LXF113.tut_gimp.pdf

Tutorial Gimp

Gimp Open source image-editing

software you can get your teeth into

Gimp: Printing & colour

Gimp, CUPS and Gutenprint are a fellowship that can lead you on your quest for

high quality prints – Michael J Hammel continues his graphics tutorial series.

been the bane of the Linux office desktop user. Advances in

2D and 3D graphics have been phenomenal thanks to the

requirements of the special effects industry in Hollywood, yet

with high-quality futuristic displays at our doorstep, we still settle

for Stone Age prints. Until recently, the lack of a unified printing

system has meant that desktop applications have either

implemented their own printing features or relied on the lowest

common denominators, often leading to low-quality prints or, even

worse, no facility for printing at all. Even when we started to

acquire good quality printer drivers and the necessary tools to use

them on the desktop, colour management in Linux applications

has been essentially non-existent.

Fortunately, that’s started to change in a big way. The Linux

printing scene has seen a wealth of printer driver support from

both open source and commercial providers. As a result, the

under-the-hood printing system has evolved into a system of

libraries and utilities that desktop systems such as Gnome and

KDE can harness – all tied together through the magic of CUPS.

Drivers, spoolers, filters and printers

Before we dive into how to use all of this with Gimp, it’s important

to have a basic understanding of how printing works under Linux.

Printers come in two forms: PostScript and non-PostScript.

PostScript is a device-independent language that describes the

data to be printed. PostScript-capable printers can understand

this language directly, whereas non-compatible printers must have

data converted into a custom printer language. Getting files into

PostScript in the first place is a multi-step process that involves a

series of software packages.

The Linux printing environment is made up of four levels of

software: drivers, spoolers, filters and applications. Printer drivers

form the backbone of printing; they send data to the printer using

a language the printer will understand. Some Epson printers, for

example, use a language called ESC/P while other manufacturers

use a language called PCL. The data must already be in that

language when the driver receives it, however; the drivers are not

responsible for doing any data conversions.

CUPS and Gutenprint

Printer drivers for Linux come primarily from the Gutenprint

and/or Ghostscript projects, although HP have their own set of

drivers (known as the HPIJS) that they actively support. Other

vendors, including Epson, offer drivers for their printers as well. All

of the drivers are intended to be fed data to print via the various

Linux print spoolers.

Drivers are at the bottom of this four-layer software stack for

printing. At the opposite end are applications, with spoolers just

below. When an application is directed to print a file, it sends it to a

print spooler, which queues it up to be processed later (when the

spooler is not busy with other print jobs). There are many spoolers

available for Linux, including LPR (Line Printer Remote), LPRng

and CUPS (Common Unix Printing System). CUPS is becoming

the standard for spoolers on Linux distributions for a variety of

reasons, although none of these make much difference to end

users. It’s important to note that you’ll be working with CUPS to

set up your printers.

Once a spooler has a file queued for printing, it runs the file

through one or more filters that convert it into various formats.

The filtering process is better know as a ‘RIP’ (Raster Image

Processing). In some cases the files need to be converted into

PostScript first, then into a printer-specific format before being

sent to the printer driver. In other cases the file may already be in

PostScript format, or may not need to be converted to another

form before being sent to the printer driver.

Filters can come from a number of software packages. The

most widely used set of filters come from the Foomatic package,

but you won’t need to worry about them much because you’ll get

the ones you need when you install CUPS.

With all that software ready and under the hood, it’s time to

configure your printer. As well as checking the OpenPrinting

website ( www.linuxfoundation.org/openprinting ) to see if the

printer is supported, you’ll also want to install CUPS. CUPS is both

a spooler and a configuration tool. It allows you to configure your

printers whether they’re directly connected to your system, located

on your local network or connected to another system elsewhere

Our

expert

Michael J

Hammel is a

contributor to the

Gimp project and

the author of three

books on the

subject, including

his latest, The

Artist’s Guide to

Gimp Effects.

Last month We saw how selection mixes can pull complex subjects from images.

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E ver since the birth of Gnome and KDE, printing support has

Gimp Tutorial

Gimp: Printing & colour

on your network. There’s a web-based config tool available, along

with desktop-specific tools for both Gnome and KDE.

CUPS uses one or more filters to rip data into a format that can

be used by a printer driver. This spooler package is designed to

hide the filtering process, so users don’t have to know whether

a particular file is being converted before printing. All you need

to know is what printer model you have and how it’ll be connected

to your system.

