GD::Polyline
GD::Polyline(3) User Contributed Perl Documentation GD::Polyline(3)
NAME
GD::Polyline - Polyline object and Polygon utilities (including
splines) for use with GD
SYNOPSIS
use GD;
use GD::Polyline;
# create an image
$image = new GD::Image (500,300);
$white = $image->colorAllocate(255,255,255);
$black = $image->colorAllocate( 0, 0, 0);
$red = $image->colorAllocate(255, 0, 0);
# create a new polyline
$polyline = new GD::Polyline;
# add some points
$polyline->addPt( 0, 0);
$polyline->addPt( 0,100);
$polyline->addPt( 50,125);
$polyline->addPt(100, 0);
# polylines can use polygon methods (and vice versa)
$polyline->offset(200,100);
# rotate 60 degrees, about the centroid
$polyline->rotate(3.14159/3, $polyline->centroid());
# scale about the centroid
$polyline->scale(1.5, 2, $polyline->centroid());
# draw the polyline
$image->polydraw($polyline,$black);
# create a spline, which is also a polyine
$spline = $polyline->addControlPoints->toSpline;
$image->polydraw($spline,$red);
# output the png
binmode STDOUT;
print $image->png;
DESCRIPTION
Polyline.pm extends the GD module by allowing you to create polylines.
Think of a polyline as "an open polygon", that is, the last vertex is
not connected to the first vertex (unless you expressly add the same
value as both points).
For the remainder of this doc, "polyline" will refer to a GD::Poly-
line, "polygon" will refer to a GD::Polygon that is not a polyline,
and "polything" and "$poly" may be either.
The big feature added to GD by this module is the means to create
splines, which are approximations to curves.
The Polyline Object
GD::Polyline defines the following class:
"GD::Polyline"
A polyline object, used for storing lists of vertices prior to
rendering a polyline into an image.
"new"
"GD::Polyline->new" class method
Create an empty polyline with no vertices.
$polyline = new GD::Polyline;
$polyline->addPt( 0, 0);
$polyline->addPt( 0,100);
$polyline->addPt( 50,100);
$polyline->addPt(100, 0);
$image->polydraw($polyline,$black);
In fact GD::Polyline is a subclass of GD::Polygon, so all polygon
methods (such as offset and transform) may be used on polylines.
Some new methods have thus been added to GD::Polygon (such as
rotate) and a few updated/modified/enhanced (such as scale) in
this module. See section "New or Updated GD::Polygon Methods"
for more info.
Note that this module is very "young" and should be considered subject
to change in future releases, and/or possibly folded in to the exist-
ing polygon object and/or GD module.
Updated Polygon Methods
The following methods (defined in GD.pm) are OVERRIDDEN if you use
this module.
All effort has been made to provide 100% backward compatibility, but
if you can confirm that has not been achieved, please consider that a
bug and let the the author of Polyline.pm know.
"scale"
"$poly->scale($sx, $sy, $cx, $cy)" object method -- UPDATE to
GD::Polygon::scale
Scale a polything in along x-axis by $sx and along the y-axis by
$sy, about centery point ($cx, $cy).
Center point ($cx, $cy) is optional -- if these are omitted, the
function will scale about the origin.
To flip a polything, use a scale factor of -1. For example, to
flip the polything top to bottom about line y = 100, use:
$poly->scale(1, -1, 0, 100);
New Polygon Methods
The following methods are added to GD::Polygon, and thus can be used
by polygons and polylines.
Don’t forget: a polyline is a GD::Polygon, so GD::Polygon methods like
offset() can be used, and they can be used in GD::Image methods like
filledPolygon().
"rotate"
"$poly->rotate($angle, $cx, $cy)" object method
Rotate a polything through $angle (clockwise, in radians) about
center point ($cx, $cy).
Center point ($cx, $cy) is optional -- if these are omitted, the
function will rotate about the origin
In this function and other angle-oriented functions in GD::Poly-
line, positive $angle corrensponds to clockwise rotation. This
is opposite of the usual Cartesian sense, but that is because the
raster is opposite of the usual Cartesian sense in that the
y-axis goes "down".
"centroid"
"($cx, $cy) = $poly->centroid($scale)" object method
Calculate and return ($cx, $cy), the centroid of the vertices of
the polything. For example, to rotate something 180 degrees
about it’s centroid:
$poly->rotate(3.14159, $poly->centroid());
$scale is optional; if supplied, $cx and $cy are multiplied by
$scale before returning. The main use of this is to shift an
polything to the origin like this:
$poly->offset($poly->centroid(-1));
"segLength"
"@segLengths = $poly->segLength()" object method
In array context, returns an array the lengths of the segments in
the polything. Segment n is the segment from vertex n to vertex
n+1. Polygons have as many segments as vertices; polylines have
one fewer.
In a scalar context, returns the sum of the array that would have
been returned in the array context.
"segAngle"
"@segAngles = $poly->segAngle()" object method
Returns an array the angles of each segment from the x-axis.
Segment n is the segment from vertex n to vertex n+1. Polygons
have as many segments as vertices; polylines have one fewer.
Returned angles will be on the interval 0 <= $angle < 2 * pi and
angles increase in a clockwise direction.
