Drawing line from thin to thick

Question

I need to draw a line where some segments of the line is thicker than the rest.
I can draw the lines, but I need some kind of smooth transition from thin line to thick and the other way around. I'm using generalpath and stroke to draw the lines.
What is the best (easiest) way to do this?

Answer 1

Check out following code if it works for you. I've used b-spline approach to generate the shape of the path. You just need to specify control points and width at them.
import java.awt.*;
import java.awt.geom.*;
import javax.swing.*;
public class FloatWidthPath {
    Point2D[] pnts1;
    Point2D[] pnts2;
    CurvedPath cp1;
    CurvedPath cp2;
    public class CurvedPath {
        Point2D[] cPoints;
        double[] params;
        CubicCurve2D[] curves;
        public CurvedPath(Point2D[] pnts) {
            this.cPoints = pnts;
            double s = 0.75;
            double [] curSeg = new double[2];
            curves = new CubicCurve2D[cPoints.length-1];
            params = new double[cPoints.length];
            for (int i = 0; i < cPoints.length-1; i++){
                curves[i] = new CubicCurve2D.Double(
                    getCP(i+1).getX(), getCP(i+1).getY(),
                    -s*getCP(i).getX()/3 + getCP(i+1).getX() +
                    s*getCP(i+2).getX()/3,
                    -s*getCP(i).getY()/3 + getCP(i+1).getY() +
                    s*getCP(i+2).getY()/3,
                    s*getCP(i+1).getX()/3 + getCP(i+2).getX() -
                    s*getCP(i+3).getX()/3,
                    s*getCP(i+1).getY()/3 + getCP(i+2).getY() -
                    s*getCP(i+3).getY()/3,
                    getCP(i+2).getX(), getCP(i+2).getY());
        Point2D getCP(int i) {
            if (i == 0) {
                return new Point2D.Double(
                    -0.5*cPoints[0].getX() + 1.5*cPoints[1].getX(),
                    -0.5*cPoints[0].getY() + 1.5*cPoints[1].getY());
            } else if (i == cPoints.length+1) {
                int l = cPoints.length;
                return new Point2D.Double(
                    -0.5*cPoints[l - 1].getX() + 1.5*cPoints[l - 2].getX(),
                    -0.5*cPoints[l - 1].getY() + 1.5*cPoints[l - 2].getY());
            return cPoints[i-1];
        public CubicCurve2D getCurve(int i) {
            return curves;
public Path2D getPath2D() {
Path2D.Double result = new Path2D.Double();
result.moveTo(cPoints[0].getX(), cPoints[0].getY());
int i = 0;
while (i < curves.length) {
result.append(curves[i], true);
i++;
return result;
public FloatWidthPath(Point2D[] pnts, double[] width) {
pnts1 = new Point2D[pnts.length];
pnts2 = new Point2D[pnts.length];
double dx = 0;
double dy = 0;
double l = 0;
for (int i = 0; i < pnts.length - 1; i++) {
dx = pnts[i + 1].getX() - pnts[i].getX();
dy = pnts[i + 1].getY() - pnts[i].getY();
l = Math.sqrt(dx*dx + dy*dy);
double dx1 = (dx*width[i])/l;
double dy1 = (dy*width[i])/l;
pnts1[i] = new Point2D.Double(pnts[i].getX()- dy1, pnts[i].getY() + dx1);
pnts2[i] = new Point2D.Double(pnts[i].getX()+ dy1, pnts[i].getY() - dx1);
int i = pnts.length - 1;
double dx1 = (dx*width[width.length - 1])/l;
double dy1 = (dy*width[width.length - 1])/l;
pnts1[i] = new Point2D.Double(pnts[i].getX()- dy1, pnts[i].getY() + dx1);
pnts2[i] = new Point2D.Double(pnts[i].getX()+ dy1, pnts[i].getY() - dx1);
for (i = 0; i < (pnts2.length >> 1); i++) {
Point2D tmp = pnts2[i];
pnts2[i] = pnts2[pnts2.length - i - 1];
pnts2[pnts2.length - i - 1] = tmp;
cp1 = new CurvedPath(pnts1);
cp2 = new CurvedPath(pnts2);
public Path2D getPath2D() {
Path2D.Double result = new Path2D.Double();
result.append(cp1.getPath2D(), false);
result.append(cp2.getPath2D(), true);
result.closePath();
return result;
public static void showPath(String frameName, Point2D[] pnts,
double [] width) {
JFrame frame = new JFrame(frameName);
final Point2D[] shownPnts = pnts;
final double [] showWidth = width;
frame.getContentPane().add(new JPanel() {
public void paint(Graphics g) {
Graphics2D g2d = (Graphics2D)g;
FloatWidthPath cp = new FloatWidthPath(shownPnts, showWidth);
g2d.fill(cp.getPath2D());
frame.setPreferredSize(new Dimension(100,100));
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.pack();
frame.setVisible(true);
public static void main(String[] args) {
showPath("CurvedPath demo", new Point2D[]{
new Point2D.Double(10,10),
new Point2D.Double(100,30),
new Point2D.Double(100,100),
new Point2D.Double(10,120),
}, new double [] {3, 4, 20, 8});