Ghostscript 9.07 Color Management
Michael J. Vrhel, Ph.D.
Artifex Software
7 Mt. Lassen Drive, A-134
San Rafael, CA 94903, USA
www.artifex.com
Abstract
This document provides information about the color architecture in Ghostscript
9.06. The document is suitable for users who wish to obtain accurate color with their
output device as well as for developers who wish to customize Ghostscript to achieve
a higher level of control and/or interface with a different color management module.
Revision 1.4
Artifex Software Inc. www.artifex.com 1
1 Introduction
With release 9.0, the color architecture of Ghostscript was updated to primarily use the
ICC[1] format for its color management needs. Prior to this release, Ghostscript’s color
architecture was based heavily upon PostScript[2] Color Management (PCM). This is due to
the fact that Ghostscript was designed prior to the ICC format and likely even before there
was much thought about digital color management. At that point in time, color management
was very much an art with someone adjusting controls to achieve the proper output color.
Today, almost all print color management is performed using ICC profiles as opposed
to PCM. This fact along with the desire to create a faster, more flexible design was the
motivation for the color architectural changes in release 9.0. Since 9.0, several new features
and capabilities have been added. As of the 9.07 release, features of the color architecture
include:
• Easy to interface different CMMs (Color Management Modules) with Ghostscript.
• ALL color spaces are defined in terms of ICC profiles.
• Linked transformations and internally generated profiles are cached.
• Easily accessed manager for ICC profiles.
• Easy to specify default profiles for source DeviceGray, DeviceRGB and DeviceCMYK
color spaces.
• Devices can readily communicate their ICC profiles and have their ICC profiles set.
• Operates efficiently in a multithreaded environment.
• Handles named colors (spots) with ICC named color profile or proprietary format.
• ICC color management of Device-N colors or alternatively customizable spot color
handing.
• Includes object type (e.g. image, graphic, text), rendering intent and black point
compensation into the computation of the linked transform.
• Ability to override document embedded ICC profiles with Ghostscript’s default ICC
profiles.
• Easy to specify unique source ICC profiles to use with CMYK and RGB graphic,
image and text objects.
• Easy to specify unique destination ICC profiles to use with graphic, image and text
objects.
Artifex Software Inc. www.artifex.com 2
• Easy to specify different rendering intents (perceptual, colorimetric, saturation, abso-
lute colorimetric) for graphic, image and text objects.
• Easy to specify different black point compensation settings for graphic, image and text
objects.
• Ability to make use of a PDF output intent ICC profile.
• Ability to use an NCLR ICC output profile when rendering to a separation device.
• Control to force gray source colors to black ink only when rendering to output devices
that support black ink.
• Ability to make use of device link ICC profiles for direct mapping of source colors to
the device color space.
• Ability to make use of device link ICC profiles for retargeting from SWOP/Fogra
standard color space to a specific device color space.
The document is organized to first provide a high level overview of the architecture. This
is followed by details of the various functions and structures, which include the information
necessary to interface other color management modules to Ghostscript as well as how to
interface specialized color handling operations.
2 Overall Architecture and Typical Flow
Figure 1 provides a graphical overview of the various components that make up the archi-
tecture. The primary components are:
• The ICC manager, which maintains the various default profiles.
• The link cache, which stores recently used linked transforms.
• The profile cache, which stores internally generated ICC profiles created from PostScript
CIE based color spaces and CalRGB, CalGray PDF color spaces.
• The profiles contained in the root folder iccprofiles, which are used as default color
spaces for the output device and for undefined source colors in the document.
• The color management module (CMM), which is the engine that provides and performs
the transformations (e.g. little CMS).
• The profiles associated with the device, which include profiles dependent upon object
type, a proofing profile and a device link profile.