"vertexAngle"
"@vertexAngles = $poly->vertexAngle()" object method
Returns an array of the angles between the segment into and out
of each vertex. For polylines, the vertex angle at vertex 0 and
the last vertex are not defined; however $vertexAngle[0] will be
undef so that $vertexAngle[1] will correspond to vertex 1.
Returned angles will be on the interval 0 <= $angle < 2 * pi and
angles increase in a clockwise direction.
Note that this calculation does not attempt to figure out the
"interior" angle with respect to "inside" or "outside" the poly-
gon, but rather, just the angle between the adjacent segments in
a clockwise sense. Thus a polygon with all right angles will
have vertex angles of either pi/2 or 3*pi/2, depending on the way
the polygon was "wound".
"toSpline"
"$poly->toSpline()" object method & factory method
Create a new polything which is a reasonably smooth curve using
cubic spline algorithms, often referred to as Bezier curves. The
"source" polything is called the "control polything". If it is a
polyline, the control polyline must have 4, 7, 10, or some number
of vertices of equal to 3n+1. If it is a polygon, the control
polygon must have 3, 6, 9, or some number of vertices of equal to
3n.
$spline = $poly->toSpline();
$image->polydraw($spline,$red);
In brief, groups of four points from the control polyline are
considered "control points" for a given portion of the spline:
the first and fourth are "anchor points", and the spline passes
through them; the second and third are "director points". The
spline does not pass through director points, however the spline
is tangent to the line segment from anchor point to adjacent
director point.
The next portion of the spline reuses the previous portion’s last
anchor point. The spline will have a cusp (non-continuous slope)
at an anchor point, unless the anchor points and its adjacent
director point are colinear.
In the current implementation, toSpline() return a fixed number
of segments in the returned polyline per set-of-four control
points. In the future, this and other parameters of the algo-
rithm may be configurable.
"addControlPoints"
"$polyline->addControlPoints()" object method & factory method
So you say: "OK. Splines sound cool. But how can I get my
anchor points and its adjacent director point to be colinear so
that I have a nice smooth curves from my polyline?" Relax! For
The Lazy: addControlPoints() to the rescue.
addControlPoints() returns a polyline that can serve as the con-
trol polyline for toSpline(), which returns another polyline
which is the spline. Is your head spinning yet? Think of it
this way:
+ If you have a polyline, and you have already put your con-
trol points where you want them, call toSpline() directly.
Remember, only every third vertex will be "on" the spline.
You get something that looks like the spline "inscribed"
inside the control polyline.
+ If you have a polyline, and you want all of its vertices on
the resulting spline, call addControlPoints() and then
toSpline():
$control = $polyline->addControlPoints();
$spline = $control->toSpline();
$image->polyline($spline,$red);
You get something that looks like the control polyline
"inscribed" inside the spline.
Adding "good" control points is subjective; this particular algo-
rithm reveals its author’s tastes. In the future, you may be
able to alter the taste slightly via parameters to the algorithm.
For The Hubristic: please build a better one!
And for The Impatient: note that addControlPoints() returns a
polyline, so you can pile up the the call like this, if you’d
like:
$image->polyline($polyline->addControlPoints()->toSpline(),$mauve);
New GD::Image Methods
"polyline"
"$image->polyline(polyline,color)" object method
$image->polyline($polyline,$black)
This draws a polyline with the specified color. Both real color
indexes and the special colors gdBrushed, gdStyled and gdStyled-
Brushed can be specified.
Neither the polyline() method or the polygon() method are very
picky: you can call either method with either a GD::Polygon or a
GD::Polyline. The method determines if the shape is "closed" or
"open" as drawn, not the object type.
"polydraw"
"$image->polydraw(polything,color)" object method
$image->polydraw($poly,$black)
This method draws the polything as expected (polygons are closed,
polylines are open) by simply checking the object type and call-
ing either $image->polygon() or $image->polyline().
Examples
Please see file "polyline-examples.pl" that is included with the dis-
tribution.
See Also
For more info on Bezier splines, see http://www.webrefer-
ence.com/dlab/9902/bezier.html.
Future Features
On the drawing board are additional features such as:
- polygon winding algorithms (to determine if a point is "inside" or "outside" the polygon)
- new polygon from bounding box
- find bounding polygon (tightest fitting simple convex polygon for a given set of vertices)
- addPts() method to add many points at once
- clone() method for polygon
- functions to interwork GD with SVG
Please provide input on other possible features you’d like to see.
Author
This module has been written by Daniel J. Harasty. Please send ques-
tions, comments, complaints, and kudos to him at harasty@cpan.org.
Thanks to Lincoln Stein for input and patience with me and this, my
first CPAN contribution.
Copyright Information
The Polyline.pm module is copyright 2002, Daniel J. Harasty. It is
distributed under the same terms as Perl itself. See the "Artistic
License" in the Perl source code distribution for licensing terms.
The latest version of Polyline.pm is available at your favorite CPAN
repository and/or along with GD.pm by Lincoln D. Stein at
http://stein.cshl.org/WWW/software/GD.
perl v5.8.8 2006-08-23 GD::Polyline(3)
