Programming Web Graphics with Perl and GNU Softwar

Name

addPt( ) — add a vertex to a polygon object

Synopsis

GD::Polygon::addPt(x, y)

This method adds a new vertex to a polygon object. Polygons must have at least three points and must be constructed one point at a time.

The GD::Polygonobject stores all of the points in the polygon, but it is also often useful to store the points separately as a string of coordinate pairs separated by whitespace. This format is easily written to or read from a file, and it is also consistent with the syntax that many web servers use to implement image maps. It is also a more concise and flexible way of constructing polygons than stringing together long series of addPt( ) method calls. It is simple to extract the points and construct a polygon from the string:

# Create a new polygon object
#
my $polywolly = new GD::Polygon;

# Store the vertices of the polygon in a string consisting of 
# x, y coordinate pairs separated by an arbitrary amount of whitespace
#
my $polyPoints = "0,0 10,10 30,10 20,0";

# The loop should iterate over each x,y pair
#
foreach (split /\s+/, $polyPoints) {
    $polywolly->addPt(split /,/, $_); # Split each pair into a two-value list
}

# Now draw the polygon in a (previously allocated) red color
#
$image->polygon($polywolly,$red);

Of course, in some cases, you will be creating a polygon dynamically, and won’t necessarily know all of the vertices at once. It still may be valuable to have a portable string representation of the polygon after the fact. See the vertices( ) method description ...