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examples/pendulum.pl  view on Meta::CPAN

# This example uses b2DistanceJoint to simulate a pendulum.
use strict;
use warnings;
use Box2D;
use SDL;
use SDL::Video;
use SDLx::App;

my $segments = $ARGV[0] || 2;

# The number of steps to apply to each frame,
# This seems to make joint correction lose less energy.
my $precision = 40;

# pixels
my $width  = 450;
my $height = 450;

# pixels per meter
my $ppm = 30;

# meters per pixel
my $mpp = 1.0 / $ppm;

# frames per second
my $fps      = 60.0;
my $timestep = 1.0 / $fps;

# velocity iterations
my $vIters = 8;

# position iterations
my $pIters = 8;

my $gravity = Box2D::b2Vec2->new( 0, -8.0 / $precision**2 );

# no sleep. don't lose energy.
my $world = Box2D::b2World->new( $gravity, 0 );

my $rodColor  = 0x00CC70FF;
my $pathColor = 0xFFFFCFFF;

my $pivot = {
    x0     => s2w( $width / 2 ),
    y0     => s2w( $height / 2 ),
    radius => s2w(4),
    color  => 0x5CCC00FF,
};
$pivot->{body}   = make_static_circle( @$pivot{qw( x0 y0 radius )} );
$pivot->{anchor} = Box2D::b2Vec2->new( @$pivot{qw( x0 y0 )} );

my $prev_pivot = $pivot;
my @bobs;
for ( 1 .. $segments ) {

    my $bob = {
        y0 => s2w( $height / 2 + .9 * $height * $_ / ( 2 * $segments ) ),
        x0 => s2w( $width / 2 + 1 ),
        radius => s2w(6),
        color  => 0xCC005CFF,
    };
    $bob->{body}   = make_dynamic_circle( @$bob{qw( x0 y0 radius )} );
    $bob->{anchor} = Box2D::b2Vec2->new( @$bob{qw( x0 y0 )} );

    my $jointDef = Box2D::b2DistanceJointDef->new();
    $jointDef->Initialize(
        $prev_pivot->{body},   $bob->{body},
        $prev_pivot->{anchor}, $bob->{anchor}
    );

    # high frequency means less energy lost from joint correction
    $jointDef->frequencyHz( 1 / $timestep );
    $jointDef->dampingRatio(0);
    $world->CreateJoint($jointDef);

    $prev_pivot = $bob;
    push @bobs, $bob;
}

my $app = SDLx::App->new(
    width  => $width,
    height => $height,
    dt     => $timestep,
    min_t  => $timestep / 2,
    flags  => SDL_DOUBLEBUF | SDL_HWSURFACE,
    eoq    => 1,
);
my $bg = $app->duplicate;
$bg->draw_rect( [ 0, 0, $width, $height ], [ 0, 0, 0, 255 ] );

my $realFps = $fps;
my $frames  = 1;