Re: Draw a scaled arrow


Yes. From math class, matrix multiplication is _not_ commutative:-) In particular, the normal concatenation of operations in AffineTransorm applies the transformations in reverse order. See concatenate() and preConcatenate().

In the example below, each arrow is rotated about its tip, then scaled, then translated to its destination. Suppose you wanted to rotate each arrow about its center. The arrow is four units wide, so translate(-2, 0) before rotating. Experiment with adding the translate() in different places to see the effect.

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Polygon;
import java.awt.Shape;
import java.awt.geom.AffineTransform;
import javax.swing.JFrame;

/**
* Test AffineTransform
* @author John B. Matthews
*/
public class Arrows extends JFrame {

private static Shape arrow = initPoly();
private static AffineTransform at = new AffineTransform();

public static void main(String args[]) {
JFrame frame = new Arrows();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setSize(400, 400);
frame.setVisible(true);
}

public void paint(Graphics g) {
Graphics2D g2D = (Graphics2D) g;
g2D.setPaint(Color.BLACK);
g2D.drawLine(0, 200, 400, 200);
g2D.drawLine(200, 0, 200, 400);
g2D.setPaint(Color.BLUE);
drawShape(g2D, 100, 100, Math.PI / 2);
drawShape(g2D, 300, 100, Math.PI);
drawShape(g2D, 100, 300, 0);
drawShape(g2D, 300, 300, -Math.PI / 2);
}

/**
* Draw a rotated, scaled and translated arrow.
* Note that the normal order of concatenation
* applies the last transforamtion first.
* @see AffineTransform#concatenate()
*/
private void drawShape(Graphics2D g2D, int x, int y, double theta) {
at.setToIdentity();
at.translate(x, y);
at.scale(30, 30);
at.rotate(theta);
Shape shape = at.createTransformedShape(arrow);
g2D.fill(shape);
}

/** Create a west pointing arrow with the tip at the origin. */
private static Polygon initPoly()
{
Polygon poly = new Polygon();
poly.addPoint( 0, 0);
poly.addPoint( 3, -1);
poly.addPoint( 2, 0);
poly.addPoint( 3, 1);
return poly;
}
}

Thanks for this, I can play with this example but I cant get it to behave like the example code I provided when dragging a line and rotating the arrow head with the mouse cursor, I still also get the problem of xy coords not aligning when going from a > 100% zoom down to a 100% zoom, the arrow does not get drawn at the correct place when scaling to 100%;

Thanks anyway
Rich
.

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