=item get_corners

Return the corners of the visible part of the canvas as a list:
(minx, miny, maxx, maxy).  This will not work if the scrollregion is
smaller than the visible region.

=cut

sub get_corners {
    my $c = shift;
    $c->view_to_coords($c->xview, $c->yview);
}

=item view_to_coords($xv1,$xv2,$yv1,$yv2)

Convert the result of xview/yview to coordinates. See comment in
C<get_corners>. The result is the list ($x1,$y1,$x2,$y2). (Please note
that the order is not the same as in the argument list! This is too
make it easier to pass the arguments from xview and yview and to use
the result in a scrollregion configuration.)

=cut

sub view_to_coords {
    my($c,$xv1,$xv2,$yv1,$yv2) = @_;
    my(@scrollregion) = ($Tk::VERSION == 800.017
                         ? $c->cget(-scrollregion)
                         : @{$c->cget(-scrollregion)});
    my $width  = ($scrollregion[2]-$scrollregion[0]);
    my $height = ($scrollregion[3]-$scrollregion[1]);
    ($xv1 * $width + $scrollregion[0],
     $yv1 * $height + $scrollregion[1],
     $xv2 * $width + $scrollregion[0],
     $yv2 * $height + $scrollregion[1],
    );
}