1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
|
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>10. A Script-Fu Tutorial</title>
<link rel="stylesheet" href="gimp-help-plain.css" type="text/css" />
<link rel="stylesheet" href="gimp-help-screen.css" type="text/css" />
<meta name="generator" content="DocBook XSL Stylesheets V1.66.1" />
<link rel="start" href="index.html" title=" " />
<link rel="up" href="ch02.html" title="Chapter 2. Using Gimp" />
<link rel="prev" href="ch02s09.html" title="9. Using Script-Fu Scripts" />
<link rel="next" href="ch02s10s02.html" title="10.2. Variables And Functions" />
</head>
<body>
<div xmlns="" class="navheader">
<table width="100%" summary="Navigation header">
<tr>
<th colspan="3" align="center" id="chaptername">10. A Script-Fu Tutorial</th>
</tr>
<tr>
<td width="20%" align="left"><a accesskey="p" href="ch02s09.html">Prev</a> </td>
<th width="60%" align="center" id="sectionname">10. A Script-Fu Tutorial</th>
<td width="20%" align="right"> <a accesskey="n" href="ch02s10s02.html">Next</a></td>
</tr>
</table>
<hr />
</div>
<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="gimp-using-script-fu-tutorial"></a>10. A Script-Fu Tutorial</h2>
</div>
</div>
</div>
<p>
In this training course, we'll introduce you to the fundamentals
of Scheme necessary to use Script-Fu, and then build a handy
script that you can add to your toolbox of scripts. The script
prompts the user for some text, then creates a new image sized
perfectly to the text. We will then enhance the script to allow
for a buffer of space around the text. We will conclude with a few
suggestions for ways to ramp up your knowledge of Script-Fu.
</p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25">
<img alt="[Note]" src="../images/note.png" />
</td>
<th align="left">Note</th>
</tr>
<tr>
<td colspan="2" align="left" valign="top">
<p>
This section as adapted from a tutorial written for the Gimp 1 User
Manual by Mike Terry.
</p>
</td>
</tr>
</table>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="id3316524"></a>10.1. Getting Acquainted With Scheme</h3>
</div>
</div>
</div>
<div class="simplesect" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id3316532"></a>Let's Start Scheme'ing</h4>
</div>
</div>
</div>
<p>
The first thing to learn is that:
</p>
<p>
<span class="bold">
<b>
Every statement in Scheme is surrounded by parentheses ().
</b>
</span>
</p>
<p>
The second thing you need to know is that:
</p>
<p>
<span class="bold">
<b>
The function name/operator is always the first item in the
parentheses, and the rest of the items are parameters to the
function.
</b>
</span>
</p>
<p>
However, not everything enclosed in parentheses is a function --
they can also be items in a list -- but we'll get to that
later. This notation is referred to as prefix notation, because
the function prefixes everything else. If you're familiar with
postfix notation, or own a calculator that uses Reverse Polish
Notation (such as most HP calculators), you should have no
problem adapting to formulating expressions in Scheme.
</p>
<p>
The third thing to understand is that:
</p>
<p>
<span class="bold">
<b>
Mathematical operators are also considered functions, and thus
are listed first when writing mathematical expressions.
</b>
</span>
</p>
<p>
This follows logically from the prefix notation that we just
mentioned.
