Programming a Custom User Interface - Beginning J2ME: From Novice to Professional - page 243

The final parameter in the drawRGB() method, processAlpha , indicates whether the integer
array is considered to contain an alpha (opacity) component. If the parameter is false , every
pixel of the image is considered fully opaque. If processAlpha is true , the opacity of each pixel
is determined by the high-order byte of the integer value, and the pixel's color will be blended
with the drawing surface appropriately. An alpha value of 0 is fully transparent, while an alpha
value of 255 is fully opaque.
Blitting
Blitting , the copying of one region of the screen to another location, is a crucial operation for
some types of games. There is one method of the Graphics class that can be used for blitting:
public void copyArea(int x_src, int y_src, int width, int height,
int x_dest, int y_dest, int anchor)
This method is pretty self-explanatory. It copies a portion of the screen, described by
x_src , y_src , width , and height , to a destination described by x_dest , y_dest , and anchor . The
anchor works the same as for the drawImage() method.
This method works only on a Graphics object that does not draw directly to the screen.
A Graphics object that draws to an image is fine, as is a Graphics object that works on a double-
buffered Canvas . A Graphics object from GameCanvas 's getGraphics() method will also work. By
contrast, a Graphics object for a non-double-buffered Canvas will throw an IllegalStateException
if the copyArea() method is called. (See the upcoming section on double buffering for more
information on the technique.)
Clipping
Graphics maintains a rectangular clipping shape . The clipping shape limits drawing, such that
any drawing that takes place outside of the clipping shape will not be displayed. It's kind of like
painting through a stencil, except you can only use a rectangular stencil. If you were writing a
game that had some kind of border on the game board, you might set the clipping rectangle to
be the inside of the game board, so that no drawing could overwrite the border.
You can find out the current clipping rectangle by calling getClipX() , getClipY() ,
getClipWidth() , and getClipHeight() .
If you would like to modify the clipping rectangle, there are two methods that you can use.
First, you can set the clipping rectangle directly by calling the following method:
public void setClip(int x, int y, int width, int height);
The other possibility is to limit the current clipping rectangle with another rectangle. The
following method takes the intersection of the current clipping rectangle and the supplied
rectangle and uses it to set the new clipping rectangle:
public void clipRect(int x, int y, int width, int height);
Key Events
Canvas handles events at a lower level than the other Displayable subclasses. Although you can
add Command s and respond to them, Canvas also includes a set of methods that handle interaction
with the individual keys of a device.