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You can modify the transform attribute in theGraphics2D
context to move, rotate, scale, and shear graphics primitives when they are rendered. The transform attribute is defined by an instance ofAffineTransform
. (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 newAffineTransform
and change theGraphics2D
transform attribute by callingtransform
.
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 directionsYou can also construct an
AffineTransform
directly and concatenate it with the current transform by calling thetransform
method.The
drawImage
method is also overloaded to allow you to specify anAffineTransform
that is applied to the image as it is rendered. Specifying a transform when you calldrawImage
does not affect theGraphics2D
transform attribute.
The following program is the same asStrokeandFill
, 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
:Before displaying the shape corresponding to the menu choices, the application first retrieves the current transform from thepublic 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; } }Graphics2D
object:This transform will be restored to theAffineTransform saveXform = g2.getTransform();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. Theat
AffineTransform
is concatenated ontotoCenterAt
:TheAffineTransform toCenterAt = new AffineTransform(); toCenterAt.concatenate(at); toCenterAt.translate(-(r.width/2), -(r.height/2));toCenterAt
transform is concatenated onto theGraphics2D
transform with thetransform
method:After rendering is completed, the original transform is restored using theg2.transform(toCenterAt);setTransform
method:g2.setTransform(saveXform);
Note: Never usesetTransform
to concatenate a coordinate transform onto an existing transform. ThesetTransform
method overwrites theGraphics2D
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
getTransform
to get the current transform.- Use
transform
,translate
,scale
,shear
, orrotate
to concatenate a transform.- Perform the rendering.
- Restore the original transform using
setTransform
.
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