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little cms Engine
http://www.littlecms.com
How to use the engine in your applications
by Marti Maria
Ver 1.17
---------------------------------------------------
Introduction
BASIC USAGE
===========
1. Basic Concepts
2. Step-by-step example
3. Embedded profiles
4. Device-link profiles
5. Built-in profiles
6. On-the-fly profiles
7. Gamma tables
8. Proofing
9.1 Black point compensation
9.2 Black preservation
10. Error handling
11. Getting information from profiles.
ADVANCED TOPICS
===============
12. Creating and writting new profiles
13. LUT handling
14. Helper functions
15. Color difference functions
16. PostScript operators
17. CIECAM02
18. Named color profiles
19. Conclusion
Sample 1: How to convert RGB to CMYK and back
Sample 2: How to deal with Lab/XYZ spaces
Annex A. About intents
Annex B. Apparent bug in XYZ -> sRGB transforms
----------------------------
Introduction:
This file has been written to present the lcms core library to
would-be writers of applications. It first describes the
concepts on which the engine is based, and then how to use it
to obtain transformations, colorspace conversions and
separations. Then, a guided step-by-step example, shows how to
use the engine in a simple or more sophisticated way.
This document doesn't even try to explain the basic concepts of
color management. For a comprehensive explanation, I will
recommend the excellent color & gamma FAQs by Charles A.
Poynton,
http://www.poynton.com
For more details about profile architecture, you can reach the
latest ICC specs on:
http://www.color.org
**PLEASE NOTE THAN lcms IS NOT A ICC SUPPORTED LIBRARY**
I will assume the reader does have a working knowledge of the C
programming language. This don't mean lcms can only be used by
C applications, but it seems the easiest way to present the API
functionality. I currently have successfully used the lcms DLL
from Delphi, C++ Builder, Visual C++, Tcl/Tk, and even Visual
Basic.
DELPHI USERS:
If you plan to use lcms from Delphi, there is a folder in the
package containing units and samples for Delphi interface. Rest
of document does refer to C API, but you can use same functions
on Delphi.
1. Basic Concepts:
============================================================================
lcms defines two kinds of structures, that are used to manage
the various abstractions required to access ICC profiles. These
are profiles and transforms.
In a care of good encapsulation, these objects are not directly
accessible from a client application. Rather, the user receives
a 'handle' for each object it queries and wants to use. This
handle is a stand-alone reference; it cannot be used like a
pointer to access directly the object's data.
There are typedef's for such handles:
cmsHPROFILE identifies a handle to an open profile.
cmsHTRANSFORM identifies a handle to a transform.
Conventions of use:
o All API functions and types have their label prefixed
by 'cms' (lower-case).
o #defined constants are always in upper case
o Some functions does accepts flags. In such cases,
you can build the flags specifier joining the values with the
bitwise-or operator '|'
o An important note is that the engine should not leak
memory when returning an error, e.g., querying the
creation of an object will allocate several internal
tables that will be freed if a disk error occurs during
a load.
Since these are a very generic conventions widely used, I will
no further discuss this stuff.
2. Step-by-step Example:
============================================================================
Here is an example to show, step by step, how a client application
can transform a bitmap between two ICC profiles using the lcms API
This is an example of how to do the whole thing:
#include "lcms.h"
int main(void)
{
cmsHPROFILE hInProfile, hOutProfile;
cmsHTRANSFORM hTransform;
int i;
hInProfile = cmsOpenProfileFromFile("HPSJTW.ICM", "r");
hOutProfile = cmsOpenProfileFromFile("sRGBColorSpace.ICM", "r");
hTransform = cmsCreateTransform(hInProfile,
TYPE_BGR_8,
hOutProfile,
TYPE_BGR_8,
INTENT_PERCEPTUAL, 0);
for (i=0; i < AllScanlinesTilesOrWatseverBlocksYouUse; i++)
{
cmsDoTransform(hTransform, YourInputBuffer,
YourOutputBuffer,
YourBuffersSizeInPixels);
}
cmsDeleteTransform(hTransform);
cmsCloseProfile(hInProfile);
cmsCloseProfile(hOutProfile);
return 0;
}
Let's discuss how it works.
a) Open the profiles
You will need the profile handles for create the transform. In
this example, I will create a transform using a HP Scan Jet
profile present in Win95 as input, and sRGB profile as output.
This task can be done by following lines:
cmsHPROFILE hInProfile, hOutProfile;
hInProfile = cmsOpenProfileFromFile("HPSJTW.ICM", "r")
hOutProfile = cmsOpenProfileFromFile("sRGBColorSpace.ICM", "r")
You surely have noticed a second parameter with a small "r".
This parameter is used to set the access mode. It describes
the "opening mode" like the C function fopen().
Currently lcms does support both read and write profiles.
WARNING!: opening with 'w' WILL OVERWRITE YOUR PROFILE!
Don't do this except if you want to create a NEW profile.
NOTES:
This only will take a small fraction of memory. The BToA or AToB tables,
which usually are big, are only loaded at transform-time, and on demand.
You can safely open a lot of profiles if you wish.
If cmsOpenProfileFromFile() fails, it raises an error signal that can
or cannot be catched by the application depending of the state of the
error handler. In this example, I'm using the "if-error-abort-whole-
application" behaviour, corresponding with the LCMS_ERROR_ABORT setting
of cmsErrorAction(). See the error handling paragraph below for more
information.
lcms is a standalone color engine, it knows nothing about where
the profiles are placed. lcms does assume nothing about
a specific directory (as Windows does, currently expects profiles
to be located on SYSTEM32/SPOOL/DRIVERS/COLOR folder in main windows
directory), so for get this example working, you need to copy the
profiles in the local directory.
b) Identify the desired format of pixels.
lcms can handle a lot of formats. In fact, it can handle:
- 8 and 16 bits per sample
- up to 16 channels
- extra channels like alpha
- swapped-channels like BGR
- endian-swapped 16 bps formats like PNG
- chunky and planar organization
- Reversed (negative) channels
- Floating-point numbers
For describing such formats, lcms does use a 32-bit value, referred
below as "format specifiers".
There are several (most usual) encodings predefined as constants, but there
are a lot more. See lcms.h to review the current list.
TYPE_RGB_DBL
TYPE_CMYK_DBL
TYPE_Lab_DBL
TYPE_XYZ_DBL
TYPE_YCbCr_DBL Takes directly the floating-point structs
TYPE_GRAY_8 Grayscale 8 bits
TYPE_GRAY_16 Grayscale 16 bits
TYPE_GRAY_16_SE Grayscale 16 bits, swap endian
TYPE_GRAYA_8 Grayscale + alpha, 8 bits
TYPE_GRAYA_16 Grayscale + alpha, 16 bits
TYPE_GRAYA_16_SE Grayscale + alpha, 16 bits
TYPE_GRAYA_8_PLANAR Grayscale + alpha, 8 bits, separate planes
TYPE_GRAYA_16_PLANAR Grayscale + alpha, 16 bits, separate planes
TYPE_RGB_8 RGB, 8 bits
TYPE_RGB_8_PLANAR RGB, 8 bits, separate planes
TYPE_BGR_8 BGR, 8 bits (windows uses this format for BMP)
TYPE_BGR_8_PLANAR BGR, 8 bits, separate planes
TYPE_RGB_16 RGB, 16 bits
TYPE_RGB_16_PLANAR ...
