Colour outside a box in Android, while keeping the box transparent?

Question

I have a box in my Android application, which needs to be transparent, while filling the rest of the frame outside the box with a specific colour.

How exactly could this be done in Canvas or OpenCV or any other means?

Answer 1

Here is an example showing a circle with transparent hole you can implement the same using Rectangle instead of circle.

public class OverlayWithHoleImageView extends ImageView {

private RectF circleRect;
private int radius;

public OverlayWithHoleImageView(Context context, AttributeSet attrs) {
    super(context, attrs);
    //In versions > 3.0 need to define layer Type
    if (android.os.Build.VERSION.SDK_INT >= 11)
    {
        setLayerType(View.LAYER_TYPE_SOFTWARE, null);
    }
}

public void setCircle(RectF rect, int radius) {
    this.circleRect = rect;
    this.radius = radius;
    //Redraw after defining circle
    postInvalidate();
}

@Override
public void onDraw(Canvas canvas) {
    super.onDraw(canvas);
    if(circleRect != null) {
        Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);
        paint.setColor(getResources().getColor(android.R.color.black));
        paint.setStyle(Paint.Style.FILL);
        canvas.drawPaint(paint);

        paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
        canvas.drawRoundRect(circleRect, radius, radius, paint);
    }
}

}

reference : Medium