2004.12_It's a Kind of Magic-Imagemagick Lets You Edit Images with Text Commands.pdf

KNOW HOW

ImageMagick

It’s a Kind of Magic!

There’s no need to start a complex program like The Gimp every time you want

to resize, crop or convert an image. ImageMagick gives you the ability to han-

dle a wide range of image manipulation jobs on the command line and helps

you to automate picture processing by using clever shell scripts.

BY HEIKE JURZIK

the filename. Click the mouse in the win-

dow you want to capture – you should

hear one beep when import begins to

capture and another beep when the job

is done. If you want to include the X win-

dow frame, you can add the option

-frame . Alternatively, you can just press

the left mouse button and then drag –

import will now capture the selected

area of the screen.

If you want to make a screenshot of

the entire desktop and save it in the

JPEG image format, you can use the

option -window :

01 Image: chickens.png

02 Format: PNG (Portable Netw

ork Graphics)

03 Geometry: 529x499

04 Class: DirectClass

05 Type: true color

06 Depth: 8 bits-per-pixel co

mponent

various image manipulation tools

that allow users to animate a

sequence of images, create a composite

image by overlaying a group of pictures,

convert one or several images to differ-

ent file formats or sizes, display and

identify pictures, capture screenshots,

and lots more.

These tools can save you a lot of time

and typing or clicking – just put your

favorite command calls into a shell script

and automate the work. This article

describes the more useful ImageMagick

utilities, introducing you to the magic of

creating, converting, and editing images

on the command line.

It’s even possible to print less infor-

mation – identify also has an option

called -format , which lets you specify the

information you want to present. So, if

you’re only interested in the picture

dimensions, you can use identify like

this:

import -window root desktop.jpg

Need more time, or maybe you want to

click on a pull-down menu to show cer-

tain aspects of a program? No problem –

by combining import with the sleep com-

mand this is easily done:

identify -format "%wx%h" U

chickens.png

529x499

%w and %h are only two of several spe-

cial format characters. Additionally, you

can ask for information on the file size

( %b ), the image comment ( %c ), the file-

name ( %f ), and many more. For a

complete list, have a look at the

ImageMagick manpage and search for

-format . Note that you have to quote

those format characters to prevent the

shell from interpreting the % .

sleep 10; import -window root U

desktop.jpg

You now have 10 seconds to arrange

everything on the desktop before import

will start its job and capture the desktop.

Import your Screen

To capture a single window, a rectangu-

lar area or the whole desktop, you don’t

have to fire up tools like The Gimp or

KDE’s KSnapshot program. You can

instead type the command, and specify

your target by ID or name, or select it by

clicking (and dragging) the mouse.

To capture a single window, type

import window.png . ImageMagick auto-

matically determines the file format to

use from the extension you choose for

Informer

To quickly determine the dimensions of

an image (and even check whether it is

corrupt or incomplete), you can use

identify . It gives you information about

the width and height, the number of col-

ors and bytes and the number of seconds

it took to read and process the informa-

tion. Together with the option -verbose

you can learn more about the file and

read additional image information:

Convert your Pictures

It is not hard to guess what convert does:

it allows you to convert between various

graphics formats on the command line.

But it can also create PostScript files, add

captions to images, apply filters, and do

much more.

To convert an image from JPEG to

PNG format, type the following simple

December 2004

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Editing images with ImageMagick

T he ImageMagick package contains

ImageMagick

KNOW HOW

command: convert image.jpg image.png.

The program evaluates the file extension

to recognize the format, and thus knows

the target format for the conversion.

This command will not remove the ori-

ginal JPEG file but will simply give you

an extra PNG file. The manpage has a list

of the numerous image formats that

convert supports. These include various

PostScript formats – that can be really

useful if you need to print out a picture.

The -page parameter makes sure

that the image is correctly positioned

on the printed page. The following

command:

picture portraits 90 degrees,

if the camera model doesn’t

do it automatically for you.

Use convert together with

the option -rotate and an

additional argument, which

specifies the number of

degrees to the right to rotate

the image:

Figure 2: xfontsel in action – helping you find the right font.

convert -rotate 90 w U

indow.jpg window_r.j U

convert -font @/var/lib/ U

defoma/x-ttcidfont-conf. U

d/dirs/TrueType/Verdana_ U

Bold.ttf -pointsize 50 - U

fill white -draw "text 5 U

0,50 'Lots of chickens'" chicke U

ns.jpg chickens_font.jpg

Alternatively, if you want to rotate to the

left, use a negative number.

convert -page A4 chickens.jpg U

document.ps

More Magic Tricks

convert helps you add captions to your

images using -font , -pointsize , -fill , -draw

options. You can choose between X

server, and TrueType fonts. You need to

add an @ prefix for the latter, as in -font

@font.ttf . The -pointsize option allows

you to specify the font size, as in -point-

size 16 for a 16 point font.

To find installed TrueType fonts on

your machine you could, for example,

run the command locate *.ttf . If you opt

for an X server font instead, you might

like to run the xfontsel tool to tell you

what fonts are available with what

attributes, character sets, and sizes.

Launch the program in an Xterm, and

select the attributes you need. The

number of fonts that match your criteria

should decrease at every step; this is

indicated by a display in the top right

corner of the window (see Figure 2): “4

names match”). If you are

happy with your selection,

click select . You can now press

the center mouse button to

insert the string that you have

created (for example -font

"-*-helvetica-medium-r-*-*-12

-*-*-*-*-*-iso8859-*" ).

