Почему мой пользовательский холст не отображается при использовании того же графического контекста? [репост]

/**
* Calin Baenen's version of a canvas element.
*/
class CPane {

    // Debug variables.
    private boolean canCatch = true; // See if this canvas can still catch errors.
    private final JPanel canvas; // Canvas.
    private BufferedImage buffer; // Buffer.
    private final Graphics ctx; // Graphics.



    // Constructor.
    public CPane(int width, int height) {
        this(0, 0, width, height);
    }
    public CPane(int width, int height, Color initialColor) {
        this(0, 0, width, height, initialColor);
    }
    public CPane(int x, int y, int width, int height) {
        this(x, y, width, height, Color.WHITE);
    }
    public CPane(int x, int y, int width, int height, Color initialColor) {
        this(x, y, width, height, initialColor, Color.WHITE);
    }
    public CPane(int x, int y, int width, int height, Color initialColor, Color baseColor) {

        this.canvas = new JPanel(); // Make an element to display the drawing on.
        canvas.setBounds(x, y, width, height); // Set the bounds of the canvas.
        canvas.setBackground(baseColor); // Make the canvas white.
        canvas.setDoubleBuffered(true); // Make the canvas double buffered.

        buffer = new BufferedImage(width, height, BufferedImage.TYPE_INT_ARGB); // Set the buffer.

        ctx = buffer.getGraphics(); // Get the graphics for the buffer.

        // Paint the image with an initisl color.
        ctx.setColor(initialColor); // Set color.
        ctx.drawRect(0, 0, width, height); // Draw.

    }



    /**
    * Draws a rectangle to the screen by filling an area with color.
    */
    public void drawRect(int x, int y, int width, int height, Color color) {
        ctx.setColor(color); // Set the context's color.
        ctx.fillRect(x, y, width, height); // Draw a rectangle.
        this.update(); // Update the canvas.
    }



    /**
    * Draw an image to the canvas.
    */
    public void drawImage(Image image, int x, int y, int width, int height) {
        Image shot = image.getScaledInstance(width, height, Image.SCALE_FAST); // Get the scaled image to draw.
        ctx.drawImage(shot, x, y, null); // Draw the image.
        this.update(); // Update the canvas.
    }



    /**
    * Update the graphics, and log the error if it fails.
    */
    public void update(boolean catcherror) {
        Graphics display = canvas.getGraphics(); // Get the canvas' display.
        try {
            display.drawImage(buffer, 0, 0, null); // Draw the buffer to the canvas.
        } catch(Exception error) {
            if(catcherror && canCatch) {Book.eog(error); canCatch = false;} // Log the error and disable error catching.
        }
        display.dispose(); // Dispose of the display.
    }

}