AffineTransform : Java Glossary

[ x']   [  m00  m01  m02  ] [ x ]   [ m00 * x + m01 * y + m02 ]
[ y'] = [  m10  m11  m12  ] [ y ] = [ m10 * x + m11 * y + m12 ]
[ 1 ]   [   0    0    1   ] [ 1 ]   [               1         ]

Tips

Graphics.drawImage will do a simple scale for you, often inadvertently. You don’t need an AffineTransform for that.

Manual Use of AffineTransform

The basic idea is this. A transform maps every point x,y onto a new point, by a combination of rotation, scaling, translation and mirroring. You can manually transform a point like this:

Drawing with AffineTransform

You can also simply plug an AffineTransform into your Graphics2D object. Then you can specify simple co-ordinates and the drawing will appear in transformed co-ordinates. Note that with Graphics2D, your co-ordinates are doubles not ints.

Correcting Mouse Co-ordinates

The catch is, the mouse knows nothing about your transforms. You need to convert mouse screen co-ordinates to your user-co-ordinates. This is the inverse of what you normally do, namely converting convenient user co-ordinates to pixel display co-ordinates. For this, you need the inverse of your transform.

You can learn more with a Google search.

Rotating Images

• AffineTransform has no flip method to turn an Image upside down. Use scale( 1.0, -1.0 ) instead.
• AffineTransform has no mirror method to reverse an image left to right. Use scale( -1.0, 1.0 ) instead.
• If you are rotating by multiples of 90 degrees, you will get faster and more accurate results if you use quadrantRotate().
• If you scale or rotate, the Image will spin around the upper left corner of the Image and disappear off the screen. You must first move the origin to the center of the image, do your rotation, then move it back.
• Try writing yourself a training program that just draws coloured rectangles. Experiment with various combinations of transforms until you see the effects, stage by stage.

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