CUPS also uses PPD (PostScript Printer Description) files,

which describe the features of printers to applications that need to

use them. Despite the name, these files are used for describing

both PostScript and non-PostScript printers. Again, in most cases

you won’t need to acquire these files if CUPS and Gutenprint

support the printer.

Gutenprint includes drivers for a huge number of printers.

However, because the package is under constant development,

the version of Gutenprint on your Linux distribution may not be up

to date. Fortunately, the version available from the OpenPrinting

website is always matched against its database of supported

printers, so if the database says the printer is supported but your

version of Gutenprint doesn’t have it, simply replace it with the

new version from the OpenPrinting site.

Colour management

Colour management is the process of matching colours between

input devices (such as scanners and cameras), editing tools (such

as Gimp) and output devices (such as your monitor or printer).

Each input or output device has a range of colours it can work

with, but you can usually edit a wider range with your applications.

The goal of colour management software is to map colours from

the input and editing phases to the range of colours that can be

displayed on the output device in order to help assure the result is

visually identical to the source.

Under Linux, colour management in applications is primarily

handled through a package known as LittleCMS. This is another of

those packages you don’t need to worry about, because your

application will install it if it needs it. Applications use LittleCMS in

conjunction with ICC colour profiles, which are files describing the

colour capabilities of a particular device, sometimes under specific

conditions. For example, most cameras have their ICC profiles

embedded in the images they take. This is why you sometimes see

a message pop up in Gimp, telling you the image is in a certain

colour space and asking if you want to convert it to a different

one. Printers, on the other hand, often deliver their profiles via

setup disks and usually provide one profile for each of the different

types of paper they can print on to.

For printers under Linux, you may run into problems trying to

find the ICC profile files on the hardware’s setup disc. In my case, I

was able to find the profiles for my new Epson Stylus Photo R280

buried in CAB archive files on the disc. You need to install the

Cabextract utility to unpack these files. Do this by copying the CAB

file to a directory and running:

cabextract filename.CAB

For my Epson

R280, there were

two CAB files that

contained identical

binary ICM files. I

unpacked one CAB

file and saved only

the ICM files.

I ran the file command on all files extracted from the archive

and found that the ICM files were ICC colour profiles. I copied

these to a directory where I can keep them safe (with my

backups). I found multiple profiles and had to open them in Gimp

to identify their purpose, which turned out to be one profile for

each type of paper the printer supported.

Install LPROF

Once you have your printer configured, have the printer ICC profile

files to hand and prepare to use colour management under Linux.

We’re not quite ready to do that yet because we don’t have an ICC

monitor profile, so first we need to create one. We can do that

easily using LPROF, but that package isn’t available for all

distributions and requires a lot of effort to compile for those it

doesn’t support. This section is for those who have problems

compiling LPROF on their systems. For those users with a

distribution that already has LPROF, such as Ubuntu, skip to the

next section to see how to use it.

First, download a copy of the latest version of LPROF from

http://lprof.sourceforge.net . I’m working with version 1.11.4.

This version has multiple requirements, the most important of

which are:

Qt 3

Specialised hardware

If the nature of your work means you need

that extra degree of precision in your

profiles, you should check out ArgyllCMS

( www.argyllcms.com/doc/ArgyllDoc.

html ), a colour management system for

Linux similar to LittleCMS. The

documentation for ArgyllCMS lists a

variety of specialised hardware that can

be used for calibrating monitors and

other devices.

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Tutorial Gimp

Vigra

LittleCMS v1.16

The latest versions of most Linux distributions are likely to

default to Qt 4, but they probably also have Qt 3 available to install

manually. On Fedora 9, I ran the following to get it:

sudo yum -y install qt3 qt3-devel

Vigra, (Vision with Genetic Algorithms) is a computer vision

library. I installed both the runtime and development libraries for

this package:

sudo yum -y install vigra vigra-devel

The most troublesome package of the three will be LittleCMS

( www.littlecms.com ), the latest version of which is 1.17. You can’t

install the prepackaged 1.16 into system directories in parallel with

newest version, so you need to download the source code for

v1.16, compile it and install in a separate directory. When I run into

issues like this, I install the specialised package under /usr/

local/<packagename> . I run whichever tools require this

package with a shell script to make sure that the package is found

before others with similar names or updated versions are found in

the normal system directories.

Again, be sure you’ve downloaded the source for version 1.16.