The -fill parameter specifies

the font color. You have a

choice of any colors supported

by your X server (you can type

showrgb for a full list of col-

ors.) The -draw parameter

specifies what to draw (circle,

rectangle, text, and so on.) The

best way to explain this is to

look at an example with a com-

bination of all these options:

This writes “Lots of chickens” in a 50

point TrueType font ( Ve rdana_Bold.ttf )

in white on the image. The origin for the

text position is the top left corner (0,0);

so the text will be 50 pixels to the right

and 50 down ( text 50,50 ). The image is

stored as chickens_font.jpg , and is

shown in Figure 3.

To add a colored frame to the image,

run convert with the -mattecolor and

-frame options. The following command

adds a blue frame that is six pixels wide

vertically and horizontally:

scales a file called chickens.jpg to fill an

A4 PostScript document (see Figure 1).

If you need a quick summary of a large

collection of image files, you can run

convert to create a so-called “visual

directory.” The following command:

convert 'vid:*.jpg' U

directory.jpg

first reduces all the JPEG files in the cur-

rent directory, then adds them to a new

image file called directory.jpg before

adding captions that tell you the original

filenames, the image size in pixels, and

the file size.

convert -mattecolor blue -fram U

e 6x6 chickens.jpg chickens_fr U

amed.jpg

The Right Size

The -geometry option allows you to scale

an image up or down and keeps the

original proportions. An image that

starts off at 320x200 is scaled to 640x400

if you type -geometry 640x480 , for exam-

ple. If you want to fill an area (and don’t

mind distorting the image), you need

instead to type -geometry '640x480!'

(note the exclamation mark). The single

quotes are not strictly necessary in

this case, but since the shell uses ! for

history expansion, it’s wise to escape

it. Alternatively, you could put a

backslash in front ( -geometry 640x480\! ).

You can also specify -geometry with

a percentage, as in -geometry 200% .

As convert is part of the ImageMagick

package, it can use multiple Image-

Magick filters. The -implode filter gives

you fun results, dragging the pixels

towards the center of the image (see

Figure 4).

Around the Clock

If you take many photos with a digital

camera, you might want to rotate some

Figure 1: convert generates PostScript files from images.

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KNOW HOW

ImageMagick

convert is really useful if you need to

automate the processing of a large num-

ber of files. For example, you might need

to convert all the JPEG images in a direc-

tory into thumbnails of the same size. A

script like the one shown in Listing 1

would do this for you.

Files with the .jpg extension are

assigned to the i variable. Then, convert

-geometry '75x75!' iterates against the

variable. The first parameter is the input

file ( $i ) and the second is the output file,

which derives its name from the base file-

name without the extension, the _small

string, and finally the extension .jpg .

A slightly easier way that requires less

typing is to use mogrify – no scripting is

needed if you choose to use this little

program to create your thumbnails for all

JPEG files in the current directory:

*.jpg . Use the [Space] key

to go to the next image,

the [backspace] key to go

back and [Q] to quit.

Alternatively, you can

open display ‘s menu by

clicking with the right

mouse button into the

image window and find

buttons for Next , Former

and Quit in the File menu.

Most of the functions

you’ve read about in this

article can be found in

the various menu entries.

So, if you wanted to crop,

chop or rotate a picture,

instead of typing the command on the

command line you could also go to the

Tr ansform menu and click on the appro-

priate button. display also understands

most of ImageMagick’s common com-

mand line options when you start the

program. For example, to scale an image

to 1024x768 pixels at program start, you

can type:

Figure 5: display creates HTML galleries for you.

The new picture shows thumbnails of

all the images, plus the filenames and

sizes. To create an HTML gallery, just

save the image with the thumbnails in

HTML format by clicking on File / Save

and choosing the HTML format. In your

current directory you will find one .html

file, one .shtml file with an image map

and one transparent GIF with the thumb-

nails. You can now have a look at your

new gallery by opening it in a browser –

you should be able to click on each

thumbnail to navigate through the pic-

tures (see Figure 5).

mogrify -sample '75x75!' *.jpg

The only disadvantage is that mogrify

will overwrite the original images. One

solution to solve this problem is that you

specify a new output format by using the

-format option:

display -geometry 1024x768 chi U

cken.png

mogrify -format png -sample '7 U

5x75!' *.jpg

Similarly, you can define that a picture

should be presented in black and white,

that ImageMagick should add a frame,

etc. One very nice feature of display is

the ability to create HTML galleries with

only a few clicks: Start the program by

typing display on the command line.

Right click the window to open the

menu. Next go to File / Visual Directory

and navigate to the directory which con-

tains the gallery pictures. Click on the

button Directory and display starts to cre-

ate thumbnails for you. Depending on

the number of pictures, this may take a

while.

The thumbnails are now being stored in

the PNG format, while the original

JPEGs remain untouched.

The Gathering

Working with factors and positions in

the command line may seem convoluted

in comparison to image manipulation

programs like The Gimp, but the

ImageMagick tools are hard to beat

when it comes to automated image pro-

cessing. Web developers and

administrators who frequently have to

manipulate large collections of pictures

can include their favorite program calls

into shell scripts – an attractive alterna-

tive to clicking with the mouse. Even if

you only need to apply one or two

changes, typing on the command line

can be much faster than a GUI and save

you a lot of time.

In the Eye of the Beholder

ImageMagick also includes a viewer pro-

gram: display is the suite’s image

processing and displaying tool. To start

the viewer, type display followed by the

images you want to load (i.e. display

image.bmp ). If you want to look at all

the JPEG files from one directory, you

can load the whole lot by typing display

■

Listing 1: Creating

Thumbnails

Figure 3: Scripted chickens.

Figure 4: Imploding chickens.

01 #!/bin/bash

03 for i in *.jpg; do

04 convert $i -geometry '75x7 U

5!' `basename $i .jpg`_small.jpg

05 done

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