TYPE_RGB_16_SE
TYPE_BGR_16
TYPE_BGR_16_PLANAR
TYPE_BGR_16_SE
TYPE_RGBA_8 These ones with alpha channel
TYPE_RGBA_8_PLANAR
TYPE_RGBA_16
TYPE_RGBA_16_PLANAR
TYPE_RGBA_16_SE
TYPE_ABGR_8
TYPE_ABGR_16
TYPE_ABGR_16_PLANAR
TYPE_ABGR_16_SE
TYPE_CMY_8 These ones for CMY separations
TYPE_CMY_8_PLANAR
TYPE_CMY_16
TYPE_CMY_16_PLANAR
TYPE_CMY_16_SE
TYPE_CMYK_8 These ones for CMYK separations
TYPE_CMYK_8_PLANAR
TYPE_CMYK_16
TYPE_CMYK_16_PLANAR
TYPE_CMYK_16_SE
TYPE_KYMC_8 Reversed CMYK
TYPE_KYMC_16
TYPE_KYMC_16_SE
TYPE_XYZ_16 XYZ, xyY and CIELab
TYPE_Yxy_16
TYPE_Lab_8
TYPE_Lab_16
TYPE_CMYKcm_8 HiFi separations
TYPE_CMYKcm_8_PLANAR
TYPE_CMYKcm_16
TYPE_CMYKcm_16_PLANAR
TYPE_CMYKcm_16_SE
TYPE_CMYK7_8
TYPE_CMYK7_16
TYPE_CMYK7_16_SE
TYPE_KYMC7_8
TYPE_KYMC7_16
TYPE_KYMC7_16_SE
TYPE_CMYK8_8
TYPE_CMYK8_16
TYPE_CMYK8_16_SE
.. etc...
For example, if you are transforming a windows .bmp to a bitmap for
display, you will use TYPE_BGR_8 for both, input and output
buffers, windows does store images as B,G,R and not as R,G,B.
Other example, you need to convert from a CMYK separation to
RGB in order to display; then you would use TYPE_CMYK_8 on
input and TYPE_BGR_8 on output. If you need to do the
separation from a TIFF, TYPE_RGB_8 on input and TYPE_CMYK_8 on
output. Please note TYPE_RGB_8 and TYPE_BGR_8 are *not* same.
The format specifiers are useful above color management. This will
provide a way to handle a lot of formats, converting them in a single,
well-known one. For example, if you need to deal with several pixel
layouts coming from a file (TIFF for example), you can use a fixed
output format, say TYPE_BGR_8 and then, vary the input format
on depending on the file parameters. lcms also provides a flag for
inhibit color management if you want speed and don't care about
profiles. see cmsFLAGS_NULLTRANSFORM for more info.
c) Create the transform
When creating transform, you are giving to lcms all information it
needs about how to translate your pixels. The syntax for simpler
transforms is:
cmsHTRANSFORM hTransform;
hTransform = cmsCreateTransform(hInputProfile,
TYPE_BGR_8,
hOutputProfile,
TYPE_BGR_8,
INTENT_PERCEPTUAL, 0);
You give the profile handles, the format of your buffers, the rendering
intent and a combination of flags controlling the transform behaviour.
It's out of scope of this document to define the exact meaning
of rendering intents. I will try to make a quick explanation
here, but often the meaning of intents depends on the profile
manufacturer. See appendix A for more information.
INTENT_PERCEPTUAL:
Hue hopefully maintained (but not required),
lightness and saturation sacrificed to maintain
the perceived color. White point changed to
result in neutral grays. Intended for images.
In lcms: Default intent of profiles is used
INTENT_RELATIVE_COLORIMETRIC:
Within and outside gamut; same as Absolute
Colorimetric. White point changed to result in
neutral grays.
In lcms: If adequate table is present in profile,
then, it is used. Else reverts to perceptual
intent.
INTENT_SATURATION:
Hue and saturation maintained with lightness
sacrificed to maintain saturation. White point
changed to result in neutral grays. Intended for
business graphics (make it colorful charts,
graphs, overheads, ...)
In lcms: If adequate table is present in profile,
then, it is used. Else reverts to perceptual
intent.
INTENT_ABSOLUTE_COLORIMETRIC:
Within the destination device gamut; hue,
lightness and saturation are maintained. Outside
the gamut; hue and lightness are maintained,
saturation is sacrificed. White point for source
and destination; unchanged. Intended for spot
colors (Pantone, TruMatch, logo colors, ...)
In lcms: relative colorimetric intent is used
with undoing of chromatic adaptation.
Not all profiles does support all intents, there is a function
for inquiring which intents are really supported, but if you
specify a intent that the profile doesn't handle, lcms will
select default intent instead. Usually perceptual one. This
will force to "look nice", no matter the intent is not the one
really desired.
lcms tries to "smelt" input and output profiles in a single
matrix-shaper or in a big 3D CLUT of 33 points. This will
improve greatly the performance of the transform, but may
induce a small delay of 1-2 seconds on some 486-based machines.
If you are willing to transform just a palette or a few
colors, you don't need this precalculations. Then, the flag
cmsFLAGS_NOTPRECALC in cmsCreateTransform() can be used to
inhibit the 3D CLUT creation.
See the API reference for a more detailed discussion of the flags.
NOTES:
Some old display profiles, only archives absolute colorimetric
intents. For these profiles, default intents are absolute
colorimetric ones. This is really a rare case.
d) Next, you can translate your bitmap, calling repeatedly the processing
function:
cmsDoTransform(hTransform, YourInputBuffer,
YourOutputBuffer,
YourBuffersSize);
This function is intended to be quite fast. You can use this
function for translating a scan line, a tile, a strip, or whole
image at time.
NOTES:
Windows, stores the bitmaps in a particular way... for speed
purposes, does align the scan lines to double word boundaries,
a bitmap has in windows always a size multiple of 4. This is
OK, since no matter if you waste a couple of bytes, but if you
call cmsDoTransform() and passes it WHOLE image, lcms doesn't
know nothing about this extra padding bytes. It assumes that
you are passing a block of BGR triplets with no alignment at
all. This result in a strange looking "lines" in obtained
bitmap.
The solution most evident is to convert scan line by scan line
instead of whole image. This is as easy as to add a for()
loop, and the time penalty is so low that is impossible to
detect.
It is safe to use same block for input and output, but only if
the input and output are coded in same format. For example,
you can safely use only one buffer for RGB to RGB but you
cannot use same buffer for RGB as input and CMYK as output.
e) Last, free all stuff.
This can be done by calling
cmsDeleteTransform(hTransform);
cmsCloseProfile(hInputProfile);
cmsCloseProfile(hOutputProfile);
And you are done!
Note that cmsDeleteTransform() does NOT automatically free
associated profiles. This works in such way to let
programmers to use a open profile in more than one transform.
3. Embedded profiles
============================================================================
Some image file formats, like TIFF, JPEG or PNG, does include
the ability of embed profiles. This means that the input
profile for the bitmap is stored inside the image file. lcms
provides a specialised profile-opening function for deal with
such profiles.
cmsHPROFILE cmsOpenProfileFromMem(LPVOID MemPtr, DWORD dwSize);
This function works like cmsOpenProfileFromFile(), but assuming
that you are given full profile in a memory block rather than a
filename. Here is not any "r", since these profiles are always
read-only. A successful call will return a handle to an opened
profile that behaves just like any other file-based.
