public class DrawingArea
extends Canvas
{
   Image passiveGround, activeGround;
   
   ...
   
   public void redrawPassiveGround()
   {
      passiveGround = new Image( getDisplay(), getBounds());
      GC gcPassive = new GC( passiveGround);

      gcPassive.draw... // drawing routines
      
      gcPassive.dispose();
   }
   
   public void redrawPassiveGround()
   {
      Image activeGround = new Image( getDisplay(), getBounds());
      GC gcActive = new GC( activeGround);

      gcActive.fill_with_transparent_color(); // how to?
      gcActive.draw... // drawing routines
      
      gcActive.dispose();
   }
   
   class MyPaintListener
   implements PaintLister
   {
      public void paintControl( PaintEvent paintEvent)
      {
         GC gc = paintEvent.gc;
         
         gc.drawImage( passiveGround, 0, 0);
         gc.drawImage( activeGround, 0, 0);
         
         gc.dispose();
      }
   }
}

Color TRANSPARENT_COLOR = getDisplay().getSystemColor( SWT.COLOR_BLACK);
Color DRAWING_COLOR = getDisplay().getSystemColor( SWT.COLOR_RED);

PaletteData paletteData = new PaletteData( new RGB[]{
   TRANSPARENT_COLOR.getRGB(),
   DRAWING_COLOR.getRGB()
   });

ImageData imageData = new ImageData( width, height, 4, paletteData);
imageData.transparentPixel = 0; // index of the palette

Image image = new Image( getDisplay(), imageData);

GC gcImage = new GC( image);

gcImage.setBackground( ??? );
gcImage.setForeground( ??? );
gcImage.fillRectangle( image.getBounds()); // clear image with transparent 
color
gcImage.draw... // drawing routines

gc.dispose();