scale transform giving inaccurate results

[gib65]

Please have a look at my code snippet below. I'm trying to do a scale transform on an Area object (called boundingArea) and I'm not getting the results I'm expecting.

System.out.println("initial width: " + getBounds().getWidth());
System.out.println("1/width = " + (1.0d / getBounds().getWidth()));
System.out.println("width * (1 + 1/width) = " + (getBounds().getWidth() * (1.0d + 1.0d / getBounds().getWidth())));
boundingArea.transform(AffineTransform.getScaleInstance(
		1.0d + (1.0d / getBounds().getWidth()), 1.0d));
System.out.println("final width: " + getBounds().getWidth());

When I run this, I get the following output:

initial width: 18.0
1/width = 0.055555555555555555
width * (1 + 1/width) = 19.0
final width: 20.0

Why does the transform operation give me the wrong value? I'm assume that what the transform operation is doing is taking 1 + 1/width and multiplying the width by that. But according to my calculations above, it should end up with 19, but it gives me 20 instead.

[Fubarable]

Can you post an SSCCE?

[DarrylBurke]

You're calling getBounds() on the current instance of whatever class that code is in, and applying a transform to a variable boundingArea.

I second the recommendation to post a SSCCE.

db

[gib65]

Here's a very simple SSCCE:

import java.awt.Rectangle;
import java.awt.geom.Area;
import java.awt.geom.AffineTransform;

public class TransformTest {

    private Area boundingArea;

    public TransformTest() {
	boundingArea = new Area(new Rectangle(1, 0, 18, 18));
    }

    public void doTest() {

	System.out.println("initial width: " + getBounds().getWidth());
	System.out.println("1/width = " + (1.0d / getBounds().getWidth()));
	System.out.println("width * (1 + 1/width) = " + 
		(getBounds().getWidth() * (1.0d + 1.0d / getBounds().getWidth())));
	boundingArea.transform(AffineTransform.getScaleInstance(
		1.0d + (1.0d / getBounds().getWidth()), 1.0d));
	System.out.println("final width: " + getBounds().getWidth());
    }

    private Rectangle getBounds() {
	return boundingArea.getBounds();
    }

    public static void main(String[] args) {

	TransformTest tranTest = new TransformTest();
	tranTest.doTest();
    }
}

Interestingly, I found that the problem disappears if I initialize boundingArea with an X value of 0. Any other value, it seems, gives me a result of 20 rather than the 19 one would expect. This includes values from 1 to 1000 and -1 to -1000.

But should I have to reposition my boundingArea to x = 0 (or y = 0, or both) just to do a scale?

[DarrylBurke]

It's a result of integer math. Set the left to 1800 and the width is 19, not 20. Better use 2D geometry.

import java.awt.geom.*;

public class ScaleTransformTest {

  public static void main(String[] args) {
    for (int x = 0; x < 1000; x++) {
      Area area = new Area(new Rectangle2D.Double(x, 0, 18, 18));
      area.transform(AffineTransform.getScaleInstance(
              1.0d + (1.0d / area.getBounds2D().getWidth()), 1.0d));
      Rectangle2D bounds = area.getBounds2D();
      if (bounds.getWidth() != 19.0) {
        System.out.println("x = " + x + "   width = " + bounds.getWidth());
      }
    }
  }
}

db

[gib65]

Yep, that worked. I guess I'm going to make use of the 2D classes from now on (at least, when it's a 2D application). Thanks for pointing me in the right direction.