Writing Games in Perl - Part 5 - Creating a Goal

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Following
posts 1,
2,
3 and
4
on the subject of writing games in Perl, now we are going to add
a goal to our game.

Currently we have a bouncing ball with that collides in walls and
have a camera following it. Now we are about to add a goal to the
game. The idea is that you should get the ball to hit some specific
point, considering the gravity and the 100% efficient bounce, making
the ball go through some small places might be an interesting
challenge.

The first thing we're going to do is change the walls
configuration, so we make a more challenging setup, currently we have
a box with a wall of half the height in the middle, let's make it a
bit more interesting, let's change the walls initialization code to
the following.


foreach my $rect ( Rect->new({ x => 0,
y => 0,
w => 20,
h => 1 }), # left wall
Rect->new({ x => 0,
y => 0,
h => 20,
w => 1 }), # bottom wall
Rect->new({ x => 20,
y => 0,
h => 20,
w => 1 }), # right wall
Rect->new({ x => 0,
y => 20,
w => 21,
h => 1 }), # top wal
Rect->new({ x => 7,
y => 0,
h => 9,
w => 1 }), # middle-left bottom
Rect->new({ x => 7,
y => 11,
h => 9,
w => 1 }), # middle-left top
Rect->new({ x => 12,
y => 0,
h => 9,
w => 1 }), # middle-right bottom
Rect->new({ x => 12,
y => 11,
h => 9,
w => 1 }), # middle-right top
) {
# ...
}

This creates two small passages in the middle of two vertical
walls, not really hard, but kinda entertaining to get the ball to go
through those. But in order to make it actually hard, let's add
another wall:


Rect->new({ x => 9.2,
y => 11,
h => 1,
w => 1.6 }), # chamber

Now we have a small chamber created between the two vertical
lines. It's kinda tricky to get the ball in there, I personally took
some minutes.

But while I was testing this map, a bug appeared, and this is
actually an important bug. Since the collision was pretty simplified
to handle just one wall at the beginning, I was inadvertedly
positioning the ball at the target destination after it bounced. This
was ok when I had just one wall, but when I have more, and more
importantly, when they are really close to each other, I might
position the ball over another wall when detecting a collision, and
that just, well, you have a ball inside a wall, unless you're watching
the X Files, this can't be good.

The problem, as you might have noticed, happens when I calculate
the target position after the bounce, so what we're going to do is
simply stop trying to guess that. We're going to position the ball in
the exactly spot before the collision with the bouncing velocities and
recalculate the whole frame from that instant on.

This will actually mean a simplification of the code, that will
look like:


foreach my $wall (@{$self->walls}) {
if (my $coll = collide($ball, $wall, $frame_elapsed_time)) {
# need to place the ball in the result after the bounce given
# the time elapsed after the collision.
my $collision_remaining_time = $frame_elapsed_time - $coll->time;
my $movement_before_collision_h = $ball->vel_h * ($coll->time - 0.001);
my $movement_before_collision_v = $ball->vel_v * ($coll->time - 0.001);
$ball->cen_h($ball->cen_h + $movement_before_collision_h);
$ball->cen_v($ball->cen_v + $movement_before_collision_v);
if ($coll->axis eq 'x') {
$ball->vel_h($ball->vel_h * -1);
} elsif ($coll->axis eq 'y') {
$ball->vel_v($ball->vel_v * -1);
} elsif (ref $coll->axis eq 'ARRAY') {
my ($xv, $yv) = @{$coll->bounce_vector};
$ball->vel_h($xv);
$ball->vel_v($yv);
} else {
warn 'BAD BALL!';
$ball->vel_h($ball->vel_h * -1);
$ball->vel_v($ball->vel_v * -1);
}
return $self->handle_frame($oldtime + ($coll->time*1000), $now);
}
}
$ball->time_lapse($oldtime, $now);

Now, to add a goal, we're going to add another set of objects, the
goal itself, which is simply a point, and the view, which I'm also
going to reuse the filled rect view. First I'm going to create a Point
object akin to the Rect I have created earlier.


package BouncingBall::Event::Point;
use Moose;

has x => ( is => 'ro',
isa => 'Num',
required => 1 );
has y => ( is => 'ro',
isa => 'Num',
required => 1 );

Now I'm going to add that point object to the controller as an
attribute:


use aliased 'BouncingBall::Event::Point';

has 'goal' => ( isa => 'rw',
isa => Point );

And now initialize both the goal and the view for it.


$self->goal(Point->new({ x => 10, y => 12.5 }));
my $goal_view = FilledRect->new({ color => 0xFFFF00,
camera => $camera,
main => $self->main_surface,
x => $self->goal->x - 0.1,
y => $self->goal->y - 0.1,
w => 0.2,
h => 0.2 });

$self->views([]);
push @{$self->views}, $background, $ball_view, $goal_view;

Ok, now that we can see our goal, we just need to detect when the
goal was achieved:


sub collide_goal {
my ($ball, $goal, $time) = @_;
my $rect = hash2point({ x => $goal->x, y => $goal->y });
my $circ = hash2circle({ x => $ball->cen_h, y => $ball->cen_v,
radius => $ball->radius,
xv => $ball->vel_h,
yv => $ball->vel_v });
return dynamic_collision($circ, $rect, interval => $time);
}
#...
sub reset_ball {
my ($self) = @_;
$self->ball(Ball->new());
}
#...
if (collide_goal($ball, $self->goal, $frame_elapsed_time)) {
$self->reset_ball();
}

Ok, not very exciting, but something does happen, and that's a
first step.

As usual, follows a small video of the game:

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About this Entry

This page contains a single entry by Daniel Ruoso published on April 11, 2010 10:51 PM.

Writing Games in Perl - Part 4 - Implementing a Camera was the previous entry in this blog.

Writing Games in Perl - Part 6 - Math for dummies is the next entry in this blog.

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