements, the IT8/CGATS format.

 IT8/CGATS is a way to encode data that is both, human and
 machine readable. You can open any of these IT8 reference sheets
 with your text editor to see it. You can even modify the
 contents. Since it is a standard, most color management systems
 does accept it. For these reasons, seems a good way to store
 measurement data.

 A IT8 file does contain measurements for a set of color patches.
 The measurement of each patch can be the device colorant values
 (the RGB or CMYK in 0..255 range) the tristimulous values (XYZ
 or Lab) and some statistic data.

A IT8 sheet looks like:

DESCRIPTOR "L* a* b* Batch average data (light D50, viewing angle 2)"
CREATED "January 07, 2002"
PROD_DATE "2001:12"
SERIAL "DIN A4  R012020"
MATERIAL "Kodak Professional Digital 3 - Non-Glossy"
SAMPLE_ID   XYZ_X   XYZ_Y   XYZ_Z          LAB_L   LAB_A   LAB_B          LAB_C   LAB_H
A1           3.25    2.71    1.94          18.86   11.18    2.87          11.55   14.41
A2           4.07    2.73    1.60          18.91   23.59    6.54          24.49   15.48
A3           5.79    3.42    1.75          21.67   33.38    9.60          34.73   16.05
A4           4.28    2.66    1.46          18.62   27.73    7.59          28.75   15.32
A5          11.97   10.30    7.42          38.38   15.04    4.16          15.61   15.47
.... etc ...

 The target manufacturer does provide a IT8 file containing these
 measurements for the target. So, the target is giving to us a set
 of KNOWN colors. We must then, tell to the profiler which RGB
 values our scanner returns for each of these patches.

 Measurement tool can generate sheets with the RGB values of each
 patch grabbing them from a Image. In order to generate a
 profile, we need the RGB and Lab values for each patch.

 let's outline the scanner profiling procedure:

      * First (and very important!) adjust controls of scanner to
        obtain a proper exchange space.

      * Scan by means of you scanning software a Image of your

      * By using measurement tool, grab the RGB values of each
        patch. Store them in a IT8 file

      * The scanner profile then will compare the RGB
        obtained by the scanner, with the known colorimetric
        measurement of target, as stored in vendor's reference

      * The scanner profiler will create a model for the
        translation RGB -> Lab or RGB -> XYZ and will store it in
        a profile.

 The math behind this model is not trivial. There is a big
 quantity of factors taken into account. Don't worry for the
 apparent missing of data. Yes, there are colors not present in
 target, however the scanner profiler is capable to "guess" or
 extrapolate them, and should produce a profile handling all
 gradients between these colors in a smooth and pleasant way.

 Lets' discuss each step in detail:

      * Step 1: Adjust controls of scanner.

 You will need 24 bits per pixel at least. Gray profiles
 are NOT supported in this revision.

 There is a big amount of controversy about how to set the
 scanner controls. Some people does advice to use linear gamma
 (gamma=1.0), which is a wrong advice unless you are working with
 16 bits per sample (48 bits per pixel)

 If you are going to scan in 8 bits per sample (24 or 32 bits)
 your best choice is a gamma between 2.2 and 3.0

 Gamma 2.2 has the additional benefit of being close to sRGB, and
 this means the uncorrected Image will "look nice" on average
 monitors. It is also near to perceptual gamma.

 Gamma 2.4 has the additional benefit of being closest to
 perceptual space, and this is a very good reason to use this

 Less that 2.2 (and of course the infamous 1.0) can generate huge
 loss of detail in shadows, only to give a slight bettering of
 highlights. Don't use this unless your are using 16 bits per
 sample, and even in such case, don't do it unless you know what
 are you doing!

 More that 2.4 (up to 3.0) drops too much highlight to be safe,
 despite you will still see the Image quite correct.

 More on controls: Turn off all color management, color enhancing
 and tweaking on scanner driver. Set brightness contrast and hue
 (if available) to default values. It is supposed this will be your 
 working scanning mode, so set all to reasonable defaults.

 IMPORTANT: The profile will only be valid with those settings!

      * Step 2: Scan the IT8 target.

