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Transforming Shapes, Text, and Images

You can modify the transform attribute in the Graphics2D context to move, rotate, scale, and shear graphics primitives when they are rendered. The transform attribute is defined by an instance of AffineTransform. (An affine transform is a transformation such as translate, rotate, scale, or shear in which parallel lines remain parallel even after being transformed.)

Graphics2D provides several methods for changing the transform attribute. You can construct a new AffineTransform and change the Graphics2D transform attribute by calling transform.

AffineTransform defines the following factory methods to make it easier to construct new transforms:

• getRotateInstance
• getScaleInstance
• getShearInstance
• getTranslateInstance

Alternatively you can use one of the Graphics2D transformation methods to modify the current transform. When you call one of these convenience methods, the resulting transform is concatenated with the current transform and is applied during rendering:

• rotate--to specify an angle of rotation in radians
• scale--to specify a scaling factor in the x and y directions
• shear--to specify a shearing factor in the x and y directions
• translate--to specify a translation offset in the x and y directions

You can also construct an AffineTransform directly and concatenate it with the current transform by calling the transform method.

The drawImage method is also overloaded to allow you to specify an AffineTransform that is applied to the image as it is rendered. Specifying a transform when you call drawImage does not affect the Graphics2D transform attribute.

Example: Transform

The following program is the same as StrokeandFill, but also allows the user to choose a transformation to apply to the selected object when it is rendered.

This is a picture of the applet's GUI. To run the applet, click the picture. The applet will appear in a new browser window.

Transform.java contains the complete code for this applet.

When a transform is chosen from the Transform menu, the transform is concatenated onto the AffineTransform at:

public void setTrans(int transIndex) {
    // Sets the AffineTransform.
    switch ( transIndex ) {
    case 0 : at.setToIdentity();
             at.translate(w/2, h/2); break;
    case 1 : at.rotate(Math.toRadians(45)); break;
    case 2 : at.scale(0.5, 0.5); break;
    case 3 : at.shear(0.5, 0.0); break;
    }
}

Before displaying the shape corresponding to the menu choices, the application first retrieves the current transform from the Graphics2D object:

AffineTransform saveXform = g2.getTransform();

This transform will be restored to the Graphics2D after rendering.

After retrieving the current transform, another AffineTransform, toCenterAt, is created that causes shapes to be rendered in the center of the panel. The at AffineTransform is concatenated onto toCenterAt:

AffineTransform toCenterAt = new AffineTransform();
toCenterAt.concatenate(at);
toCenterAt.translate(-(r.width/2), -(r.height/2));

The toCenterAt transform is concatenated onto the Graphics2D transform with the transform method:

g2.transform(toCenterAt);

After rendering is completed, the original transform is restored using the setTransform method:

g2.setTransform(saveXform);

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Note: Never use setTransform to concatenate a coordinate transform onto an existing transform. The setTransform method overwrites the Graphics2D object's current transform, which might be needed for other reasons, such as positioning Swing and lightweight components in a window. Use these steps to perform transformations:

1. Use getTransform to get the current transform.
2. Use transform, translate, scale, shear, or rotate to concatenate a transform.
3. Perform the rendering.
4. Restore the original transform using setTransform.

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