A C# function for changing the opacity on an image to the specified value. It uses the System.Drawing, System.Drawing.Imaging and System.Drawing.Drawing2D namespaces.

1. public static Image SetImgOpacity(Image imgPic, float imgOpac)
2. {
3.       Bitmap bmpPic = new Bitmap(imgPic.Width, imgPic.Height);
4.       Graphics gfxPic = Graphics.FromImage(bmpPic);
5.       ColorMatrix cmxPic = new ColorMatrix();
6.       cmxPic.Matrix33 = imgOpac;
7. 
8.       ImageAttributes iaPic = new ImageAttributes();
9.       iaPic.SetColorMatrix(cmxPic, ColorMatrixFlag.Default, ColorAdjustType.Bitmap);
10.       gfxPic.DrawImage(imgPic, new Rectangle(0, 0, bmpPic.Width, bmpPic.Height), 0, 0, imgPic.Width, imgPic.Height, GraphicsUnit.Pixel, iaPic);
11.       gfxPic.Dispose();
12. 
13.       return bmpPic;
14. }