Using System.Drawing, how can I draw something that imitates the effect of a yellow highlighting marker?

Question

I want what's "behind" the yellow to show through.

EDIT 1: But, if I'm drawing on "white", I'd like the marker color to retain its pure yellowness.

EDIT 2: @Kevin's answer is probably correct, and I marked it correct, even though I didn't code it up. In my code, I'm settling for @Guffa's answer, using Color.FromArgb.

Answer 1

You use a semi transparent color, for example with a 50% alpha opacity:

Color.FromArgb(128, Color.Yellow)

With a lower alpha value more of the background will show through.

Answer 2

Try using a SolidBrush initialised with a Color value with an alpha of less then 255. This should create a semi-transparent brush of the colour you use.

Answer 3

A highlighter is pigment, so it's essentially subtractive — you want to turn white into yellow, but not black into yellow. I don't know .NET, but what you want is a non-standard “blending mode”, specifically subtract. Specifically, set the blending mode to subtract and your color to pure blue (the color you want to subtract, leaving yellow). Black will be left alone, since there's no blue to subtract, and white will become yellow.

Unfortunately, many modern drawing interfaces leave out blending modes other than alpha, and it looks like this might be one of them. If you have access to the bitmap, you can implement it yourself — take each pixel value and set the blue component to zero. Or if the area you want to highlight is complicated, then: make a copy of your image, draw black over the highlighted area in the copy, then combine the red and green channels from the original and the blue channel from the copy into the final result image.

Answer 4

This is the code you need:

    protected override void OnPaint( PaintEventArgs e )
    {
        using ( var bmp = new Bitmap( 100, 100 ) )
        using ( var g = Graphics.FromImage( bmp ) )
        using ( var ia = new ImageAttributes() )
        {
            // 1. create a sample bitmap
            g.Clear( Color.White );
            var p = Point.Empty;
            foreach ( var color in new Color[] { Color.Black, Color.Gray, Color.LightBlue, Color.Green, Color.Red, Color.Magenta } )
                using ( var brush = new SolidBrush( color ) )
                {
                    g.DrawString( "Some sample text", SystemFonts.DefaultFont, brush, p );
                    p.Offset( 0, 16 );
                }
            // 2. transfer the bitmap on screen
            e.Graphics.DrawImage( bmp, Point.Empty );
            // 3. transfer a part of the bitmap on screen again, this time removing all blue
            ia.SetColorMatrix( new ColorMatrix( new float[][] {
                        new float[] {1, 0, 0, 0, 0},
                        new float[] {0, 1, 0, 0, 0},
                        new float[] {0, 0, 0, 0, 0},
                        new float[] {0, 0, 0, 1, 0},
                        new float[] {0, 0, 0, 0, 1}} ) );
            e.Graphics.DrawImage(
                bmp,
                new Rectangle( 30, 0, 40, 100 ),
                30, 0, 40, 100,
                GraphicsUnit.Pixel,
                ia );
        }
    }