Java Algorithms How to - Find Angle from transform matrix

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Question

We would like to know how to find Angle from transform matrix.

Answer

import java.awt.Point;
import java.awt.geom.AffineTransform;
import java.awt.geom.Point2D;
import java.util.Random;
//from  w  w  w.jav  a 2s  .  c  o m
public class Main {
  public static void main(String[] args) {
    for (int i = 0; i <= 180; i++) {
      double angleRad = Math.toRadians(i);
      AffineTransform at = createRandomTransform(angleRad);
      double extractedAngleRad = extractAngle(at);
      System.out.println("In: " + Math.toDegrees(angleRad) + " " + "Out "
          + Math.toDegrees(extractedAngleRad));
    }
  }
  private static double extractAngle(AffineTransform at) {
    Point2D p0 = new Point();
    Point2D p1 = new Point(1, 0);
    Point2D pp0 = at.transform(p0, null);
    Point2D pp1 = at.transform(p1, null);
    double dx = pp1.getX() - pp0.getX();
    double dy = pp1.getY() - pp0.getY();
    double angle = Math.atan2(dy, dx);
    return angle;
  }

  private static AffineTransform createRandomTransform(double angleRad) {
    AffineTransform at = new AffineTransform();
    double scale = 1.0;
    at.translate(randomDouble(), randomDouble());
    scale = Math.abs(randomDouble());
    at.scale(scale, scale);
    at.rotate(angleRad);
    at.translate(randomDouble(), randomDouble());
    scale = Math.abs(randomDouble());
    at.scale(scale, scale);
    return at;
  }
  private static double randomDouble() {
    Random random = new Random(0);
    return -5.0 + random.nextDouble() * 10;
  }
}

The code above generates the following result.

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