Memory based profiles does not waste more resources than memory,
so you can have tons of profiles opened sumultaneously using this
function.
NOTES:
Once opened, you can safely FREE the memory block. lcms keeps a
temporary copy.
You can retrieve information of this profile, but generally
these are minimal shaper-matrix profiles with little if none
handy info present.
Be also warned that some software does embed WRONG profiles,
i.e., profiles marked as using different colorspace that
one the profile really manages. lcms is NOT likely to understand
these profiles since they will be wrong at all.
4. Device-link profiles
============================================================================
Device-link profiles are "smelted" profiles that represents
a whole transform rather than single-device profiles. In theory,
device-link profiles may have greater precision that single ones
and are faster to load. If you plan to use device-link profiles,
be warned there are drawbacks about its inter-operability and the
gain of speed is almost null.
Perhaps their only advantage is when restoration from CMYK
with great precision is required, since CMYK to pcs CLUTs can
become very, very big.
For creating a device-link transform, you must open the device link
profile as usual, using cmsOpenProfileFromFile(). Then, create
the transform with the device link profile as input and the output
profile parameter equal to NULL:
hDeviceLink = cmsOpenProfileFromFile("MYDEVLINK.ICM", "r");
hTransform = cmsCreateTransform(hDeviceLink, TYPE_RGB_8,
NULL, TYPE_BGR_8,
INTENT_PERCEPTUAL,
0);
That's all. lcms will understand and transparently handle the
device-link profile.
There is also a function for dumping a transform into a devicelink
profile
cmsHPROFILE cmsTransform2DeviceLink(cmsHTRANSFORM hTransform,
DWORD dwFlags);
This profile can be used in any other transform or saved to disk/memory.
5. - Built-in profiles
============================================================================
In order to make things ease, there are several built-in profiles that
programmer can use without the need of any disk file. These does include:
- sRGB profile
- L*a*b profiles
- XYZ profile
- Gray profiles
- RGB matrix-shaper.
- Linearization device link
- Ink-Limiting
- Bright/Contrast/Hue/Saturation/White point adjust devicelink.
sRGB, Lab and XYZ are very usefull for tricking & trapping. For example,
creating a transform from sRGB to Lab could be done without any disk file.
Something like:
hsRGB = cmsCreate_sRGBProfile();
hLab = cmsCreateLabProfile()
xform = cmsCreateTransform(hSRGB, TYPE_RGB_DBL, hLab, TYPE_Lab_DBL,
INTENT_PERCEPTUAL, cmsFLAGS_NOTPRECALC);
Then you can convert directly form double sRGB values (in 0..1.0 range) to
Lab by using:
double RGB[3];
cmsCIELab Lab;
RGB[0] = 0.1; RGB[1] = 0.2 RGB[2] = 0.3;
cmsDoTransform(xform, RGB, &Lab, 1);
.. get result on "Lab" variable ..
Even more, you can create your own RGB or Gray profiles "on the fly" by
using cmsCreateRGBProfile() and cmsCreateGrayProfile(). See next section
for a explanation on how to do.
6. - On-the-fly profiles.
============================================================================
There are several situations where it will be useful to build
a minimal profile using adjusts only available at run time.
Surely you have seen the classical pattern-gray trick for adjusting
gamma: the end user moves a scroll bar and when pattern seems to
match background gray, then gamma is adjusted.
Another trick is to use a black background with some gray rectangles.
The user chooses the most neutral grey, giving the white point or the
temperature in °K.
All these visual gadgets are not part of lcms, you must implement
them by yourself if you like. But lcms will help you with a function for
generating a virtual profile based on the results of these tests.
Another usage would be to build colorimetric descriptors for
file images that does not include any embedded profile, but
does include fields for identifying original colorspace.
One example is TIFF files. The TIFF 6.0 spec talks about
"RGB Image Colorimetry" (See section 20) a "colorimetric" TIFF
image has all needed parameters (WhitePointTag=318,
PrimaryChromacitiesTag=318, TransferFunction=301,TransferRange=342)
Obtain a emulated profile from such files is easy since the contents
of these tags does match the cmsCreateRGBProfile() parameters.
Also PNG can come with information for build a virtual profile,
See the gAMA and cHRM chunks.
This is the main function for creating virtual RGB profiles:
cmsHPROFILE cmsCreateRGBProfile(LPcmsCIExyY WhitePoint,
LPcmsCIExyYTRIPLE Primaries,
LPGAMMATABLE TransferFunction[3]);
It takes as arguments the white point, the primaries and 3
gamma curves. The profile emulated is always operating in RGB
space. Once created, a handle to a profile is returned. This
opened profile behaves like any other file or memory based
profile.
Virtual RGB profiles are implemented as matrix-shaper, so they
cannot compete against CLUT based ones, but generally are good enough
to provide a reasonable alternative to generic profiles.
For simplify the parameters construction, there are additional
functions:
LCMSBOOL cmsWhitePointFromTemp(int TempK, LPcmsCIExyY WhitePoint);
This function computes the xyY chromacity of white point using
the temperature. Screen manufacturers often includes a white
point hard switch in monitors, but they refer as "Temperature"
instead of chromacity. Most extended temperatures are 5000K,
6500K and 9300K
It returns TRUE if a valid white point can be computed, or FALSE
if the temperature were non valid. You must give a pointer to a
cmsCIExyY struct for holding resulting white point.
For primaries, currently I don't know any trick or proof for
identifying primaries, so here are a few chromacities of most
extended. Y is always 1.0
RED GREEN BLUE
x y x y x y
---- ---- ---- ---- ---- ----
NTSC 0.67, 0.33, 0.21, 0.71, 0.14, 0.08
EBU(PAL/SECAM) 0.64, 0.33, 0.29, 0.60, 0.15, 0.06
SMPTE 0.630, 0.340, 0.310, 0.595, 0.155, 0.070
HDTV 0.670, 0.330, 0.210, 0.710, 0.150, 0.060
CIE 0.7355,0.2645,0.2658,0.7243,0.1669,0.0085
These are TRUE primaries, not colorants. lcms does include a
white-point balancing and a chromatic adaptation using a method
called Bradford Transform for D50 adaptation.
NOTE: Additional information about Bradford transform math can be found
on the sRGB site:
http://www.srgb.com
Another kind of profiles that can be built on runtime are GrayScale
profiles. This can be accomplished by the function:
cmsHPROFILE cmsCreateGrayProfile(LPcmsCIExyY WhitePoint,
LPGAMMATABLE TransferFunction);
This one is somehow easier, since it only takes the gray curve (transfer
function) and the media white point. Of course gray scale does not need
primaries!
7. - Gamma tables
============================================================================
The gamma tables or transfer functions are stored in a simple way,
let's examine the GAMMATABLE typedef:
typedef struct {
int nEntries;
WORD GammaTable[1];
} GAMMATABLE, FAR* LPGAMMATABLE;
That is, first it comes a 32 integer for entry count, followed of
a variable number of words describing the table. The easiest way to
generate a gamma table is to use the function
LPGAMMATABLE cmsBuildGamma(int nEntries, double Gamma);
You must specify the number of entries your table will consist of,
and the float value for gamma. The generated table has linear and non-linear
steps, the linear ramp near 0 is for minimising noise.
If you want to fill yourself the values, you can allocate space for your
table by using
LPGAMMATABLE cmsAllocGamma(int nEntries);
This function only creates memory for the table. The entries does
not contain any useful value (garbage) since it is expected you will fill
this table after created.
