3 <h3 id="TOP">Index</h3>
5 <ul><li><a href="#NAME">NAME</a></li>
6 <li><a href="#INTRODUCTION">INTRODUCTION</a>
9 <ul><li><a href="#CREATING_A_DEMO">CREATING A DEMO</a></li>
12 <li><a href="#FIRST_VERSION">FIRST VERSION</a></li>
13 <li><a href="#CONTROLLING_THE_SHIP">CONTROLLING THE SHIP</a></li>
14 <li><a href="#HOW_ABOUT_THE_GRAPHICS">HOW ABOUT THE GRAPHICS?</a>
15 <ul><li><a href="#THE_GRAPHICS">THE GRAPHICS</a></li>
18 <li><a href="#USING_SDL">USING SDL</a></li>
21 <li><a href="#COPYRIGHT_amp_LICENSE">COPYRIGHT & LICENSE</a>
26 <h1 id="NAME">NAME</h1><p><a href="#TOP" class="toplink">Top</a></p>
27 <div id="NAME_CONTENT">
28 <p>Lunar Lander - a small tutorial on Perl SDL</p>
31 <h1 id="INTRODUCTION">INTRODUCTION</h1><p><a href="#TOP" class="toplink">Top</a></p>
32 <div id="INTRODUCTION_CONTENT">
33 <p>This is a quick introduction to Games, Perl, and SDL (Simple
34 DirectMedia Layer, a cross-platform multimedia programming
35 library). We'll write a small game -- Lunar Lander -- in 100
36 lines of code, or less.</p>
39 <h3 id="CREATING_A_DEMO">CREATING A DEMO</h3>
40 <div id="CREATING_A_DEMO_CONTENT">
41 <p>You can see the final version of the demo code by doing:</p>
43 <pre> perl -MSDL::Tutorial::LunarLander=lander.pl -e1
47 <p>this will create all three files used in the tutorial:</p>
54 <h2 id="FIRST_VERSION">FIRST VERSION</h2>
55 <div id="FIRST_VERSION_CONTENT">
56 <p>We'll start with a text version of the game.</p>
57 <p>"What?", you may ask. "I thought it was a SDL tutorial".</p>
58 <p>Yes, it is -- thank you for reminding me. But we'll leave the SDL part for
59 later. We must build the game logic first!</p>
60 <p>One of the traps of game programming is focusing too much on the interface.
61 If we start with a simpler simulation, we can worry with the presentation
63 <p>So, here's the initial code:</p>
70 my $height = 1000; # m
71 my $velocity = 0; # m/s
72 my $gravity = 1; # m/s^2
76 while ( $height > 0 ) {
77 print "at $t s height = $height m, velocity = $velocity m/s\n";
79 $height = $height - $velocity;
80 $velocity = $velocity + $gravity;
84 if ( $velocity > 10 ) {
85 print "CRASH!!!\n";
87 print "You landed on the surface safely! :-D\n";
92 <p>Run the code and you'll see something like this:</p>
94 <pre> at 0 s height = 1000 m, velocity = 0 m/s
95 at 1 s height = 1000 m, velocity = 1 m/s
96 at 2 s height = 999 m, velocity = 2 m/s
97 at 3 s height = 997 m, velocity = 3 m/s
98 at 4 s height = 994 m, velocity = 4 m/s
99 at 5 s height = 990 m, velocity = 5 m/s
101 at 43 s height = 97 m, velocity = 43 m/s
102 at 44 s height = 54 m, velocity = 44 m/s
103 at 45 s height = 10 m, velocity = 45 m/s
109 <p>"What happened? How do I control the ship???"</p>
112 <h2 id="CONTROLLING_THE_SHIP">CONTROLLING THE SHIP</h2>
113 <div id="CONTROLLING_THE_SHIP_CONTENT">
114 <p>The problem with our first spaceship is that it had no controls!</p>
115 <p>So, let's fix this problem, making the spaceship scriptable. (We
116 could write some code to handle keyboard and joysticks now, but
117 an scriptable spaceship will be easier to start. Remember, focus
118 on the game logic!)</p>
119 <p>So, create add this simple script to the end of your file:</p>
122 at 41s, accelerate 10 m/s^2 up
130 <p>The script is straightforward: it simply states a time when we
131 will push the spaceship up with a given acceleration. It accepts
132 free text: any two numbers you type will work.</p>
133 <p>We can parse the script using this regular expression:</p>
135 <pre> my $script_re = qr/(\d+) \D+ (\d+)/x;
139 <p>And we can build a hash of ( time => acceleration ) with:</p>
141 <pre> my %up = map { $_ =~ $script_re } <DATA>;
145 <p>So the middle section of the program will become:</p>
147 <pre> my $script_re = qr/(\d+) \D+ (\d+)/x;