To compile LittleCMS and install it under its own directory under

/usr/local , I run the following commands:

./configure --prefix=/usr/local/lcms

make

sudo make install

With the prerequisites installed I can now compile LPROF and

install it under its own directory under /usr/local :

export PATH=/usr/local/lcms/bin:$PATH

export LD_LIBRARY_PATH=/usr/local/lcms/lib:$LD_

LIBRARY_PATH

python scons.py cxxflags=”-I/usr/local/lcms/include”

PREFIX=/usr/local/lprof

sudo python scons.py install

Management with LPROF

To run LPROF, I use this simple script:

#!/bin/bash

export LD_LIBRARY_PATH=/usr/local/lcms/lib:$LD_

LIBRARY_PATH

export PATH=/usr/local/lprof/bin:$PATH

lprof

LPROF enables you to create ICC profiles for scanners,

cameras and monitors. You need to use specialised hardware to

get highly accurate profiles, but we can get a rough estimate for a

monitor just using LPROF.

You can make a coarse approximation of a monitor profile just

using your eyes. To begin, you should be in a dark room with the

The printing ecosystem

Find out if your printer is supported, get the latest news

and learn more about printing under Linux by visiting the

OpenPrinting section at the Linux Foundation’s website:

www.linuxfoundation.org/en/OpenPrinting .

monitor’s contrast control set to its highest level. If you can’t be in

a completely dark room, try placing a dark-coloured cloth over the

monitor and your head while you’re doing this. You might look a

little silly, but you’ll get better results. Don’t leave the cloth over

the monitor (especially a CRT monitor) for very long because it

may overheat.

Coarse approximations

The process of creating a coarse approximation profile is

explained on the Monitor page of the LPROF documentation

( http://lprof.sourceforge.net/help/monitor.html ), but this

page is a little hard to follow, so I’ll walk you through the process.

The LPROF window opens with a set of tabs on the right. In the

Monitor Profile tab you’ll see that the first option under Profile

Method is to create a coarse profile. Click the Enter Monitor Values

button to open the Monitor Values dialog. Click the Set Gamma

And Black Point button under the Gamma entry. The window that

opens has a small window labelled ‘Gamma = 2.2’ and a larger

window to the right that’s labelled ‘Gamma And Black Level. You’ll

see a slider at the the bottom, along with some other buttons.

Adjust the slider so that the Gamma = 2.2 window shows the

horizontal lines and the box around them as close to a solid grey

as you can make them. The lines probably won’t go away when

you do this; the trick is to stare at the box so your vision goes

slightly out of focus and make sure that the grey gradient you see

is as smooth as possible. This adjustment sets the gamma level

for your monitor. Your monitor profile tells software to adjust

images so they display as if your monitor were at 2.2.

After setting the gamma level, you need to adjust the

Brightness control of your monitor, which may also be labelled

‘Black Point’ (as it is on my Westinghouse LCD monitor). Adjust it

so that the bars in the Black Level window on the right are barely

visible across from 2.2 on the scale just to the left and disappear

completely from view by 1.9. After making the Brightness or Black

Point adjustment on your monitor, you may need to adjust the

Gamma slider again. Repeat this process a few times until you’re

satisfied with the result.

You may need to click the Link Channels button to adjust the

Red, Green and Blue channels separately. With each channel, try

to adjust the gamma first, then adjust the Brightness on the

monitor. Close the window by clicking on the OK button and then

close the Rough Monitor Values window.

Create a new profile

Back in the LPROF main window, you’ll see the Profile

Identification button in the top-left. Click on this to fill in the

monitor make and model information, which is often found on a

label on the back of the monitor.

At the bottom-right of the LPROF window, just above the

Create Profile button is the Output Profile field. Type in the path

and name of the ICC output file, then click on Create Profile. You

can check the profile using the Profile Checker button. Information

on how to use this is found on the Profile Checker page of the

LPROF documentation.

The LPROF

window (which

titles itself as

LittleCMS Profiler)

has multiple tabs

on the right. Start

with the Monitor

Profile tab.

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Gimp Tutorial

Use the Gamma/Black Point window to visually configure

your monitor profile without specialised hardware.

the best for most home and small business use. Professional

designers can read the Gimp help documentation for details on

the other options.

The File Open Behaviour option determines what Gimp will do

with images it opens that have ICC profile data embedded in them.