You can find the inverse of a tabulated curve by using:
LPGAMMATABLE cmsReverseGamma(int nResultSamples, LPGAMMATABLE InGamma);
This function reverses the gamma table if it can be done. lcms does not
detect whatever a non-monotonic function is given, so wrong input can
result in ugly results: not to be a problem since "normal" gamma curves
are not collapsing inputs at same output value. The new curve will be
re-sampled to nResultSamples entries.
You can also smooth the curve by using:
LCMSBOOL cmsSmoothGamma(LPGAMMATABLE Tab, double lambda);
"Smooth" curves does work better and are more pleasant to eyes.
You can join two gamma curves with:
LPGAMMATABLE cmsJoinGamma(LPGAMMATABLE InGamma,
LPGAMMATABLE OutGamma);
This will let you to "refine" the generic gamma for monitors (2.1 or 2.2
are usual values) to match viewing conditions of more or less background
light. Note that this function uses TABULATED functions, so very exotic
curves can be obtained by combining transfer functions with reversed
gamma curves. Normally there is no need of worry about such gamma
manipulations, but the functionality is here if you wish to use.
There is a Extended join function that let specify the point it will have:
LPGAMMATABLE cmsJoinGammaEx(LPGAMMATABLE InGamma,
LPGAMMATABLE OutGamma,
int nPoints);
You must free all gamma tables you allocate (or create via
cmsReverseGamma() or cmsJoinGamma()) by using:
void cmsFreeGamma(LPGAMMATABLE Gamma);
Another functions for dealing with gamma curves are:
LPGAMMATABLE cmsDupGamma(LPGAMMATABLE Src);
Duplicates a gamma table, allocatine a new memory block
double cmsEstimateGamma(LPGAMMATABLE t);
This one does a coarse estimation of the apparent gamma of a given curve.
It is intended mainly for informational purposes.
double cmsEstimateGammaEx(LPGAMMATABLE t, double Thereshold);
This is similar to anterior, but it let specify the Standard deviation
below that the curve is considered pure exponential. cmsEstimateGamma()
does use a default value of 0.7
LPGAMMATABLE cmsBuildParametricGamma(int nEntries,
int Type,
double Params[]);
This one is intended to build parametric curves, as stated in ICC
4.0 spec. "Type" refers to ICC type plus one. If type is negative, then
the curve is analitically reversed. This function is still experimental.
8. Proofing.
============================================================================
An additional ability of lcms is to create "proofing" transforms.
A proofing transform does emulate the colors that will appear if
a image is rendered on a specific device. That is, for example,
with a proofing transform I can see how will look a photo of my
little daughter if rendered on my HP. Since most printer profiles
does include some sort of gamut-remapping, it is likely colors
will not look *exactly* as the original. Using a proofing
transform, it can be done by using the appropriate function.
Note that this is an important feature for final users, it is worth
of all color-management stuff if the final media is not cheap.
The creation of a proofing transform involves three profiles, the input
and output ones as cmsCreateTransform() plus another, representing the
emulated profile.
cmsHTRANSFORM cmsCreateProofingTransform(cmsHPROFILE Input,
DWORD InputFormat,
cmsHPROFILE Output,
DWORD OutputFormat,
cmsHPROFILE Proofing,
int Intent,
int ProofingIntent,
DWORD dwFlags);
Also, there is another parameter for specifying the intent for
the proof. The Intent here, represents the intent the user will select
when printing, and the proofing intent represent the intent system is
using for showing the proofed color. Since some printers can archive
colors that displays cannot render (darker ones) some gamut-remapping must
be done to accommodate such colors. Normally INTENT_ABSOLUTE_COLORIMETRIC
is to be used: is is likely the user wants to see the exact colors on screen,
cutting off these unrepresentable colors. INTENT_RELATIVE_COLORIMETRIC could
serve as well.
Proofing transforms can also be used to show the colors that are out of the
printer gamut. You can activate this feature by using the
cmsFLAGS_GAMUTCHECK flag in dwFlags field.
Then, the function:
void cmsSetAlarmCodes(int r, int g, int b);
Can be used to define the marker. rgb are expected to be integers
in range 0..255
NOTES: For activating the preview or gamut check features, you MUST
include the corresponding flags
cmsFLAGS_SOFTPROOFING
cmsFLAGS_GAMUTCHECK
This is done in such way because the user usually wants to compare
with/without softproofing. Then, you can share same code. If any of the flags
is present, the transform does the proofing stuff. If not, the transform
ignores the proofing profile/intent and behaves like a normal input-output
transform. In practical usage, you need only to associate the check boxes of
"softproofing" and "gamut check" with these flags.
9.1 Black point compensation
============================================================================
The black point compensation feature does work in conjunction
with relative colorimetric intent. Perceptual intent should make no
difference, although it affects some profiles.
The mechanics are simple. BPC does scale full image across gray
axis in order to accommodate the darkest tone origin media can
render to darkest tone destination media can render. As a such,
BPC is primarily targeting CMYK.
Let's take an example. You have a separation (CMYK image) for,
say, SWOP. Then you want to translate this separation to another
media on another printer. The destination media/printer can deliver
a black darker that original SWOP. Now you have several options.
a) use perceptual intent, and let profile do the gamut remapping for
you. Some users complains about the profiles moving too much the
colors. This is the "normal" ICC way.
b) use relative colorimetric.This will not move any color, but
depending on different media you would end with "flat" images,
taking only a fraction of available grays or a "collapsed" images,
with loss of detail in dark shadows.
c) Use relative colorimetric + black point compensation. This is
the discussion theme. Colors are unmoved *except* gray balance
that is scaled in order to accommodate to the dynamic range of new
media. Is not a smart CMM, but a fist step letting the CMM to do
some remapping.
The algorithm used for black point compensation is a XYZ linear scaling
in order to match endpoints.
You can enable the BPC feature by using this in the dwFlags field, it works
on softproofs too.
cmsFLAGS_BLACKPOINTCOMPENSATION
9.2 - Black preserving transforms
===========================================================================
Black preservation deals with CMYK -> CMYK transforms, and is intended
to preserve, as much as possible, the black (K) channel
whilst matching color by using CMY inks. There is a tradeoff between
accuracy and black preservation, so you lost some accuracy
in order to preserve the original separation. Not to be a big problem
in most cases, benefits of keeping K channel are huge!
For sure you have seen prints of gray images with a huge
color cast towards magenta or green. That's very unpleasant.
Mainly, this happens because metamerism is not taken into
account when doing the profile. Black ink chromaticity changes on
different illuminants. For example, a profile is done measuring black
ink under D50, then, under D50 this black ink have tendency to
magenta. Ok, the profile captures such chroma and, when
reproducing colorimetrically, replaces the destination black with
CMY reproducing this magenta. Now, If you take the original K ink
and examine it under sunlight, it is not magenta anymore! But the
reproduction using CMY keeps going magenta. Result: a nasty color
cast .
And this is just one of the reasons why keeping black ink is so important.
Maybe there is a slight discontinuity, as big as the chromaticity of
blacks differ, but it is so small that the smoothing induced by the
CLUT is enough to compensate. And this is almost nothing when
compared with the huge cast on grays a CMYK->Lab->CMYK
may create.
