How to Achieve Transparent Corners in a Rounded Rectangle Border?

Border with Rounded Corners & Transparency: Eliminating Background Corners

Problem Statement:

The query revolves around TextBubbleBorder, a custom border designed for text areas. However, when the border is used to create a rounded rectangle (by setting the pointer size to zero), the corners outside the rectangle remained opaque, displaying the default panel color instead of the desired transparency.

Solution Overview:

The key to achieving transparency in the border corners lies in painting the background color of the parent component over the areas outside the rounded rectangle. This involves:

1. Identifying the region outside the border (using an Area).
2. Creating a clipping path that excludes this region.
3. Setting the graphics context's clip to the clipping path.
4. Drawing the parent background.
5. Restoring the original clip.

Modified Code:

The following modified paintBorder method within the TextBubbleBorder class incorporates the above steps:

@Override
public void paintBorder(
        Component c,
        Graphics g,
        int x, int y,
        int width, int height) {

    Graphics2D g2 = (Graphics2D) g;

    // ... (unchanged code)

    // Paint the BG color of the parent, everywhere outside the clip
    // of the text bubble.
    Component parent  = c.getParent();
    if (parent!=null) {
        Color bg = parent.getBackground();
        Rectangle rect = new Rectangle(0,0,width, height);
        Area borderRegion = new Area(rect);
        borderRegion.subtract(area);
        g2.setClip(borderRegion);
        g2.setColor(bg);
        g2.fillRect(0, 0, width, height);
        g2.setClip(null);
    }

    // ... (unchanged code)
}

Result:

This modification effectively draws the parent background in the areas outside the rounded corners, producing the desired transparent border. You can now create a rounded rectangle with a transparent border that seamlessly integrates with its parent component.

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