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      my $collision = 0;
      for my $old (@occupied) {
	last if $old->right + 2 < $g->left;
	next if $old->bottom < $pos;
	next if $old->top > $bottom;
	$collision = $old;
	last;
      }
      last unless $collision;
      if ($bump_direction > 0) {
	$pos += $collision->height + 2;                    # collision, so bump
      } else {
	$pos -= $g->height + 2;
      }
    }

    $g->move(0,$pos);
    @occupied = sort { $b->right <=> $a->right } ($g,@occupied);
    $rightmost = $g->right if $g->right > $rightmost;
  }
}

# return list of glyphs -- only after they are laid out
sub glyphs { shift->{glyphs} }

# height is determined by the layout, and cannot be externally controlled
sub height {
  my $self = shift;
  return $self->{cache_height} if defined $self->{cache_height};

  $self->layout;
  my $glyphs = $self->{glyphs} or croak "Can't lay out";
  return 0 unless @$glyphs;

  my ($topmost)    = sort { $a->top    <=> $b->top }    @$glyphs;
  my ($bottommost) = sort { $b->bottom <=> $a->bottom } @$glyphs;

  return $self->{cache_height} = $bottommost->bottom - $topmost->top;
}

sub make_factory {
  my ($class,$type,@options) = @_;
  Ace::Graphics::GlyphFactory->new($type,@options);
}


1;
__END__

=head1 NAME

Ace::Graphics::Track - PNG graphics of Ace::Sequence::Feature objects

=head1 SYNOPSIS

  use Ace::Sequence;
  use Ace::Graphics::Panel;

  my $db     = Ace->connect(-host=>'brie2.cshl.org',-port=>2005) or die;
  my $cosmid = Ace::Sequence->new(-seq=>'Y16B4A',
				  -db=>$db,-start=>-15000,-end=>15000) or die;

  my @transcripts = $cosmid->transcripts;

  my $panel = Ace::Graphics::Panel->new(
				      -segment => $cosmid,
				      -width  => 800
				     );


  my $track = $panel->add_track('transcript'
   		                -fillcolor =>  'wheat',
				-fgcolor   =>  'black',
				-bump      =>  +1,
				-height    =>  10,
				-label     =>  1);
  foreach (@transcripts) {
     $track->add_feature($_);
  }

  my $boxes = $panel->boxes;
  print $panel->png;


=head1 DESCRIPTION

The Ace::Graphics::Track class is used by Ace::Graphics::Panel to lay
out a set of sequence features using a uniform glyph type. You will
ordinarily work with panels rather than directly with tracks.

=head1 METHODS

This section describes the class and object methods for
Ace::Graphics::Panel.

=head2 CONSTRUCTORS

There is only one constructor, the new() method.  It is ordinarily
called by Ace::Graphics::Panel, and not in end-developer code.

=over 4

=item $track = Ace::Graphics::Track->new($glyph_name,$features,@options)

The new() method creates a new track object from the provided glyph
name and list of features.  The arguments are similar to those in
Ace::Graphics::Panel->new().

If successful new() will return a new Ace::Graphics::Track.
Otherwise, it will return undef.

If the specified glyph name is not a valid one, new() will throw an
exception.

=back

=head2 OBJECT METHODS

Once a track is created, the following methods can be invoked.

=over 4

=item $track->add_feature($feature)

This adds a new feature to the track.  The feature can either be a
single object that implements the Bio::SeqFeatureI interface (such as
an Ace::Sequence::Feature or Das::Segment::Feature), or can be an
anonymous array containing a set of related features.  In the latter
case, the track will attempt to keep the features in the same
horizontal band and will not allow any other features to overlap.

=item $track->add_group($group)

This behaves the same as add_feature(), but requires that its argument
be an array reference containing a list of grouped features.

=item $track->draw($gd,$left,$top)

Render the track on a previously-created GD::Image object.  The $left
and $top arguments indicate the position at which to start rendering.

=item $boxes = $track->boxes($left,$top)

=item @boxes = $track->boxes($left,$top)

Return an array of array references indicating glyph coordinates for
each of the render features.  $left and $top indicate the offset for
the track on the image plane.  In a scalar context, this method
returns an array reference of glyph coordinates.  In a list context,
it returns the list itself.

See Ace::Graphics::Panel->boxes() for the format of the result.

=back

=head2 ACCESSORS

The following accessor methods provide access to various attributes of
the track object.  Called with no arguments, they each return the
current value of the attribute.  Called with a single argument, they
set the attribute and return its previous value.

Note that in most cases you must change attributes before the track's
layout() method is called.

   Accessor Name      Description
   -------------      -----------

   scale()	      Get/set the track scale, measured in pixels/bp
   lineheight()	      Get/set the height of each glyph, pixels
   width()	      Get/set the width of the track
   bump()	      Get/set the bump direction

=head2 INTERNAL METHODS

The following methods are used internally, but may be useful for those
implementing new glyph types.

=over 4

=item $glyphs = $track->layout

Layout the features, and return an anonymous array of
Ace::Graphics::Glyph objects that have been created and correctly
positioned.

Because layout is an expensive operation, calling this method several
times will return the previously-cached result, ignoring any changes
to track attributes.

=item $height = $track->height

Invokes layout() and returns the height of the track.

=item $glyphs = $track->glyphs

Returns the glyph cache.  Returns undef before layout() and a
reference to an array of glyphs after layout().

=item $factory = $track->make_factory(@options)