You can enable the black preservation feature by using this flag:
cmsFLAGS_PRESERVEBLACK
10. Error handling
============================================================================
lcms primary goal is to be quite simple, so error handling is managed
in a simple way. If you are using lcms as a DLL, you can tell lcms what is
supposed to happen when an error is detected. For doing that, you can use
this function.
void cmsErrorAction(int nAction);
'nAction' can be one of the following values:
LCMS_ERROR_ABORT 0
LCMS_ERROR_SHOW 1
LCMS_ERROR_IGNORE 2
Default is LCMS_ERROR_ABORT. That is, if an error is detected, lcms
will show a MessageBox with a small explanation about the error and
then WILL ABORT WHOLE APPLICATION. This behaviour is desirable when
debugging, but not in final releases. For inhibit such aborting,
you can use LCMS_ERROR_SHOW. This setting will show the error text, but
doesn't finish the application. Some functions like cmsOpenProfileFromFile()
or cmsCreateTransform() will return NULL instead of a valid handle as
error-marker. Others will return FALSE.
The last setting is LCMS_ERROR_IGNORE, that is, no message is displayed
and only a NULL or FALSE is returned if operation fails.
Note that if you use LCMS_ERROR_SHOW or LCMS_ERROR_IGNORE, your code
must check the return code. This is not necessary if you are using
LCMS_ERROR_ABORT, since the application will be terminated as soon as
the error is detected.
If you doesn't like this scheme, you can provide your own error handling,
function by using:
void cmsSetErrorHandler(cmsErrorHandlerFunction Fn);
You need to write your own error handling function, in the form:
int MyErrorHandlerFunction(int ErrorCode, const char *ErrorText)
{
... do whatsever you want with error codes ..
}
And then register the error handler by using:
cmsSetErrorHandler(MyErrorHandlerFunction);
ErrorCode can be one of the following values:
LCMS_ERRC_WARNING 0x1000
LCMS_ERRC_RECOVERABLE 0x2000
LCMS_ERRC_ABORTED 0x3000
ErrorText is a text holding an english description of error.
You should return 1 if you are handling the error. Returning 0,
throws the error back to the default error handler.
WARNING: lcms is *not* supposed to recover from all errors. If you are
using your own error handling, please note that you have to abort
all process if you recive LCMS_ERRC_ABORTED in ErrorCode parameter.
Incoming versions will handle this issue more properly.
11. Getting information from profiles.
============================================================================
There are some functions for retrieve information on opened profiles.
These are:
LCMSBOOL cmsIsTag(cmsHPROFILE hProfile, icTagSignature sig);
This one does check if a particular tag is present. Remaining does take
useful information about general parameters.
LCMSBOOL cmsTakeMediaWhitePoint(LPcmsCIEXYZ Dest, cmsHPROFILE hProfile);
LCMSBOOL cmsTakeMediaBlackPoint(LPcmsCIEXYZ Dest, cmsHPROFILE hProfile);
LCMSBOOL cmsTakeIluminant(LPcmsCIEXYZ Dest, cmsHPROFILE hProfile);
LCMSBOOL cmsTakeColorants(LPcmsCIEXYZTRIPLE Dest, cmsHPROFILE hProfile);
const char* cmsTakeProductName(cmsHPROFILE hProfile);
const char* cmsTakeProductDesc(cmsHPROFILE hProfile);
int cmsTakeRenderingIntent(cmsHPROFILE hProfile);
icColorSpaceSignature cmsGetPCS(cmsHPROFILE hProfile);
icColorSpaceSignature cmsGetColorSpace(cmsHPROFILE hProfile);
icProfileClassSignature cmsGetDeviceClass(cmsHPROFILE hProfile);
These functions are given mainly for building user interfaces,
you don't need to use them if you just want a plain translation.
Other usage would be to identify "families" of profiles.
The functions returning strings are using an static buffer that is
overwritten in each call, others does accept a pointer to an specific
struct that is filled if function is successful.
#define LCMS_USED_AS_INPUT 0
#define LCMS_USED_AS_OUTPUT 1
#define LCMS_USED_AS_PROOF 2
LCMSBOOL cmsIsIntentSupported(cmsHPROFILE hProfile, int Intent, int UsedDirection);
This one helps on inquiring if a determinate intent is
supported by an opened profile. You must give a handle to
profile, the intent and a third parameter specifying how the
profile would be used. The function does return TRUE if intent
is supported or FALSE if not. If the intent is not supported,
lcms will use default intent (usually perceptual).
ADVANCED TOPICS
===============
The following section does describe additional functions for advanced
use of lcms. Several of these are expanding the capabilities of lcms
above a 'pure' CMM. In this way, they do not belong to the CMM layer,
but as a low level primitives for the CMS formed by the CMM and the
profilers. There is no need of these function on "normal" usage.
12. Creating and writting new profiles.
============================================================================
Since version 1.09, lcms has the capability of writting profiles. The
interface is very simple, despite its use does imply certain knowleage
of profile internals.
These are the functions needed to create a new profile:
void cmsSetDeviceClass(cmsHPROFILE hProfile, icProfileClassSignature sig);
void cmsSetColorSpace(cmsHPROFILE hProfile, icColorSpaceSignature sig);
void cmsSetPCS(cmsHPROFILE hProfile, icColorSpaceSignature pcs);
void cmsSetRenderingIntent(cmsHPROFILE hProfile, int RenderingIntent);
LCMSBOOL cmsAddTag(cmsHPROFILE hProfile, icTagSignature sig, void* data);
And the generic file management ones:
cmsOpenProfileFromFile() and cmsCloseProfile()
Device class, colorspace and PCS type does affect to header and overall
profile. cmsSetRenderingIntent() sets the (informative) intent field on
header. Rest of information is included using cmsAddTag().
Of course you must know which tags to include and the meaning of each tag.
LittleCms does nothing to check the validity of newly created profiles.
These functions are only intended to be a low level interface to profile
creation.
Creating a new profile
-----------------------
When you open a profile with 'w' as access mode, you got a simpler
Lab identity. That is, a profile marked as Lab colorspace that passes
input untouched to output. You need to fully qualify your profile by
setting its colorspace, device class, PCS and then add the required
tags. cmsAddTag() does understand following tags:
Signature Expected data type
========================== ==================
icSigCharTargetTag const char*
icSigCopyrightTag const char*
icSigProfileDescriptionTag const char*
icSigDeviceMfgDescTag const char*
icSigDeviceModelDescTag const char*
icSigRedColorantTag LPcmsCIEXYZ
icSigGreenColorantTag LPcmsCIEXYZ
icSigBlueColorantTag LPcmsCIEXYZ
icSigMediaWhitePointTag LPcmsCIEXYZ
icSigMediaBlackPointTag LPcmsCIEXYZ
icSigRedTRCTag LPGAMMATABLE
icSigGreenTRCTag LPGAMMATABLE
icSigBlueTRCTag LPGAMMATABLE
icSigGrayTRCTag LPGAMMATABLE
icSigAToB0Tag LPLUT
icSigAToB1Tag LPLUT
icSigAToB2Tag LPLUT
icSigBToA0Tag LPLUT
icSigBToA1Tag LPLUT
icSigBToA2Tag LPLUT
icSigGamutTag LPLUT
icSigPreview0Tag LPLUT
icSigPreview1Tag LPLUT
icSigPreview2Tag LPLUT
icSigChromaticityTag LPcmsCIExyYTRIPLE
icSigNamedColor2Tag LPcmsNAMEDCOLORLIST
icSigColorantTableTag LPcmsNAMEDCOLORLIST
icSigColorantTableOutTag LPcmsNAMEDCOLORLIST
icSigCalibrationDateTimeTag const struct tm*
More tags are expected to be added in future revisions.
