SDL::Tutorial::LunarLaUser(Contributed Perl DocumSDL::Tutorial::LunarLander(3)NAME
Lunar Lander - a small tutorial on Perl SDL
INTRODUCTION
This is a quick introduction to Games, Perl, and SDL (Simple
DirectMedia Layer, a cross-platform multimedia programming library).
We'll write a small game -- Lunar Lander -- in 100 lines of code, or
less.
CREATING A DEMO
You can see the final version of the demo code by doing:
perl -MSDL::Tutorial::LunarLander=lander.pl -e1
this will create all three files used in the tutorial:
FIRST VERSION
We'll start with a text version of the game.
"What?", you may ask. "I thought it was a SDL tutorial".
Yes, it is -- thank you for reminding me. But we'll leave the SDL part
for later. We must build the game logic first!
One of the traps of game programming is focusing too much on the
interface. If we start with a simpler simulation, we can worry with
the presentation later.
So, here's the initial code:
#!/usr/bin/perl
use strict;
use warnings;
my $height = 1000; # m
my $velocity = 0; # m/s
my $gravity = 1; # m/s^2
my $t = 0;
while ( $height > 0 ) {
print "at $t s height = $height m, velocity = $velocity m/s\n";
$height = $height - $velocity;
$velocity = $velocity + $gravity;
$t = $t + 1;
}
if ( $velocity > 10 ) {
print "CRASH!!!\n";
} else {
print "You landed on the surface safely! :-D\n";
}
Run the code and you'll see something like this:
at 0 s height = 1000 m, velocity = 0 m/s
at 1 s height = 1000 m, velocity = 1 m/s
at 2 s height = 999 m, velocity = 2 m/s
at 3 s height = 997 m, velocity = 3 m/s
at 4 s height = 994 m, velocity = 4 m/s
at 5 s height = 990 m, velocity = 5 m/s
...
at 43 s height = 97 m, velocity = 43 m/s
at 44 s height = 54 m, velocity = 44 m/s
at 45 s height = 10 m, velocity = 45 m/s
CRASH!!!
"What happened? How do I control the ship???"
CONTROLLING THE SHIP
The problem with our first spaceship is that it had no controls!
So, let's fix this problem, making the spaceship scriptable. (We could
write some code to handle keyboard and joysticks now, but an scriptable
spaceship will be easier to start. Remember, focus on the game logic!)
So, create add this simple script to the end of your file:
__DATA__
at 41s, accelerate 10 m/s^2 up
at 43s, 10 m/s^2
at 45s, 10
at 47s, 10
at 49s, 10
The script is straightforward: it simply states a time when we will
push the spaceship up with a given acceleration. It accepts free text:
any two numbers you type will work.
We can parse the script using this regular expression:
my $script_re = qr/(\d+) \D+ (\d+)/x;
And we can build a hash of ( time => acceleration ) with:
my %up = map { $_ =~ $script_re } <DATA>;
So the middle section of the program will become:
my $script_re = qr/(\d+) \D+ (\d+)/x;
my %up = map { $_ =~ $script_re } <DATA>;
while ( $height > 0 ) {
print "at $t s height = $height m, velocity = $velocity m/s\n";
if ( $up{$t} ) {
my $a = $up{$t};
print "(accellerating $a m/s^2)\n";
$velocity = $velocity - $a;
}
$height = $height - $velocity;
$velocity = $velocity + $gravity;
$t = $t + 1;
}
That's it!
Try to run the program, and the ship should land safely:
./lunar.pl autopilot.txt
at 0 s height = 1000 m, velocity = 0 m/s
at 1 s height = 1000 m, velocity = 1 m/s
at 2 s height = 999 m, velocity = 2 m/s
at 3 s height = 997 m, velocity = 3 m/s
at 4 s height = 994 m, velocity = 4 m/s
at 5 s height = 990 m, velocity = 5 m/s
...
at 54 s height = 19 m, velocity = 4 m/s
at 55 s height = 15 m, velocity = 5 m/s
at 56 s height = 10 m, velocity = 6 m/s
at 57 s height = 4 m, velocity = 7 m/s
You landed on the surface safely! :-D
Cool, but...
HOW ABOUT THE GRAPHICS?
Okay, okay... now that we have a working prototype, we can work on the
graphics. But, first of all, we'll need...
THE GRAPHICS
Yes, the graphics.
We won't use anything fancy here, just two images: a large one, for the
background, and a smaller one for the spaceship.
Create the images using the Gimp, or use the images provided by this
tutorial; Save these images in a subdirectory called "images":
(""images/background.jpg"" and ""images/ship.png"").
USING SDL
First step: use the required libraries:
use SDL; #needed to get all constants
use SDL::App;
use SDL::Surface;
use SDL::Rect;
Second step: initialize "SDL::App":
my $app = SDL::App->new(
-title => "Lunar Lander",
-width => 800,
-height => 600,
-depth => 32,
);
Third step: load the images and create the necessary "rectangles":
my $background = SDL::Surface->new( -name => 'images/background.jpg', );
my $ship = SDL::Surface->new( -name => 'images/ship.png', );
my $background_rect = SDL::Rect->new(
-height => $background->height(),
-width => $background->width(),
);
my $ship_rect = SDL::Rect->new(
-height => $ship->height(),
-width => $ship->width(),
);
Fourth step: create a sub to draw the spaceship and background:
sub draw {
my ( $x, $y ) = @_; # spaceship position
# fix $y for screen resolution
$y = 450 * ( 1000 - $y ) / 1000;
# background
$background->blit( $background_rect, $app, $background_rect );
# ship
my $ship_dest_rect = SDL::Rect->new(
-height => $ship->height(),
-width => $ship->width(),
-x => $x,
-y => $y,
);
$ship->blit( $ship_rect, $app, $ship_dest_rect );
$app->update($background_rect);
}
Note that this sub first combines all the bitmaps, using a blit ("Block
Image Transfer") operation -- which is quite fast, but does not update
the display.
The combined image is displayed in the last line. This process of
combining first, and displaying later, avoids that annoying fading
between cycles ("flickering").
Finally, add the following lines to the end of the main loop, so that
we call the "draw()" function with the correct spaceship coordinates:
while ( $height > 0 ) {
# ...
draw( 100, $height );
$app->delay(10);
}
That's it!
Run the program and watch the spaceship landing safely on the surface
of the moon.
COPYRIGHT & LICENSE
Copyright 2009 Nelson Ferraz, all rights reserved.
This program is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
perl v5.14.1 2011-07-18 SDL::Tutorial::LunarLander(3)