</p>
</div>
<div class="simplesect" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id3316392"></a>Examples Of Prefix, Infix, And Postfix Notations</h4>
</div>
</div>
</div>
<p>
Here are some quick examples illustrating the differences
between <span class="emphasis"><em>prefix</em></span>,
<span class="emphasis"><em>infix</em></span>, and <span class="emphasis"><em>postfix</em></span>
notations. We'll add a 1 and 3 together:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
Prefix notation: + 1 3 (the way Scheme will want it)
</p>
</li>
<li>
<p>
Infix notation: 1 + 3 (the way we "normally" write it)
</p>
</li>
<li>
<p>
Postfix notation: 1 3 + (the way many HP calculators will
want it)
</p>
</li>
</ul>
</div>
</div>
<div class="simplesect" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id3316645"></a>Practicing In Scheme</h4>
</div>
</div>
</div>
<p>
Now, let's practice what we have just learned. Start up Gimp,
if you have not already done so, and choose
Xtns/Script-Fu/Console. This will start up the Script-Fu
Console window, which allows us to work interactively in
Scheme. In a matter of moments, the Script-Fu Console will
appear:
</p>
</div>
<div class="simplesect" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id3316396"></a>The Script-Fu Console Window</h4>
</div>
</div>
</div>
<p>
At the bottom of this window is an entry-field entitled
Current Command. Here, we can test out simple Scheme commands
interactively. Let's start out easy, and add some numbers:
</p>
<pre class="programlisting">
(+ 3 5)
</pre>
<p>
Typing this in and hitting Return yields the expected answer
of 8 in the center window.
</p>
<p>
Now, what if we wanted to add more than one number? The "+"
function can take two or more arguments, so this is not a
problem:
</p>
<pre class="programlisting">
(+ 3 5 6)
</pre>
<p>
This also yields the expected answer of 14.
</p>
<p>
So far, so good -- we type in a Scheme statement and it's
executed immediately in the Script-Fu Console window. Now for
a word of caution....
</p>
</div>
<div class="simplesect" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id3316355"></a>Watch Out For Extra Parens</h4>
</div>
</div>
</div>
<p>
If you're like me, you're used to being able to use extra
parentheses whenever you want to -- like when you're typing a
complex mathematical equation and you want to separate the
parts by parentheses to make it clearer when you read it. In
Scheme, you have to be careful and not insert these extra
parentheses incorrectly. For example, say we wanted to add 3
to the result of adding 5 and 6 together:
</p>
<pre class="programlisting">
3 + (5 + 6) + 7= ?
</pre>
<p>
Knowing that the + operator can take a list of numbers to add,
you might be tempted to convert the above to the following:
</p>
<pre class="programlisting">
(+ 3 (5 6) 7)
</pre>
<p>
However, this is incorrect -- remember, every statement in
Scheme starts and ends with parens, so the Scheme interpreter
will think that you're trying to call a function named "5" in
the second group of parens, rather than summing those numbers
before adding them to 3.
</p>
<p>
The correct way to write the above statement would be:
</p>
<pre class="programlisting">
(+ 3 (+ 5 6) 7)
</pre>
</div>
<div class="simplesect" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id3316852"></a>Make Sure You Have The Proper Spacing, Too</h4>
</div>
</div>
</div>
<p>
If you are familiar with other programming languages, like
C/C++, Perl or Java, you know that you don't need white space
around mathematical operators to properly form an expression:
</p>
<pre class="programlisting">
<tt class="literal">3+5, 3 +5, 3+ 5</tt>
</pre>
<p>
These are all accepted by C/C++, Perl and Java
compilers. However, the same is not true for Scheme. You must
have a space after a mathematical operator (or any other
function name or operator) in Scheme for it to be correctly
interpreted by the Scheme interpreter.
</p>
<p>
Practice a bit with simple mathematical equations in the
Script-Fu Console until you're totally comfortable with these
initial concepts.
</p>
</div>
</div>
</div>
<div class="navfooter">
<hr />
<table width="100%" summary="Navigation footer">
<tr>
<td width="40%" align="left"><a accesskey="p" href="ch02s09.html">Prev</a> </td>
<td width="20%" align="center">
<a accesskey="u" href="ch02.html">Up</a>
</td>
<td width="40%" align="right"> <a accesskey="n" href="ch02s10s02.html">Next</a></td>
</tr>
<tr>
<td width="40%" align="left" valign="top">9. Using Script-Fu Scripts </td>
<td width="20%" align="center">
<a accesskey="h" href="index.html">Home</a>
</td>
<td width="40%" align="right" valign="top"> 10.2. Variables And Functions</td>
</tr>
</table>
</div>
</body>
</html>
|
