你如何使SWT Scale Component具有透明背景？

Question

我正在尝试在带有绘制图像的画布上使用SWT Scale Component（滑块thingy），缩放用于在画布上放大和缩小，所以我希望标尺周围的灰色区域继承从画布后面绘制的图像，我不能，我已经尝试了here的建议，但似乎继承了背景颜色，但仍然围绕它绘制正方形而不是在该空间上绘制任何东西。

以下是此问题示例中的屏幕截图：

此代码生成上述窗口：

import org.eclipse.swt.SWT;
import org.eclipse.swt.events.PaintEvent;
import org.eclipse.swt.events.PaintListener;
import org.eclipse.swt.graphics.GC;
import org.eclipse.swt.graphics.Point;
import org.eclipse.swt.layout.GridData;
import org.eclipse.swt.layout.GridLayout;
import org.eclipse.swt.widgets.Canvas;
import org.eclipse.swt.widgets.Composite;
import org.eclipse.swt.widgets.Display;
import org.eclipse.swt.widgets.Scale;
import org.eclipse.swt.widgets.Shell;

public class ScaleExample extends Canvas{

    public ScaleExample(Composite parent, int style) {
        super(parent, style);

        addPaintListener(new PaintListener() {
            public void paintControl(PaintEvent e) {
                ScaleExample.this.paintControl(e);
            }
        });

        this.setBackgroundMode(SWT.INHERIT_FORCE);
        Scale zoomRule = new Scale(this, SWT.VERTICAL);
        zoomRule.setMinimum(1);
        zoomRule.setMaximum(18);
        zoomRule.setIncrement(1);
        Point p = zoomRule.computeSize(40, 200);
        zoomRule.setBounds(10, 10, p.x, p.y);
    }

    protected void paintControl(PaintEvent e) {
        GC gc = e.gc;

        Point size = getSize();
        int width = size.x, height = size.y;

        gc.drawOval(0, 0, width, height);        

    }


    public static void main(String[] args) {
        final Display display = new Display();
        final Shell shell = new Shell(display);
        shell.setText("Scale Example");
        shell.setSize(new Point(300, 300));
        GridLayout layout = new GridLayout(1, true);
        shell.setLayout(layout);
        ScaleExample se = new ScaleExample(shell, SWT.BORDER);
        se.setLayoutData(new GridData(SWT.FILL, SWT.FILL, true, true));

        shell.open();
        while (!shell.isDisposed()) {
            if (!display.readAndDispatch())
                display.sleep();

        }
        display.dispose();
    }


}