Extracting transparent image from canvas

Question

Hi Folks,
Given an image painted on a graphics2D, I need to copy some parts of this image to be able to process them and repaint them in place, giving to the original image a different look.
Theses parts are not rectangles but donuts like shapes (circular surface with a hole in he middle).
The approach I tried in the following:
- Combining 2 ellipses Shape to be able to create an Area representing the donut shaped surface
- Clipping the Graphics2D this Area
[at this point, if the panel is repainted with the clip set, the original image is only visible through the donut shape clip, which is exactly was I want]
- creating a new image with transparency, painting the clipped original image onto this new image
[that's where it does not work, if I paint the new image on the canvas, it's blank, or when I disabled transparency, it's a squared shape, and not a donut like shape]
Any help/hint would be gladly appreciated, below in the paintComponent method
Thanks
Elie
public synchronized void paintComponent(Graphics g) {
        g2d = (Graphics2D)g;
        if ( readyToDisplayImage == null ) {
            g2d.setColor(getBackground());
            g2d.fillRect(0, 0, getWidth(), getHeight());
            return;
        // account for borders
        Insets insets = getInsets();
        int tx = insets.left + originX;
        int ty = insets.top  + originY;
        // clear damaged component area
        Rectangle clipBounds = g2d.getClipBounds();
        g2d.setColor(getBackground());
        g2d.fillRect(clipBounds.x,
                     clipBounds.y,
                     clipBounds.width,
                     clipBounds.height);
          g2d.drawRenderedImage(readyToDisplayImage,
                              AffineTransform.getTranslateInstance(tx, ty));
   if(test) {
        Shape oldClip = g2d.getClip() ;
        Shape s1 = (new Circle(500,500, 150)).toEllipse2D();
        Shape s2 = (new Circle(500,500, 250)).toEllipse2D();
        Area a1 = new Area(s1);
        Area a2 = new Area(s2);
        a2.subtract(a1); // donut shaped mask
         g2d.setColor(getBackground());
         g2d.fillRect(0, 0, getWidth(), getHeight());
        g2d.setClip(a2) ;
        g2d.drawRenderedImage(readyToDisplayImage,AffineTransform.getTranslateInstance(tx, ty));
        Rectangle bounds = a2.getBounds();
        BufferedImage subimage = new BufferedImage(bounds.width, bounds.height, BufferedImage.TYPE_INT_ARGB);
        Graphics2D g2 = (Graphics2D) subimage.createGraphics();
        // Clear image with transparent alpha by drawing a rectangle  - commented beacause worsens things
       // g2.setComposite(AlphaComposite.getInstance(AlphaComposite.CLEAR, 0.0f));
       // Rectangle2D.Double rect = new Rectangle2D.Double(0,0,bounds.width, bounds.height);
        //g2.fill(rect);
        g2.drawRenderedImage(readyToDisplayImage, AffineTransform.getTranslateInstance(bounds.x, bounds.y));
        g2.dispose();
        g2d.setClip(oldClip) ; //back to normal
     // Now checking whether the extracted sub image has the expected shape : this is not the case
         g2d.setColor(getBackground());
         g2d.fillRect(0, 0, getWidth(), getHeight());
         g2d.drawImage(subimage, bounds.x, bounds.y, this );
    }

Answer 1

I found another way, which is using ROIShape class as mask.
But an exception is raised by the last line of the method.
Apparently the resulting image form and "add" between a oneband and a 3 band image is not supported.
Also, if I do convert the one band image to color, it does not solve the problem
ColorModel colorModel = cropped.getColorModel();
pb = new ParameterBlock();
     pb.addSource(maskimage);
     pb.add(colorModel);
maskimage = JAI.create("colorconvert", pb);
What would be the right way to do this ?
Thanks
trace:
image mask bounds=java.awt.Rectangle[x=250,y=250,width=500,height=500]
cropped image bounds=java.awt.Rectangle[x=250,y=250,width=500,height=500]
subimage image bounds=java.awt.Rectangle[x=250,y=250,width=500,height=500]
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at com.sun.media.jai.opimage.AddOpImage.computeRectByte(AddOpImage.java:268)
at com.sun.media.jai.opimage.AddOpImage.computeRect(AddOpImage.java:220)
at javax.media.jai.PointOpImage.computeTile(PointOpImage.java:974)
at com.sun.media.jai.util.SunTileScheduler.scheduleTile(SunTileScheduler.java:912)
at javax.media.jai.OpImage.getTile(OpImage.java:1139)
at javax.media.jai.RenderedOp.getTile(RenderedOp.java:2268)
at sun.java2d.SunGraphics2D.drawTranslatedRenderedImage(SunGraphics2D.java:2864)
at sun.java2d.SunGraphics2D.drawRenderedImage(SunGraphics2D.java:2751)
at circleops.ImagePanel.paintComponent(ImagePanel.java:253)
public synchronized void paintComponent(Graphics g) {
        g2d = (Graphics2D)g;
        if ( readyToDisplayImage == null ) {
            g2d.setColor(getBackground());
            g2d.fillRect(0, 0, getWidth(), getHeight());
            return;
        // account for borders
        Insets insets = getInsets();
        int tx = insets.left + originX;
        int ty = insets.top  + originY;
        // clear damaged component area
        Rectangle clipBounds = g2d.getClipBounds();
        g2d.setColor(getBackground());
        g2d.fillRect(clipBounds.x,
                     clipBounds.y,
                     clipBounds.width,
                     clipBounds.height);
            Translation moves the entire image within the container
        g2d.drawRenderedImage(readyToDisplayImage,
                              AffineTransform.getTranslateInstance(tx, ty));
        g2d.setColor(Color.red);                     
        g2d.fillRect(lastClickedX-4, lastClickedY-4, 8, 8);
        //System.out.println("x="+currentX+" y="+currentY);
        Iterator<ConcentricTurbulence> it = concentrics.iterator();
        while(it.hasNext()) {
        if(test) {
        Shape oldClip = g2d.getClip() ;
        Shape s1 = (new Circle(500,500, 150)).toEllipse2D();
        Shape s2 = (new Circle(500,500, 250)).toEllipse2D();
        Area a1 = new Area(s1);
        Area a2 = new Area(s2);
        a2.subtract(a1);
        ROIShape roishape = new ROIShape(a2);
        PlanarImage maskimage = roishape.getAsImage();
        Rectangle bounds = maskimage.getBounds();
        System.out.println("image mask bounds="+bounds.toString());
        ParameterBlock pbc = new ParameterBlock();
        pbc.addSource(readyToDisplayImage);
        pbc.add((float)bounds.x);
        pbc.add((float)bounds.y);
        pbc.add((float)bounds.width);
        pbc.add((float)bounds.height);
        PlanarImage  cropped = JAI.create("Crop", pbc, null);
        System.out.println("cropped image  bounds="+cropped.getBounds());
        ParameterBlock pbd = new ParameterBlock();
        pbd.addSource(cropped);
        pbd.addSource(maskimage);
        PlanarImage  subimage = JAI.create("add", pbd, null);
        System.out.println("subimage image  bounds="+subimage.getBounds());
         g2d.setColor(getBackground());
         g2d.fillRect(0, 0, getWidth(), getHeight());
         g2d.drawRenderedImage(subimage, AffineTransform.getTranslateInstance(bounds.x, bounds.y));
    }