SWT Java：在画布上通过矩形选择图像而无需快速重绘

Question

I have an image on a canvas:

    canvas = new Canvas(top, SWT.V_SCROLL | SWT.H_SCROLL | SWT.NO_REDRAW_RESIZE);
    GridData canvasGridData = new GridData(GridData.VERTICAL_ALIGN_FILL);
    canvasGridData.verticalSpan = 15;
    canvasGridData.horizontalAlignment = GridData.FILL;
    canvasGridData.verticalAlignment = GridData.FILL;
    canvasGridData.grabExcessHorizontalSpace = true;
    canvasGridData.grabExcessVerticalSpace = true;
    canvas.setLayoutData(canvasGridData);
    canvas.addPaintListener(new PaintListener() {
        public void paintControl(PaintEvent e) {
            if (buttonPressed) {
                GC gc = e.gc;
                image = GetMyImagetoDisplay();
                gc.drawImage(image, 10, 10);
            }
        }
    });

When a GUI button is pressed, reDraw() is called and image is loaded.

I also have a mouse events for getting a rectangular selection. They draw a rectangular selection over the image: canvas.addListener(SWT.MouseDown, new Listener() {

        @Override
        public void handleEvent(Event event) {
            startX = event.x;
            startY = event.y;

            drag = true;
        }
    });

    canvas.addListener(SWT.MouseUp, new Listener() {

        @Override
        public void handleEvent(Event event) {
            endX = event.x;
            endY = event.y;

            drag = false;

            canvas.redraw();
        }
    });

    canvas.addListener(SWT.MouseMove, new Listener() {

        @Override
        public void handleEvent(Event event) {


            if(drag)
            {
                endX = event.x;
                endY = event.y;


                canvas.redraw();
            }
        }
    });

    canvas.addListener(SWT.Paint, new Listener() {

        @Override
        public void handleEvent(Event event) {
            if(drag)
            {
                GC gc = event.gc;

                //gc.setBackground(top.getDefault().getSystemColor(SWT.COLOR_BLACK));
                gc.setAlpha(128);

                int minX = Math.min(startX, endX);
                int minY = Math.min(startY, endY);

                int maxX = Math.max(startX, endX);
                int maxY = Math.max(startY, endY);

                int width = maxX - minX;
                int height = maxY - minY;

                gc.fillRectangle(minX, minY, width, height);
            }
        }
    });

The problem is, that the image keeps flickering as I drag the mouse across.

Can you suggest a fix?

Update:

By passing SWT.DOUBLE_BUFFERED to the Canvas constructor, it enabled double buffering!!!

imageCanvas = new Canvas(top, SWT.V_SCROLL | SWT.H_SCROLL | SWT.NO_REDRAW_RESIZE | SWT.DOUBLE_BUFFERED);

Answer 1

Looks like you're adding two SWT.Paint listeners to the same Canvas and they are competing against each other.

Your first listener paints the "background" and the second one paints the "foreground". The flickering you see is most likely the two of them painting alternatingly.

Consider something like this with just one paint listener:

canvas.addListener(SWT.Paint, new Listener() {
    @Override
    public void handleEvent(Event event)
    {
        // paint background

        if(drag)
        {
            // paint foreground
        }
    }
});

---

If this doesn't change anything, try using SWT.DOUBLE_BUFFERED as the style of the Canvas .

Answer 2

To answer to this was very simple!!! By passing SWT.DOUBLE_BUFFERED to the Canvas constructor, it enabled double buffering!!!

imageCanvas = new Canvas(top, SWT.V_SCROLL | SWT.H_SCROLL | SWT.NO_REDRAW_RESIZE | SWT.DOUBLE_BUFFERED);

This completely removed the flicker effect