Question

I am trying to flatten a png with transparency over a bitmap with a solid colour.

so far i have this

    using (System.Drawing.Image backImage = System.Drawing.Image.FromFile(layer1imagename))

    using (System.Drawing.Image frontImage = System.Drawing.Image.FromFile(HttpContext.Current.Server.MapPath("~/") + layer2))

    using (Image IMG1 = new Bitmap(251, 400))
    using (Graphics compositeGraphics = Graphics.FromImage(IMG1))
    {
        compositeGraphics.CompositingMode = System.Drawing.Drawing2D.CompositingMode.SourceCopy;
        compositeGraphics.DrawImageUnscaled(backImage, 0, 0);
        compositeGraphics.DrawImageUnscaled(frontImage, 0, 0);
        compositeGraphics.Dispose();
        frontImage.Dispose();
        backImage.Dispose();
        IMG1.Save(layer1imagename, System.Drawing.Imaging.ImageFormat.Png);
    }

However this turns removes the transparency on the top layer. How do i retain a transparent png on the top?

Answer 1

change using (Image IMG1 = new Bitmap(251, 400)) to using (Image IMG1 = new Bitmap(251, 400, PixelFormat.Format32bppArgb)).

For MSDN references see:

• http://msdn.microsoft.com/en-us/library/3z132tat.aspx
• http://msdn.microsoft.com/en-us/library/system.drawing.imaging.pixelformat.aspx

EDIT – as per comment:

To draw the PNG you want you need to use a different CompositingMode – SourceOver instead of SourceCopy.