Cameras often embed profiles in the images they create. There’s

little reason not to set this to Convert To RGB Workspace; leaving it

as ‘Ask What To Do’ will cause a prompt to be displayed asking you

what to do when you open images with embedded profiles.

The most important option in the Preferences is the Mark Out

Of Gamut setting. With this option enabled, and the Mode of

Operation set to Print Simulation, the Gimp Canvas window will

highlight any pixels that won’t print properly with a specified

colour (which defaults to grey). This enables you to see which

pixels will have to be converted to another colour by the printer. It

will, in effect, tell you how different your image is likely to be from

what you see when you’re working on the image on screen.

With out-of-gamut colours marked, you may wonder how you

can fix an image to print properly (without having the printer

convert them to different colours). Unfortunately, there is no fix-all

option. For my images, I adjusted the different Colour Channels in

the Curves dialog by moving the end points of the graph away from

the corners. When you do this with Preview enabled in the Curves

dialog you can watch the grey, out-of-gamut colours change. In this

case, the fix took only a few seconds, but you may find that the

Curves dialog is not sufficient for your needs. In that case, you’ll

need to experiment with other tools in the Colour menu.

Printing

With your image colour corrected you’re ready to print. Remember

we used CUPS and Gutenprint to set up our printer. Gimp has its

own print dialog built into it. Gutenprint also provides a separate

print plugin for use with Gimp. With both enabled, you can choose

which is more appropriate for your needs.

Gimp’s built-in Print window offers a print preview, but it

doesn’t make clear whether the preview is colour-adjusted based

on ICC profiles. Still, for most home printing, you’ll probably be

happy with the preview it provides.

Gutenprint’s Print window offers much finer control over the

print process itself. The print preview is purposely very low quality

to make it easier to drag it around the page for alignment. This

preview does not make use of your printer’s ICC profiles. What’s

most important about the Gutenprint dialog is that you can

configure any printer you might have connected to your computer

for use with Gimp, even if that computer isn’t configured through

CUPS. This is useful, for example, if you have a printer used

specifically for Gimp prints, but not shared with others in your

household or business.

Summary

Printing support under Linux has become easily good enough for

the home and small business user due to advances in Gutenprint,

CUPS and the availability of drivers from commercial printer

vendors. Colour management under

Gimp is still a work in progress, but the 2.4

release has brought us closer to colour

Nirvana with the ability to work directly

with ICC profiles. While there’s still work to

be done with regard to correcting of out-

of-gamut issues, we can at least know

when an image won’t fit and can manually

work at colour correction to fit the image

into gamut for the printer. LXF

Using profiles is very straightforward, but interpreting the

results and adjusting an image accordingly takes some

experimentation. Gimp 2.4 introduced support for ICC profiles in

the Colour Management page of the Preferences dialog. This is

where you select the profiles for your printer and the monitor

profile that we just created with LPROF.

The Mode of Operation controls how you will view your images

in the Canvas window. Setting this to Colour Managed Display

will cause Gimp to display images using your monitor profile

(which we’ll configure in a moment), while setting it to Print

Simulation will cause Gimp to display the image using the printer

profile. This is what you want to use when you want to preview

your image to check for out-of-gamut colours. These are colours

that can’t be reproduced by your printer given the printer profile

you’ll be using. The purpose of the RGB and CMYK settings isn’t

clear from Gimp documentation, but for now you can leave these

set to None.

Interpreting profiles

Use the Monitor Profile setting to select the profile you created

using LPROF. The Print Simulation profile is used to select the

profile for your printer. Notice that I have multiple profiles and the

Gimp dialog for selecting the profile provides information about

the profile. For my Epson printer, I have profiles set up for different

papers, most of which were created using a White Point setting of

5000K (which is shown here as D50).

Gimp has multiple methods available for interpreting profiles.

These can be set with the Display Rendering Intent (for

interpreting how images are shown on the Canvas) and Softproof

Rendering Intent (for how printed images will be previewed on the

Canvas) options in the Preferences dialog. The various options

available are quite technical and the Perceptual option is probably

To convert or not to convert

Unless you’re a graphic designer working

with a client who has specific colour

requirements, you probably won’t worry

much about colour management and ICC

profiles when using Gimp. If you don’t care

about such things, it won’t matter if you

convert images from their original colour

space. If you do care, you should convert

the image before working on it. Be certain

to save your work into a different file so

you can maintain the colour profile

information in the original file.

Next month We’ll use quick colour fixes to adjust tints and whiten a smile.

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