Question

I want to replace all pixels in a rectangular region of a Bitmap with ‘transparent white’ – i.e., a=0, b=255, r=255, g=255.

FillRectangle doesnt do this – given a transparent brush, the existing pixels are unchanged.

Do I need to use SetPixel individually for each pixel in the rectangle?

Answer 1

You’ll have to set the Graphics.CompositingMode property. For example:

protected override void OnPaint(PaintEventArgs e) {
    using (var img = Properties.Resources.Chrysanthemum) {
        e.Graphics.DrawImage(img, 0, 0); 
        e.Graphics.CompositingMode = System.Drawing.Drawing2D.CompositingMode.SourceCopy;
        using (var br = new SolidBrush(Color.FromArgb(0, 255, 255, 255))) {
            e.Graphics.FillRectangle(br, new Rectangle(50, 50, 100, 100));
        }
    }
}

The actual color you use doesn’t matter, you’ll get a black rectangle with an alpha of 0.