Programming Web Graphics with Perl and GNU Softwar

Name

toPt( ) — define a polygon’s vertices using relative coordinates

Synopsis

GD::Polygon::toPt(dx, dy)

This method will allow you to specify the points of a polygon relative to a starting point, just like Logo-type turtle graphics. Use addPt( ) to define the first point in the polygon, then subsequent calls to toPt( ) will create new points by adding the dx and dy parameters to the x and y values of the previous point. Note that the polygon object still stores only the absolute values of the coordinates, so getPt( ) will return the same value as if you had added the point with addPt( ).

To draw a parallelogram whose vertices are at (10, 10), (20, 20), (30, 20), and (20, 10), use:

my $parallelogram = new GD::Polygon;       # create a new polygon
$parallelogram->addPt(10, 10);             # add the first point
$poly->toPt(10, 10);                       # add (20, 20)
$poly->toPt(10, 0);                        # add (30, 20)
$poly->toPt(-10, -10);                     # add (20, 10)
$image->polygon($parallelogram, $black);   # draw the polygon in black