 Store it in PNG, TIFF, BMP on depending of supported formats of
 your platform. DON'T use JPEG! Its lousy nature makes it
 unsuitable for this purpose. You could use JPEG on final images,
 but not in profiling process.

      * Step 3: Extract the RGB values for each patch.

 Without help, this could be a huge task. There are plenty of
 patches and taking each sample by hand would be tedious. With
 the help of measurement tool, this is a easy task.

 Start the measurement tool by typing


 Press "Load Image" button. Select the Image of target you scanned 
 in step 2. I will use the scandmo.png image sited on "pics" 
 directory as sample. This is a Kodak film target, scanned with a 
 scanner that adds a huge violet cast. Don't worry, the profile 
 will turn this one to a nice looking Image.

 Since this is a Kodak target that has some additional patches
 (those on I,J,K,L - 20,21,22) we can choose a picker template
 according the nature of target. Press "Options" tab, and in the
 combo labeled "Pick template" select:

               "IT8.7 19+3 (Kodak) column picker"

 Press "Image" tab.

 You will see a lot of green rectangles. Using the mouse you 
 must place the pick template over the patches. Use left button to
 move whole template and right button to resize it. Since this
 Image is not perfectly horizontal, you may want to "unshear" the 
 layout: You can do it by pressing "Ctrl" key while dragging with 
 left mouse button.

 The "hot area" is marked with green rectangle. Assure all green
 rectangles are inside patches. Failure to do so could result in
 wrong profile! You can also adjust the relative size of "hot
 areas" by setting the % in "safe frame" on "Options" tab.
 Normally there is no need to touch anything.

 Once you were happy with the location of patch grabber zones,
 press the "Pick" button, placed on left side of window.

 The measurement tool will build a IT8 sheet containing RGB
 values of each of your patches. Now you can save this sheet by
 pressing the "Save IT8 sheet" button. For our tutorial I will
 name this measurement "scandmo.it8" and will place it on
 "pics" directory. (There is already one scandmo.it8, that you
 can overwrite)

 Our work with measurement tool is now done. We have right now
 the reference sheet of target, which is given by Kodak, and the
 RGB values obtained by our scanner. Now we can compare these
 files to create a profile.

  * Step 4: Close the measurement tool, and launch the 
    scanner profiler by typing:


 From here is quite easy. First we must select the target
 reference. In this tutorial, we have used a film target of
 reference 1996-08, so, press the "Select target & vendor type"
 button and select "targets", "Kodak" folder, then "Emissive"
 folder (since it is film). Press Ok. The left combo will be
 filled with all target references. Then select 1996-08 (is on
 last part of list)

 Once selected, you need to specify your measurement. Press the
 "..." button placed on right side of "Measurement" edit. Select
 "picts" folder and "scandmo.it8" file.

 Now we have to specify the profile filename. Press the "..." button
 placed on right of "output profile file", select the "profiles"
 folder. The name could be "scandmo.icm"

 You can now, as we did in the monitor profile, fill some info to
 help us to locate the profile latter. Press the "profile
 identification" tab, and fill in the fields.

 You are ready to generate the profile. Press "GO!" button to
 launch the profiler process. After a few seconds, you will have
 a small profile for your scanner. In the demo, the obtained dE is
 about 1.24, which is reasonably. Now,  we can better a lot the 
 error level, but this will take more time to generate the profile.
 Press the "Parameters and tweaking" tab, and select on
 "Resolution" group "33 points". Then check the checkbox labeled
 "local convergence analysis". Press the "GO!" button. Now the
 generation will take a while, but the obtained profile has
 bettered a lot... maximum dE is about 0.7 and the peak value is
 as small as 3!!!

    * Step 5: Let's check how this profile does affect the 
     original Image.

 Start the measurement tool, select "options" tab and make sure
 picker template is set to "empty picker", "input profile" should
 be set to this newly created scanner profile, and "monitor
 profile" should be the profile we have made for monitor. Press
 "Image" tab, press "Load Image" button, load the scanned Image
 (scandmo.png in our sample) and then press the "proof" button to
 see the results. The girl's Image is dark, this is normal.
 However, rest of patches are hugely modified, the grayscale
 should appear smooth and without any cast, as well as all violet
 tint should disappear.

