3 <h3 id="TOP">Index</h3>
5 <ul><li><a href="#NAME">NAME</a>
6 <ul><li><a href="#CATEGORY">CATEGORY</a></li>
9 <li><a href="#INTRODUCTION">INTRODUCTION</a>
12 <ul><li><a href="#CREATING_A_DEMO">CREATING A DEMO</a></li>
15 <li><a href="#FIRST_VERSION">FIRST VERSION</a></li>
16 <li><a href="#CONTROLLING_THE_SHIP">CONTROLLING THE SHIP</a></li>
17 <li><a href="#HOW_ABOUT_THE_GRAPHICS">HOW ABOUT THE GRAPHICS?</a>
18 <ul><li><a href="#THE_GRAPHICS">THE GRAPHICS</a></li>
21 <li><a href="#USING_SDL">USING SDL</a></li>
24 <li><a href="#COPYRIGHT_amp_LICENSE">COPYRIGHT & LICENSE</a>
29 <h1 id="NAME">NAME</h1><p><a href="#TOP" class="toplink">Top</a></p>
30 <div id="NAME_CONTENT">
31 <p>Lunar Lander - a small tutorial on Perl SDL</p>
34 <h2 id="CATEGORY">CATEGORY</h2>
35 <div id="CATEGORY_CONTENT">
39 <h1 id="INTRODUCTION">INTRODUCTION</h1><p><a href="#TOP" class="toplink">Top</a></p>
40 <div id="INTRODUCTION_CONTENT">
41 <p>This is a quick introduction to Games, Perl, and SDL (Simple
42 DirectMedia Layer, a cross-platform multimedia programming
43 library). We'll write a small game -- Lunar Lander -- in 100
44 lines of code, or less.</p>
47 <h3 id="CREATING_A_DEMO">CREATING A DEMO</h3>
48 <div id="CREATING_A_DEMO_CONTENT">
49 <p>You can see the final version of the demo code by doing:</p>
51 <pre> perl -MSDL::Tutorial::LunarLander=lander.pl -e1
55 <p>this will create all three files used in the tutorial:</p>
62 <h2 id="FIRST_VERSION">FIRST VERSION</h2>
63 <div id="FIRST_VERSION_CONTENT">
64 <p>We'll start with a text version of the game.</p>
65 <p>"What?", you may ask. "I thought it was a SDL tutorial".</p>
66 <p>Yes, it is -- thank you for reminding me. But we'll leave the SDL part for
67 later. We must build the game logic first!</p>
68 <p>One of the traps of game programming is focusing too much on the interface.
69 If we start with a simpler simulation, we can worry with the presentation
71 <p>So, here's the initial code:</p>
78 my $height = 1000; # m
79 my $velocity = 0; # m/s
80 my $gravity = 1; # m/s^2
84 while ( $height > 0 ) {
85 print "at $t s height = $height m, velocity = $velocity m/s\n";
87 $height = $height - $velocity;
88 $velocity = $velocity + $gravity;
92 if ( $velocity > 10 ) {
93 print "CRASH!!!\n";
95 print "You landed on the surface safely! :-D\n";
100 <p>Run the code and you'll see something like this:</p>
102 <pre> at 0 s height = 1000 m, velocity = 0 m/s
103 at 1 s height = 1000 m, velocity = 1 m/s
104 at 2 s height = 999 m, velocity = 2 m/s
105 at 3 s height = 997 m, velocity = 3 m/s
106 at 4 s height = 994 m, velocity = 4 m/s
107 at 5 s height = 990 m, velocity = 5 m/s
109 at 43 s height = 97 m, velocity = 43 m/s
110 at 44 s height = 54 m, velocity = 44 m/s
111 at 45 s height = 10 m, velocity = 45 m/s
117 <p>"What happened? How do I control the ship???"</p>
120 <h2 id="CONTROLLING_THE_SHIP">CONTROLLING THE SHIP</h2>
121 <div id="CONTROLLING_THE_SHIP_CONTENT">
122 <p>The problem with our first spaceship is that it had no controls!</p>
123 <p>So, let's fix this problem, making the spaceship scriptable. (We
124 could write some code to handle keyboard and joysticks now, but
125 an scriptable spaceship will be easier to start. Remember, focus
126 on the game logic!)</p>
127 <p>So, create add this simple script to the end of your file:</p>
130 at 41s, accelerate 10 m/s^2 up
138 <p>The script is straightforward: it simply states a time when we
139 will push the spaceship up with a given acceleration. It accepts
140 free text: any two numbers you type will work.</p>
141 <p>We can parse the script using this regular expression:</p>
143 <pre> my $script_re = qr/(\d+) \D+ (\d+)/x;
147 <p>And we can build a hash of ( time => acceleration ) with:</p>
149 <pre> my %up = map { $_ =~ $script_re } <DATA>;
153 <p>So the middle section of the program will become:</p>