Another way to create profiles is by using _cmsSaveProfile() or
_cmsSaveProfileToMem(). See the API reference for details.
13. LUT handling
============================================================================
LUT stands for L)ook U)p T)ables. This is a generalizad way to handle
workflows consisting of a quite complex stages. Here is the pipeline
scheme, Don't panic!
[Mat] -> [L1] -> [Mat3] -> [Ofs3] -> [L3] ->
[CLUT] -> [L4] -> [Mat4] -> [Ofs4] -> [L2]
Mat{n} are matrices of 3x3
Ofs{n} are offsets
CLUT is a multidimensional LUT table
Some or all of these stages may be missing. LUTS can handle up to
8 channels on input and 16 channels of output.
The programmer can allocate a LUT by calling:
NewLUT = cmsAllocLUT();
This allocates an empty LUT. Input is passed transparently to output
by default. The programmer can optionally add pre/post linearization
tables by using:
cmsAllocLinearTable(LPLUT NewLUT, LPGAMMATABLE Tables[], int nTable);
Being Table:
1 - Prelinearization 1D table L1
2 - Postlinearization 1D table L2
3 - linearization 1D table L3
4 - linearization 1D table L4
The 3x3 matrices can be set by:
LPLUT cmsSetMatrixLUT4(LPLUT Lut, LPMAT3 M, LPVEC3 off, DWORD dwFlags);
Flags define the matrix to set:
LUT_HASMATRIX: Mat
LUT_HASMATRIX3: Mat3
LUT_HASMATRIX4: Mat4
The CLUT is a multidimensional table where most of the magic of
colormanagement is done. It holds as many (hyper)cubes as ouput
channels and the dimension of these hypercubes is the number of
input channels.
To fill the CLUT with sampled values, the programmer can call:
LCMSBOOL cmsSample3DGrid(LPLUT Lut,
_cmsSAMPLER Sampler,
LPVOID Cargo,
DWORD dwFlags)
This function builds the CLUT table by calling repeatly a
callback function:
typedef int (* _cmsSAMPLER)(register WORD In[],
register WORD Out[],
register LPVOID Cargo);
The programmer has to write his callback function. This function
should calculate Out[] values given a In[] set. For example, if we
want a LUT to invert (negate) channels, a sampler could be:
int InvertSampler(register WORD In[],
register WORD Out[],
register LPVOID Cargo)
{
for (i=0; i < 3; i++)
Out[i] = ~ In[i];
return TRUE;
}
cmsSample3DGrid does call this function to build the CLUT.
Pre/post linearization tables may be taken into account across
flags parameter
Flags Meaning
================ =======================================
LUT_HASTL1 Do reverse linear interpolation on
prelinearization table before calling
the callback.
LUT_HASTL2 Do reverse linear interpolation on
postlinearization table after calling
the callback.
LUT_INSPECT Does NOT write any data to the 3D CLUT,
instead, retrieve the coordinates for inspection
only. If using such flag, Out[] will hold
the CLUT contents.
In[] -> The CLUT indexes
Out[] -> The CLUT contents
HASTL1 and HASTL2 Flags are intended to be used as an aid for
building non-uniformly spaced CLUTs. Using these flags results in
"undoing" any linearization that tables could apply. In such way,
the callback is expected to be called with In[] always the original
colorspace, and must return Out[] values always in original
(non-postlinearized) space as well. The linearization cooking is done
automatically.
The callback must return TRUE if all is ok, or zero
to indicate error. If error condition is raised, whole CLUT
construction is aborted.
Once builded, programmer can evaluate the LUT by using:
void cmsEvalLUT(LPLUT Lut, WORD In[], WORD Out[]);
That does interpolate values according on pipeline tables.
Finally, a LUT can be freed by
void cmsFreeLUT(LPLUT Lut);
Or retrieved from a profile using:
LPLUT cmsReadICCLut(cmsHPROFILE hProfile, icTagSignature sig);
To include a LUT in a profile, use cmsAddTag() with tag signature and
a pointer to LUT structure as parameters. See cmsAddTag() in the API
reference for more info.
14. Helper functions
============================================================================
Here are some functions that could be useful. They are not needed
in "normal" usage.
Colorimetric space conversions:
void cmsXYZ2xyY(LPcmsCIExyY Dest, const LPcmsCIEXYZ Source);
void cmsxyY2XYZ(LPcmsCIEXYZ Dest, const LPcmsCIExyY Source);
void cmsXYZ2Lab(LPcmsCIEXYZ WhitePoint, LPcmsCIELab Lab, const LPcmsCIEXYZ xyz);
void cmsLab2XYZ(LPcmsCIEXYZ WhitePoint, LPcmsCIEXYZ xyz, const LPcmsCIELab Lab);
void cmsLab2LCh(LPcmsCIELCh LCh, const LPcmsCIELab Lab);
void cmsLCh2Lab(LPcmsCIELab Lab, const LPcmsCIELCh LCh);
Notation conversion: (converts from PT_* colorspaces to ICC notation)
icColorSpaceSignature _cmsICCcolorSpace(int OurNotation);
Channels of a given colorspace:
int _cmsChannelsOf(icColorSpaceSignature ColorSpace);
Chromatic Adaptation:
LCMSBOOL cmsAdaptToIlluminant(LPcmsCIEXYZ Result, LPcmsCIEXYZ SourceWhitePt,
LPcmsCIEXYZ Illuminant, LPcmsCIEXYZ Value);
Build a balanced transfer matrix with chromatic adaptation, this
is equivalent to "cooking" required to conform a colorant matrix.
LCMSBOOL cmsBuildRGB2XYZtransferMatrix(LPMAT3 r,
LPcmsCIExyY WhitePoint,
LPcmsCIExyYTRIPLE Primaries);
15. Color difference functions
============================================================================
These functions does compute the difference between two Lab colors,
using several difference spaces
double cmsDeltaE(LPcmsCIELab Lab1, LPcmsCIELab Lab2);
double cmsCIE94DeltaE(LPcmsCIELab Lab1, LPcmsCIELab Lab2);
double cmsBFDdeltaE(LPcmsCIELab Lab1, LPcmsCIELab Lab2);
double cmsCMCdeltaE(LPcmsCIELab Lab1, LPcmsCIELab Lab2);
double cmsCIE2000DeltaE(LPcmsCIELab Lab1, LPcmsCIELab Lab2, double Kl, double Kc, double Kh);
16. PostScript generation
============================================================================
3 functions carry the task of obtaining CRD and CSA.
DWORD cmsGetPostScriptCSA(cmsHPROFILE hProfile, int Intent, LPVOID Buffer, DWORD dwBufferLen);
DWORD cmsGetPostScriptCRD(cmsHPROFILE hProfile, int Intent, LPVOID Buffer, DWORD dwBufferLen);
DWORD cmsGetPostScriptCRDEx(cmsHPROFILE hProfile, int Intent, DWORD dwFlags, LPVOID Buffer, DWORD dwBufferLen);
cmsGetPostScriptCRDEx allows black point compensation using
cmsFLAGS_WHITEBLACKCOMPENSATION in flags field.
