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 the AffineTransform
class. An affine transform is a transformation such as translate, rotate, scale, or shear in which parallel lines remain parallel even after being transformed.
The Graphics2D
class 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 radiansscale
— to specify a scaling factor in the x and y directionsshear
— to specify a shearing factor in the x and y directionstranslate
— to specify a translation offset in the x and y directions
You can also construct an AffineTransform
object 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.
Note: If you don't see the applet running, you need to install at least the Java SE Development Kit (JDK) 7 release.
contains the complete code for this applet.Transform.java
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);
Note: Never use the
setTransform
method 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:
- Use the
getTransform
method to get the current transform. - Use
transform
,translate
,scale
,shear
, orrotate
to concatenate a transform. - Perform the rendering.
- Restore the original transform using the
setTransform
method.