Draw a shape in GDI+ that overlays opaque pixels with transparency

Question

I have a mask bitmap (bmpMask) that I am drawing onto a destination bitmap (bmpDest). Both bitmaps have alpha channels but are already full of opaque content.

What I want to do is produce transparent areas using GDI+ 'Draw...' methods on bmpMask so that bmpDest shows through when I draw bmpMask over it.

Of course gMask.DrawLine(Pens.Transparent, 0, y, wMax, y) causes no change to bmpMask, because GDI+ works as designed and doesn't draw anything when transparent. Even with semitransparent colors, only the r,g,b values of bmpMask's pixels are updated.

But what I want to do is issue a draw method that will change the alpha channel of bmpMask so it is transparent when drawn onto bmpDest. I know I can do this with SetPixel or speedier unsafe or Marshall alternatives, but that would cause a much more complicated solution. Thanks.

Answer 1

To my knowledge, the only way to alter the alpha channel is, as you say, to use SetPixel or (of course, much better) pointers. Depending on what you're trying to do to, you could get by with creative use of a ColorMatrix. That will let you alter a DrawImage operation in such a way that you can interchange the meaning of R/G/B/A as you draw it. Try this to get an idea:

        using ( var bmp = new Bitmap( 100, 100 ) )
        using ( var g = Graphics.FromImage( bmp ) )
        using ( var ia = new ImageAttributes() )
        {
            float R = 1;
            float G = 0;
            float B = 0;
            ia.SetColorMatrix( new ColorMatrix( new float[][] {
                new float[] {1, 0, 0, R, 0},
                new float[] {0, 1, 0, G, 0},
                new float[] {0, 0, 1, B, 0},
                new float[] {0, 0, 0, 0, 0},
                new float[] {0, 0, 0, 0, 1}} ) );
            g.Clear( Color.White );
            g.FillEllipse( Brushes.Blue, 10, 10, 30, 30 );
            g.FillEllipse( Brushes.Red, 60, 10, 30, 30 );
            g.FillEllipse( Brushes.Green, 10, 60, 30, 30 );
            g.FillEllipse( Brushes.Black, 60, 60, 30, 30 );
            e.Graphics.DrawImage(
                bmp,
                new Rectangle( 0, 0, 100, 100 ),
                0, 0, 100, 100,
                GraphicsUnit.Pixel,
                ia );
        }

Try setting R/G/B to 1 respectively and see how that changes what colors that come out solid/transparent. But, if you should like to alter the alpha channel of the bitmap directly, here's how to do it: Soft edged images in GDI+.