A raster operation applies a logical operation to the display of a GDI primitive, such as a pen, brush, image, or shape to achieve a visual effect. Raster operations that you can perform using the Graphics object are defined in the RasterOp object. When you review the methods supported by the Graphics object, you'll see that for each basic operation, such as the drawing of lines or the display of images, there's a method that takes a RasterOp as a parameter.
In their simplest incarnations, the drawing methods the Graphics object supports merely writes or copies pixels to some area of the display, overwriting what's currently displayed. Add raster operations to the equation and this overwrite becomes more complex.
Suppose, for example, that you want to draw a black rectangle to an area currently covered by an image, but you want to logically combine the black pixels with their corresponding pixels in the target image and to write the result to the display. Raster operations make such combinations possible.
The variations on the logic that the RasterOps object supports are too numerous to be covered thoroughly in this document. The following statements, however, demonstrate the basic syntax of a call using a RasterOps object. These statements set the background color of a form, and then draw a line that represents a color inversion of the background color:
protected void onPaint(PaintEvent e)
{
this.setBackColor(new Color(255, 255, 255);
e.graphics.drawLine(new Point(10, 10), new Point(100, 10),
RasterOp.TARGET.invert());
}