  Making workspace profiles

 Some applications, for example Adobe Photoshop, does use what is
 called "Workspaces". These are RGB color spaces, which are not
 intended to be representing any device, nor storage but a
 "working space". That is, Image processing operations are
 performed in such space, and also can be used as a way to 
 interchange image data between applications.

 The monitor profiler, despite its name, can be effectively used
 to build such profiles. 

 Just select the gamma, primaries set and white point. Press GO!
 and the workspace profile will be generated.

 Building accurate monitor profiles by hardware measurement devices

 To measure your monitor, you need to fill a IT8 sheet,
 containing XYZ measurements for several RGB combinations. A
 sample sheet of this kind can be found in "pics" directory. The
 file is called "sRGB.IT8" and it measures an ideal sRGB monitor.

 You need to change the XYZ values by those read by your
 measurement device. Don't put any Lab value, just RGB and XYZ.

 Please keep the RGB values the sRGB.IT8 has. Optionally, you can 
 use more patches to assure consistent color reproduction, but make 
 sure to include at least that ones.

 Note the big amount of gray patches needed. You can also add as
 many as you wish, on more gray patches, better linearization
 curves control. You must specify at least 16 gray patches.

 Now we will try this procedure, no matter you have not any
 hardware device. As we will see below, this method could be used
 to dissect profiles and do some tweaking.

 We are going to generate a sRGB profile, by using the sRGB.IT8
 measurement sheet. For that, we will use monitor profiler, so
 start it by typing:


 Now, we are going to use it in advanced mode. Select the radio
 button labeled:

    "I want to build accurate profile from measurement sheet"

 Then press the "..." button located at right of edit. Select in
 "pics" directory the file called "sRGB.IT8". In the "Output
 profile file", select a new profile in "profiles" directory.
 Call it "my_sRGB.icm".

 I will use this sample to present another utility, the profile
 checker. Since the profile checker can understand advanced tags,
 we want to include all information in the profile, so select the
 "parameters and tweaking" tab, and make sure to select "Verbose,
 store anything" in the "profile verbosity level" group.

 Now, fill the info fields if you want (again, I recommend to do
 that) and press the GO button. A new "my_srgb.icm" profile will
 be created.

 We will check now how accurate is this profile. Start the
 profile checker program by typing


 The profile checker is a tool for inspecting some of profile
 internals. In our case, we want to check the accurancy. For
 selecting the profile, press the ".." button on the right side
 of the "profile" edit. Select "pics" directory, and choose
 "my_srgb.icm" profile. Press "GO!" button.

 On "dE report tab" you will see the error statistics. (unless
 you forgot to set the profile verbosity to "store anything") 
 Our sample scored 0.18, which is a incredibly low value, not 
 to be even reached in real world cases, but this could give 
 an idea of the precision the profiler can give with only a 
 very few measurements.

 The profile checker has also the capability of draw chromaticity
 diagram, to show primaries, color displacements, to guess the
 gamma of the profile and many other functionality. Feel free to
 experiment, the profile checker will never write on your
 profile, so you can play with it in a safe way.

 Additional utilities

 Finally, there is a additional command-line utility that can be
 used in conjunction of monitor profiler. It is called icc2it8
 and is placed on ICCtoIT8 directory. This utility is capable of
 generate IT8 measurements from profiles, so, it can be used as a
 "profile tweacker". You can generate a IT8 sheet from a given
 profile, then edit manually XYZ values and then build a new
 profile using the modified sheet. This right now is limited to
 matrix-shaper profiles, but even with this small limitation, 
 it can become a valuable tool. 

 Other useful tools are included in littlecms CMM package. 
 You have TIFFICC that can be used to apply profile transforms
 to TIFF files, and JPEGICC that does same on JPEG. Also, 
 you have ICCTRANS, that can be used to check profiles
 by numbers. See the littlecms documentation for more details.

 That's all for now. There are still many options that this small
 introduction don't cover, like the CIECAM97s appearance model,
 and a more advanced usage. However, you have now a basic
 knowledge of how this stuff works. There is a mailing list on
 littlecms for asking and sharing your tricks on how get the best
 from littlecms profilers. Feel free to ask.