155 <pre> my $script_re = qr/(\d+) \D+ (\d+)/x;
156 my %up = map { $_ =~ $script_re } <DATA>;
158 while ( $height > 0 ) {
159 print "at $t s height = $height m, velocity = $velocity m/s\n";
163 print "(accellerating $a m/s^2)\n";
164 $velocity = $velocity - $a;
167 $height = $height - $velocity;
168 $velocity = $velocity + $gravity;
175 <p>Try to run the program, and the ship should land safely:</p>
177 <pre> ./lunar.pl autopilot.txt
178 at 0 s height = 1000 m, velocity = 0 m/s
179 at 1 s height = 1000 m, velocity = 1 m/s
180 at 2 s height = 999 m, velocity = 2 m/s
181 at 3 s height = 997 m, velocity = 3 m/s
182 at 4 s height = 994 m, velocity = 4 m/s
183 at 5 s height = 990 m, velocity = 5 m/s
185 at 54 s height = 19 m, velocity = 4 m/s
186 at 55 s height = 15 m, velocity = 5 m/s
187 at 56 s height = 10 m, velocity = 6 m/s
188 at 57 s height = 4 m, velocity = 7 m/s
190 You landed on the surface safely! :-D
197 <h2 id="HOW_ABOUT_THE_GRAPHICS">HOW ABOUT THE GRAPHICS?</h2>
198 <div id="HOW_ABOUT_THE_GRAPHICS_CONTENT">
199 <p>Okay, okay... now that we have a working prototype, we can work on
200 the graphics. But, first of all, we'll need...</p>
203 <h3 id="THE_GRAPHICS">THE GRAPHICS</h3>
204 <div id="THE_GRAPHICS_CONTENT">
205 <p>Yes, the graphics.</p>
206 <p>We won't use anything fancy here, just two images: a large one, for
207 the background, and a smaller one for the spaceship.</p>
208 <p>Create the images using the Gimp, or use the images provided by
209 this tutorial; Save these images in a subdirectory called "images":
210 ("<code>images/background.jpg</code>" and "<code>images/ship.png</code>").</p>
213 <h2 id="USING_SDL">USING SDL</h2>
214 <div id="USING_SDL_CONTENT">
215 <p>First step: use the required libraries:</p>
217 <pre> use SDL; #needed to get all constants
224 <p>Second step: initialize <code>SDL::App</code>:</p>
226 <pre> my $app = SDL::App->new(
227 -title => "Lunar Lander",
235 <p>Third step: load the images and create the necessary "rectangles":</p>
237 <pre> my $background = SDL::Surface->new( -name => 'images/background.jpg', );
238 my $ship = SDL::Surface->new( -name => 'images/ship.png', );
240 my $background_rect = SDL::Rect->new(
241 -height => $background->height(),
242 -width => $background->width(),
245 my $ship_rect = SDL::Rect->new(
246 -height => $ship->height(),
247 -width => $ship->width(),
252 <p>Fourth step: create a sub to draw the spaceship and background:</p>
255 my ( $x, $y ) = @_; # spaceship position
257 # fix $y for screen resolution
258 $y = 450 * ( 1000 - $y ) / 1000;
261 $background->blit( $background_rect, $app, $background_rect );
264 my $ship_dest_rect = SDL::Rect->new(
265 -height => $ship->height(),
266 -width => $ship->width(),
271 $ship->blit( $ship_rect, $app, $ship_dest_rect );
273 $app->update($background_rect);
278 <p>Note that this sub first combines all the bitmaps, using a blit
279 ("Block Image Transfer") operation -- which is quite fast, but does
280 not update the display.</p>
281 <p>The combined image is displayed in the last line. This process of
282 combining first, and displaying later, avoids that annoying fading
283 between cycles ("flickering").</p>
284 <p>Finally, add the following lines to the end of the main loop, so that
285 we call the <code>draw()</code> function with the correct spaceship
288 <pre> while ( $height > 0 ) {
292 draw( 100, $height );
299 <p>Run the program and watch the spaceship landing safely on the surface
303 <h1 id="COPYRIGHT_amp_LICENSE">COPYRIGHT & LICENSE</h1><p><a href="#TOP" class="toplink">Top</a></p>
304 <div id="COPYRIGHT_amp_LICENSE_CONTENT">
305 <p>Copyright 2009 Nelson Ferraz, all rights reserved.</p>
306 <p>This program is free software; you can redistribute it and/or modify it
307 under the same terms as Perl itself.</p>