Moving Shapes along an arc path with java graphics

问题

Pardon my naivety, its my first time here and first time dealing with animation of graphics in java. I'm trying to accomplish an animation of star shapes that moves along a sort of arc(trying to simulate an orbit on a 2d wise). The orbit Action is used with a Timer to animate the stars.

Simply put, I have drawn several stars at various positions in a jpanel. The translation of the stars y position depends on how far that star is away from the the x axis of decline which is initialized to 300(the center of the jpanel). The closer a star is to the point of decline, the less their y position is going to change. When a star reaches or passes the right side of the panel(or goes out of view), reset to the left side at its original y position(ugly, i know). I chose to do it this way since the stars are placed at random. I cant have all the stars start with the same dy, if it were so, all the stars would move along their own arc instead.

However, when I run this, after the third pass, the x positions of all the stars become smaller(into the negative ranges and out of view). Any suggestions for a better way to accomplish the original task are welcome. Thanks.

private Action orbit = new AbstractAction() {

    int declineAxis = 300; //if a stars top left x is greater than this, move downwards
    double distFromDecline;
    AffineTransform at = new AffineTransform();

    @Override
    public void actionPerformed(ActionEvent ae) {
        for (int i = 0; i < 12; i++) {
            distFromDecline = Math.abs(declineAxis - stars.getStar(i).getBounds().getCenterX());
            if (distFromDecline <= 50) {
                if (stars.getStar(i).getBounds().getX() < declineAxis) {
                    at.translate(5, -2);
                } else {
                    at.translate(5, 2);
                }
            } else if (distFromDecline <= 100 && distFromDecline > 50) {
                if (stars.getStar(i).getBounds().getX() < declineAxis) {
                    at.translate(5, -3);
                } else {
                    at.translate(5, 3);
                }
            } else if (distFromDecline <= 200 && distFromDecline > 100) {
                if (stars.getStar(i).getBounds().getX() < declineAxis) {
                    at.translate(5, -4);
                } else {
                    at.translate(5, 4);
                }
            } else if (distFromDecline >200) {
                if (stars.getStar(i).getBounds().getX() < declineAxis) {
                    at.translate(5, -5);
                } else {
                    at.translate(5, 5);
                }
            }
            stars.move(at, i);
        }
    }
};
public class Stars {

private int[] yOrigins;
private Path2D[] stars;
private Random rand = new Random();

public Stars(int n) {
    stars = new Path2D[n];
    yOrigins = new int[n];
    int dx = 700 / n;
    int x = 0;
    for (int i = 0; i < n; i++) {
        int y = rand.nextInt(401);
        generateStar(i, x, y);
        yOrigins[i] = y;
        x += dx;
    }
}

private void generateStar(int i, int x, int y) {
    stars[i] = new Path2D.Double();
    Path2D.Double cur = (Path2D.Double) stars[i];
    cur.moveTo(x, y);
    cur.lineTo(cur.getCurrentPoint().getX() + 6, y - 2);
    cur.lineTo(cur.getCurrentPoint().getX() + 2, cur.getCurrentPoint().getY() - 6);
    cur.lineTo(cur.getCurrentPoint().getX() + 2, cur.getCurrentPoint().getY() + 6);
    cur.lineTo(cur.getCurrentPoint().getX() + 6, cur.getCurrentPoint().getY() + 2);
    cur.lineTo(cur.getCurrentPoint().getX() - 6, cur.getCurrentPoint().getY() + 2);
    cur.lineTo(cur.getCurrentPoint().getX() - 2, cur.getCurrentPoint().getY() + 6);
    cur.lineTo(cur.getCurrentPoint().getX() - 2, cur.getCurrentPoint().getY() - 6);
    cur.closePath();
}

public void paintStars(Graphics2D g) {
    //super.paintComponent(g);
    g.setColor(new Color(246, 246, 255));
    for (int i = 0; i < stars.length; i++) {
        g.fill(stars[i]);
    }
}

public Shape getStar(int i) {
    return stars[i];
}

void move(AffineTransform at, int i) {
    stars[i] = (Path2D) stars[i].createTransformedShape(at);
    System.out.println(i+": " + stars[i].getBounds());
    if(stars[i].getBounds().getX()>700){
        at.translate(-(stars[i].getBounds().x+stars[i].getBounds().getWidth()), yOrigins[i]);
        stars[i] = (Path2D) at.createTransformedShape(stars[i]);
    }
}

}

回答1:

java.awt.geom.FlatteningPathIterator http://docs.oracle.com/javase/6/docs/api/java/awt/geom/FlatteningPathIterator.html

You pass your arc (or any another Shape) and use the points to position star.

You can use stars frm here http://java-sl.com/shapes.html

回答2:

WarpImage from the Sun/Oracle Java2D demo, java2d/demos/Images/WarpImage.java, is an appealing example of an animation that follows a CubicCurve2D using PathIterator. You might see if it offers any guidance.

来源：https://stackoverflow.com/questions/9238061/moving-shapes-along-an-arc-path-with-java-graphics