PostScrip colorflow is often done in a different way. Insted of creating a
transform, it is sometimes desirable to delegate the color management to
PostScript interpreter. These functions does translate input and output
profiles into Color Space Arrays (CSA) and Color Rendering Dictionaries (CRD)
· CRD are equivalent to output (printer) profiles. Can be
loaded into printer at startup and can be stored as resources.
· CSA are equivalent to input and workspace profiles, and are
intended to be included in the document definition.
These functions does generate the PostScript equivalents. Since the lenght of
the resultant PostScript code is unknown in advance, you can call the
functions with len=0 and Buffer=NULL to get the lenght. After that, you need
to allocate enough memory to contain the whole block
Example:
Size = cmsGetPostScriptCSA(hProfile, INTENT_PERCEPTUAL, NULL, 0);
If (Size == 0) error()
Block = malloc(Size);
cmsGetPostScriptCSA(hProfile, INTENT_PERCEPTUAL, Block, Size);
Devicelink profiles are supported, as long as input colorspace matches
Lab/XYZ for CSA or output colorspace matches Lab/XYZ for CRD. This can
be used in conjuntion with cmsCreateMultiprofileTransform(),
and cmsTransform2DeviceLink() to embed complex color flow into PostScript.
WARNING: Preccision of PostScript is limited to 8 bits per sample. If you
can choose between normal transforms and CSA/CRD, normal transforms will
give more accurancy. However, there are situations where there is no
chance.
17. CIECAM02
============================================================================
The model input data are the adapting field luminance in cd/m2
(normally taken to be 20% of the luminance of white in the adapting field),
La , the relative tristimulus values of the stimulus, XYZ, the relative
tristimulus values of white in the same viewing conditions, "whitePoint",
and the relative luminance of the background, Yb . Relative tristimulus
values should be expressed on a scale from Y = 0 for a perfect black
to Y = 100 for a perfect reflecting diffuser.
All CIE tristimulus values are obtained using the CIE 1931 Standard
Colorimetric Observer (2°).
typedef struct {
cmsCIEXYZ whitePoint; // The media white in XYZ
double Yb;
double La;
int surround;
double D_value;
} cmsViewingConditions, FAR* LPcmsViewingConditions;
Surround can be one of these
#define AVG_SURROUND 1
#define DIM_SURROUND 2
#define DARK_SURROUND 3
D_value (adaptation degree) is any value between 0 and 1
The functions for dealing with CAM02 appearance model are:
LCMSHANDLE cmsCIECAM02Init(LPcmsViewingConditions pVC);
void cmsCIECAM02Done(LCMSHANDLE hModel);
void cmsCIECAM02Forward(LCMSHANDLE hModel, LPcmsCIEXYZ pIn, LPcmsJCh pOut);
void cmsCIECAM02Reverse(LCMSHANDLE hModel, LPcmsJCh pIn, LPcmsCIEXYZ pOut);
For example, to convert XYZ values from a given viewing condition to another:
a) Create descriptions of both viewing conditions by using cmsCIECAM02Init
b) Convert XYZ to JCh using cmsCIECAM02Forward for viewing condition 1
c) Convert JCh back to XYZ using cmsCIECAM02Reverse for viewing condition 2
d) when done, free both descriptions
cmsViewingConditions vc1, vc2;
cmsJCh Out;
cmsCIEXYZ In;
HANDLE h1, h2;
vc.whitePoint.X = 98.88;
vc.whitePoint.Y = 90.00;
vc.whitePoint.Z = 32.03;
vc.Yb = 18;
vc.La = 200;
vc.surround = AVG_SURROUND;
vc.D_value = 1.0;
h1 = cmsCIECAM02Init(&vc);
vc2.whitePoint.X = 98.88;
vc2.whitePoint.Y = 100.00;
vc2.whitePoint.Z = 32.03;
vc2.Yb = 20;
vc2.La = 20;
vc2.surround = AVG_SURROUND;
vc2.D_value = 1.0;
h2 = cmsCIECAM02Init(&vc);
In.X= 19.31;
In.Y= 23.93;
In.Z =10.14;
cmsCIECAM02Forward(h1, &In, &Out);
cmsCIECAM02Reverse(h2, &Out, &In);
cmsCIECAM02Done(h1);
cmsCIECAM02Done(h2);
See the CIECAM02 paper on CIE site for further details.
18. Named color profiles
============================================================================
Named color profiles are a special kind of profiles handling lists of spot
colors. The typical example is PANTONE. CMM deals with named color profiles like
all other types, except they must be in input stage and the encoding supported
is limited to a one single channel of 16-bit indexes.
Let's assume we have a Named color profile holding only 4 colors:
· CYAN
· MAGENTA
· YELLOW
· BLACK
We create a transform using:
hTransform = cmsCreateTransform(hNamedColorProfile,
TYPE_NAMED_COLOR_INDEX,
hOutputProfile,
TYPE_BGR_8,
INTENT_PERCEPTUAL, 0);
"TYPE_NAMED_COLOR_INDEX" is a special encoding for these profiles, it
represents a single channel holding the spot color index. In our case
value 0 will be "CYAN", value 1 "MAGENTA" and so one.
For converting between string and index there is an auxiliary function:
int cmsNamedColorIndex(cmsHTRANSFORM hTransform, const char* ColorName);
That will perform a look up on the spot colors database and return the color
number or -1 if the color was not found. Other additional functions for named
color transforms are:
int cmsNamedColorCount(cmsHTRANSFORM hTransform);
That returns the number of colors present on transform database.
LCMSBOOL cmsNamedColorInfo(cmsHTRANSFORM xform, int nColor,
char* Name, char* Prefix, char* Suffix);
That returns extended information about a given color. Named color profiles
can also be grouped by using multiprofile transforms. In such case, the database
will be formed by the union of all colors in all named color profiles present in
transform.
Named color profiles does hold two coordinates for each color, let's
take our PANTONE example. This profile would contain for each color
the CMYK colorants plus its PCS coordinates, usually in Lab space.
lcms can work with named color using both coordinates. Creating a
transform with two profiles, if the input one is a named color, then you
obtain the translated color using PCS.
Example, named color -> sRGB will give the color patches in sRGB
In the other hand, setting second profile to NULL, returns the device
coordinates, that is, CMYK colorants in our PANTONE sample.
Example: Named color -> NULL will give the CMYK amount for each spot color.
The transform must use TYPE_NAMED_COLOR_INDEX on input. That is, a single
channel containing the 0-based color index.
Then you have:
cmsNamedColorIndex(cmsHTRANSFORM xform, const char* Name)
for obtaining index from color name, and
cmsNamedColorInfo(), cmsNamedColorCount() to retrieve the list.
The profile is supposed to be for a unique device. Then the CMYK
values does represent the amount of inks THIS PRINTER needs to render
the spot color. The profile also has the Lab values corresponding to
the color. This really would have no sense if gamut of printer were
infinite, but since printers does have a limited gamut a PANTONE-certified
printer renders colors near gamut boundaries with some limitations.
The named color profile is somehow explaining which are these limitation
for that printer.
So, you can use a named color profile in two different ways, as output,
giving the index and getting the CMYK values or as input and getting the
Lab for that color.
A transform named color -> NULL will give the CMYK values for the spot
color on the printer the profile is describing. This would be the normal usage.
A transform Named color -> another printer will give on the output printer
the spot colors as if they were printed in the printer named color profile
is describing. This is useful for soft proofing.