148 my %up = map { $_ =~ $script_re } <DATA>;
150 while ( $height > 0 ) {
151 print "at $t s height = $height m, velocity = $velocity m/s\n";
155 print "(accellerating $a m/s^2)\n";
156 $velocity = $velocity - $a;
159 $height = $height - $velocity;
160 $velocity = $velocity + $gravity;
167 <p>Try to run the program, and the ship should land safely:</p>
169 <pre> ./lunar.pl autopilot.txt
170 at 0 s height = 1000 m, velocity = 0 m/s
171 at 1 s height = 1000 m, velocity = 1 m/s
172 at 2 s height = 999 m, velocity = 2 m/s
173 at 3 s height = 997 m, velocity = 3 m/s
174 at 4 s height = 994 m, velocity = 4 m/s
175 at 5 s height = 990 m, velocity = 5 m/s
177 at 54 s height = 19 m, velocity = 4 m/s
178 at 55 s height = 15 m, velocity = 5 m/s
179 at 56 s height = 10 m, velocity = 6 m/s
180 at 57 s height = 4 m, velocity = 7 m/s
182 You landed on the surface safely! :-D
189 <h2 id="HOW_ABOUT_THE_GRAPHICS">HOW ABOUT THE GRAPHICS?</h2>
190 <div id="HOW_ABOUT_THE_GRAPHICS_CONTENT">
191 <p>Okay, okay... now that we have a working prototype, we can work on
192 the graphics. But, first of all, we'll need...</p>
195 <h3 id="THE_GRAPHICS">THE GRAPHICS</h3>
196 <div id="THE_GRAPHICS_CONTENT">
197 <p>Yes, the graphics.</p>
198 <p>We won't use anything fancy here, just two images: a large one, for
199 the background, and a smaller one for the spaceship.</p>
200 <p>Create the images using the Gimp, or use the images provided by
201 this tutorial; Save these images in a subdirectory called "images":
202 ("<code>images/background.jpg</code>" and "<code>images/ship.png</code>").</p>
205 <h2 id="USING_SDL">USING SDL</h2>
206 <div id="USING_SDL_CONTENT">
207 <p>First step: use the required libraries:</p>
209 <pre> use SDL; #needed to get all constants
216 <p>Second step: initialize <code>SDL::App</code>:</p>
218 <pre> my $app = SDL::App->new(
219 -title => "Lunar Lander",
227 <p>Third step: load the images and create the necessary "rectangles":</p>
229 <pre> my $background = SDL::Surface->new( -name => 'images/background.jpg', );
230 my $ship = SDL::Surface->new( -name => 'images/ship.png', );
232 my $background_rect = SDL::Rect->new(
233 -height => $background->height(),
234 -width => $background->width(),
237 my $ship_rect = SDL::Rect->new(
238 -height => $ship->height(),
239 -width => $ship->width(),
244 <p>Fourth step: create a sub to draw the spaceship and background:</p>
247 my ( $x, $y ) = @_; # spaceship position
249 # fix $y for screen resolution
250 $y = 450 * ( 1000 - $y ) / 1000;
253 $background->blit( $background_rect, $app, $background_rect );
256 my $ship_dest_rect = SDL::Rect->new(
257 -height => $ship->height(),
258 -width => $ship->width(),
263 $ship->blit( $ship_rect, $app, $ship_dest_rect );
265 $app->update($background_rect);
270 <p>Note that this sub first combines all the bitmaps, using a blit
271 ("Block Image Transfer") operation -- which is quite fast, but does
272 not update the display.</p>
273 <p>The combined image is displayed in the last line. This process of
274 combining first, and displaying later, avoids that annoying fading
275 between cycles ("flickering").</p>
276 <p>Finally, add the following lines to the end of the main loop, so that
277 we call the <code>draw()</code> function with the correct spaceship
280 <pre> while ( $height > 0 ) {
284 draw( 100, $height );
291 <p>Run the program and watch the spaceship landing safely on the surface
295 <h1 id="COPYRIGHT_amp_LICENSE">COPYRIGHT & LICENSE</h1><p><a href="#TOP" class="toplink">Top</a></p>
296 <div id="COPYRIGHT_amp_LICENSE_CONTENT">
297 <p>Copyright 2009 Nelson Ferraz, all rights reserved.</p>
298 <p>This program is free software; you can redistribute it and/or modify it
299 under the same terms as Perl itself.</p>