Java：AffineTransform旋转多边形，然后获取其点

Question

I'd like to draw a Polygon and rotate it with AffineTransform like so.

float theta = 90;
Polygon p = new Polygon(new int[]{0, 4, 4, 0}, new int[]{0, 0, 4, 4}, 4);
AffineTransform transform = new AffineTransform();
transform.rotate(Math.toRadians(theta), p.xpoints[0], p.ypoints[0]);
Shape transformed = transform.createTransformedShape(p);
g2.fill(transformed);

However, then I'd like to be able to access the points (transformed.xpoints[0]) in the same way I did as a Polygon. One way to look at this would be transforming the Shape into a Polygon--but as far as I know this isn't possible.

What's the best option?

As a sidenote: this is an exercise in creating a fractal tree made of 4-sided Polygons (rectangles). I've chosen to use Polygons in order to anchor branches to the top left and top right points, respectively. If this is needlessly complicated, let me know.

Answer 1

You can use the AffineTransform to transform the individual points as well, like so:

Point2D[] srcPoints = new Point2D[] { new Point(0, 0), new Point(4, 0), new Point(4, 4), new Point(4, 0) };
Point2D[] destPoints = new Point2D[4];
transform.transform(srcPoints, 0, destPoints, 0, 4);

The resulting destPoints array then looks like this:

[Point2D.Float[-0.0, 0.0], Point2D.Float[-0.0, 4.0], Point2D.Float[-4.0, 4.0], Point2D.Float[-0.0, 4.0]]

Answer 2

You can get the coordinates form the path returned by createTransformedShape(...) .

Path2D.Double transformed = (Path2D.Double) transform.createTransformedShape(p);

List<Double> xpointsList = new ArrayList<>();
List<Double> ypointsList = new ArrayList<>();
PathIterator pi = transformed.getPathIterator(null);

while(!pi.isDone()){
    double[] coords = new double[6];
    int type = pi.currentSegment(coords);
    if(type == PathIterator.SEG_MOVETO || type == PathIterator.SEG_LINETO){ // The only types we're interested in given the original shape
        xpointsList.add(coords[0]);
        ypointsList.add(coords[1]);
    }
    pi.next();
}

I haven't done this in a while, there may be an easier way to achieve what you want. Also, the casting to Path2D.Double is not ideal.

Answer 3

Its been a while, but I couldn't resist putting this here for those who will have this issue later on.

So how about we just use a teenie-weenie bit of math instead of AffineTransform?

public class ROTOR {

/**
 * Method for rotating a Point object in the XY plane or in any plane
 * parallel to the XY plane. The Point object to be rotated and the one
 * about which it is rotating must have the same Z coordinates.
 *
 * @param p a Point object in the xy plane The z coordinates of both Point
 * objects must be either the same or must both be zero.
 * @param cen a Point object about which the rotation of the first Point
 * object will occur.
 * @param angle the angle of rotation
 * @return
 */
     public static Point planarXYRotate(Point p,Point cen,double angle){

    double sin = Math.sin(angle);
    double cos = Math.cos(angle);
    double X = p.x*cos-p.y*sin+cen.x*(1-cos)+cen.y*sin;
    double Y = p.x*sin+p.y*cos+cen.y*(1-cos)-cen.x*sin;
    return new Point( (int) X, (int) Y );


    }//end method



/**
 * 
 * @param polygon The polygon to rotate
 * @param origin The point about which to rotate it.
 * @param angle The angle of rotation.
 * @return a new Polygon rotated through the specified angle and about the said point.
 */
public static Polygon rotate(Polygon polygon , Point origin , double angle){

    Point cen = new Point(origin.x , origin.y);
    Polygon newPolygon = new Polygon();

    for(int i=0;i<polygon.npoints;i++){
        Point p = new Point( polygon.xpoints[i] , polygon.ypoints[i] );
        Point point = ROTOR.planarXYRotate(p, cen, angle);
        newPolygon.addPoint((int) point.x, (int) point.y);

    }

    return newPolygon;
}

}

You may then easily obtain your rotated polygon and get its points as you wish, using:

Polygon rotatedPolygon = ROTOR.rotate(polygon, origin, angle);