As an additional feature, lcms can "group" several named color profiles
into a single database by means of cmsCreateMultiprofileTransform().
Such case works as described above, but joining all named colors as they
were in a single profile.
19. Conclusion.
============================================================================
That's almost all you must know to begin experimenting with profiles,
just a couple of words about the possibilities ICC profiles can
give to programmers:
o ColorSpace profiles are valuable tools for converting
from/to exotic file formats. I'm using lcms to read
Lab TIFF using the popular Sam Leffler's TIFFLib. Also,
the ability to restore separations are much better that
the infamous 1-CMY method.
o Abstract profiles can be used to manipulate color of
images, contrast, brightness and true-gray reductions can
be done fast and accurately. Grayscale conversions can be
done exceptionally well, and even in tweaked colorspaces
that does emulate more gray levels that the output device
can effectively render.
o lcms does all calculation on 16 bit per component basis,
the display and output profiles can take advantage of these
precision and efficiently emulate more than 8 bits per sample.
You probably will not notice this effect on screen, but it can
be seen on printed or film media.
o There is a huge quantity of profiles moving around the net,
and there is very good software for generating them, so
future compatibility seems to be assured.
I thank you for your time and consideration.
Enjoy!
Sample 1: How to convert RGB to CMYK and back
=============================================
This is easy. Just use a transform between RGB profile to CMYK profile.
#include "lcms.h"
int main(void)
{
cmsHPROFILE hInProfile, hOutProfile;
cmsHTRANSFORM hTransform;
int i;
hInProfile = cmsOpenProfileFromFile("sRGBColorSpace.ICM", "r");
hOutProfile = cmsOpenProfileFromFile("MyCmyk.ICM", "r");
hTransform = cmsCreateTransform(hInProfile,
TYPE_RGB_8,
hOutProfile,
TYPE_CMYK_8,
INTENT_PERCEPTUAL, 0);
for (i=0; i < AllScanlinesTilesOrWatseverBlocksYouUse; i++)
{
cmsDoTransform(hTransform, YourInputBuffer,
YourOutputBuffer,
YourBuffersSizeInPixels);
}
cmsDeleteTransform(hTransform);
cmsCloseProfile(hInProfile);
cmsCloseProfile(hOutProfile);
return 0;
}
And Back....? Same. Just exchange profiles and format descriptors:
int main(void)
{
cmsHPROFILE hInProfile, hOutProfile;
cmsHTRANSFORM hTransform;
int i;
hInProfile = cmsOpenProfileFromFile("MyCmyk.ICM", "r");
hOutProfile = cmsOpenProfileFromFile("sRGBColorSpace.ICM", "r");
hTransform = cmsCreateTransform(hInProfile,
TYPE_CMYK_8,
hOutProfile,
TYPE_RGB_8,
INTENT_PERCEPTUAL, 0);
for (i=0; i < AllScanlinesTilesOrWatseverBlocksYouUse; i++)
{
cmsDoTransform(hTransform, YourInputBuffer,
YourOutputBuffer,
YourBuffersSizeInPixels);
}
cmsDeleteTransform(hTransform);
cmsCloseProfile(hInProfile);
cmsCloseProfile(hOutProfile);
return 0;
}
Sample 2: How to deal with Lab/XYZ spaces
==========================================
This is more elaborated. There is a Lab identity Built-In profile involved.
// Converts Lab(D50) to sRGB:
int main(void)
{
cmsHPROFILE hInProfile, hOutProfile;
cmsHTRANSFORM hTransform;
int i;
BYTE RGB[3];
cmsCIELab Lab[..];
hInProfile = cmsCreateLabProfile(NULL);
hOutProfile = cmsOpenProfileFromFile("sRGBColorSpace.ICM", "r");
hTransform = cmsCreateTransform(hInProfile,
TYPE_Lab_DBL,
hOutProfile,
TYPE_RGB_8,
INTENT_PERCEPTUAL, 0);
for (i=0; i < AllLabValuesToConvert; i++)
{
// Fill in the Float Lab
Lab[i].L = Your L;
Lab[i].a = Your a;
Lab[i].b = Your b;
cmsDoTransform(hTransform, Lab, RGB, 1);
.. Do whatsever with the RGB values in RGB[3]
}
cmsDeleteTransform(hTransform);
cmsCloseProfile(hInProfile);
cmsCloseProfile(hOutProfile);
return 0;
}
Annex A. About intents
============================================================================
Charles Cowens gives to me a clear explanation about
accomplished intents. Since it is very useful to understand how
intents are internally implemented, I will reproduce here.
AtoBX/BtoAX LUTs and Rendering Intents
The ICC spec is pretty detailed about the LUTs and their
varying meaning according to context in tables 20, 21, and 22
in section 6.3. My reading of this is that even though there
are 4 rendering intent selectors there are really 6 rendering
styles:
Relative Indefinite
(Relative) Perceptual
Relative Colorimetric
(Relative) Saturation
Absolute Indefinite
Absolute Colorimetric
If a device profile has a single-LUT or matrix:
* Perceptual, Relative Colorimetric, Saturation selectors
produce the same Relative Indefinite rendering style
* Absolute Colorimetric selector produces an Absolute
Indefinite rendering style derived from the single LUT or
matrix, the media white point tag, and the inverse of a
white point compensation method designated by the CMS
If a device profile has 3 LUTs:
* Perceptual, Relative Colorimetric, Saturation selectors
produce the appropriate rendering styles using the 0, 1, and
2 LUTs respectively
* Absolute Colorimetric selector produces an Absolute
Colorimetric rendering style derived from the Relative
Colorimetric LUT (numbered "1"), the media white point tag,
and the inverse of a white point compensation method
designated by the CMS
This would explain why perceptual is the default rendering style
because a single-LUT profile's LUT is numbered "0".
Annex B Apparent bug in XYZ -> sRGB transforms
============================================================================
John van den Heuvel warns me about an apparent bug on
XYZ -> sRGB transforms. Ver 1.10 should minimize this effect.
The obtained results are visually OK, but numbers seems to be wrong.
It appears only when following conditions:
a) You are using a transform from a colorspace with
a gamut a lot bigger that output space, i.e. XYZ.
Note than sRGB -> XYZ does work OK.
b) You are using absolute colorimetric intent.
c) You transform a color near gamut hull boundary
d) The output profile is implemented as a matrix-shaper,
i.e. sRGB.
e) You are using precalculated device link tables.
The numbers lcms returns doesn't match closely that color, but other
perceptually close to the intended one.
It happens that since XYZ has a very big gamut, and sRGB a narrow
one on compared to XYZ, when lcms tries to compute the device link
between XYZ -> sRGB, got most values as negative RGB (out of gamut).
lcms assumes this is effectively out of gamut and clamps to 0.
Then, since (127, 0, 0) is just over gamut boundary (for example
(127, -1, -1) would be out of gamut), lcms does interpolate
wrongly, not between -n to n but between 0 to n.
I could put an If() in the code for dealing with such situation,
but I guess it is better not touch anything and document this
behaviour.
XYZ is almost never used as a storage space, and since most monitor
profiles are implemented as matrix shaper touching this would slow
down common operations. The solution is quite simple,
if you want to deal with numbers, then use cmsFLAGS_NOTPRECALC.
If you deal with images, let lcms optimize the transform.
Visual results should appear OK, no matter numbers doesn't match.