Artifex Software Inc. www.artifex.com 3
In the typical flow, when a thread is ready to transform a buffer of data, it will request
a linked transform from the link cache. When requesting a link, it is necessary to provide
information to the CMM, which consists of a source color space, a destination color space, an
object state (e.g. text, graphic, or image), black point compensation setting and a rendering
type (e.g. perceptual, saturation, colorimetric). The linked transform provides a mapping
directly from the source color space to the destination color space. If a linked transform for
these settings does not already exist in the link cache, a linked transform from the CMM will
be obtained (assuming there is sufficient memory – if there is not sufficient memory then the
requesting thread will need to wait). Depending upon the CMM, it is possible that the CMM
may create a lazy linked object (i.e. create the real thing when it is asked to transform data).
At some point, a linked transform will be returned to the requesting thread. The thread can
then use this mapping to transform buffers of data through calls through an interface to the
external CMM. Once the thread has completed its use of the link transform, it will notify
the link cache. The link cache will then be able to release the link when it needs additional
cache space due to other link requests.
3 PDL Color Definitions and ICC Profiles
To help reduce confusion, it is worthwhile to clarify terminology. In particular, the use of the
terms process color and device color need to be defined in the context of ICC profiles. Both
PDF[3] and PostScript (PS) have a distinction between process colors and device colors. In
PS, there is a conversion (e.g. via UCR/BG) from device colors to process colors. In an
ICC work flow, the colors are transformed directly from an input color space (often called
the source space) to an output color space (often called the destination space). The output
color space defined by the device’s ICC profile is a mapping to what PDF and PS define as
the process color space of the device. In other words, the “device color space” as defined by
the device’s ICC profile IS the process color space of PDF and PS. The ICC profile of the
device is a mapping from a CIE color space to the process color space AND from the process
color space to a CIE color space.
To understand this better, it may help to understand the method by which a print based
ICC profile is created. To create an ICC profile for a device, a chart is printed using its process
colors (e.g. CMYK). This chart is measured using a colorimeter or a spectrophotometer. This
provides the forward mapping from process colors to CIELAB values. The inverse mapping
(from CIELAB to process colors) is obtained by inverting this table usually through a brute
force search and extrapolation method. These mappings are both packed into an ICC format,
thereby defining mappings between the device “process colors” and the CIE color space.
Artifex Software Inc. www.artifex.com 4
gsicc_init_buffer
gsicc_get_link
gsicc_release_link
gsicc_set_icc_directory
gsicc_set_profile
gsicc_init_device_profile
gsicc_set_gscs_profile
gsicc_get_gscs_profile
gsicc_profile_new
gsicc_get_profile_handle_buffer
Each thread could
have access to a
common ICC cache
or create its own
Graphics
Library
&
Interpreter
CMM
gscms_error
gscms_create
gscms_destroy
gscms_get_profile_handle_mem
gscms_get_profile_handle_file
gscms_release_profile
gscms_get_link
gscms_get_link_proof_devlink
gscms_get_name2device_link
gscms_release_link
gscms_transform_color_buffer
gscms_transform_color
gscms_transform_named_color
gscms_get_numberclrtnames
gscms_get_clrtname
gscms_get_input_channel_count
gscms_get_output_channel_count
gscms_get_profile_data_space
gsicc_set_device_profile
gsicc_set_device_profile_intent
gx_default_get_profile
Device
User profile directory
gsicc_set_icc_directory
gsicc_set_profile
gsicc_init_device_profile
TextProfile.icc
Device Profiles For
Various Rendering
Cases and Object Types
GraphicsProfile.icc
ImageProfile.icc
ICC Manager
Link Cache
Named Color Profile
DeviceN Profiles [ ]
DefaultGray Profile
DefaultRGB Profile
DefaultCMYK Profile
Profile Cache
SoftMask Profiles
Source Profiles
(override)
default_gray.icc
default_rgb.icc
default_cmyk.icc
iccprofiles
lab.icc
sRGB.icc
s-gray.icc
ps_gray.icc
ps_rgb.icc
ps_cmyk.icc
gray_to_k.icc
ProofProfile.icc
DevicLinkProfile.icc
Figure 1: Graphical Overview of Ghostscript’s Color Architecture
