Using Graphics.DrawImage() to Draw Image with Transparency/Alpha Channel

Question

I'm copying an image. (My actual code is resizing the image but that's not relevant to my question.) My code looks something like this.

Image src = ...

using (Image dest = new Bitmap(width, height))
{
    Graphics graph = Graphics.FromImage(dest);
    graph.InterpolationMode = InterpolationMode.HighQualityBicubic;
    graph.DrawImage(src, 0, 0, width, height);
    dest.Save(filename, saveFormat);
}

This seems to work great unless src is loaded from an image with transparencies (such as GIF) or an alpha channel (such as PNG).

How can I get DrawImage() to transfer the transparencies/alpha channel to the new image, and then keep them when I save the file?

The question is what is the value of saveFormat variable? It should be PixelFormat.Format32bppArgb (or another format with alpha channel handled). Try also: using (Image dest = new Bitmap(width, height, PixelFormat.Format32bppArgb)). — Lukasz M
This seems to work for alpha channels, but not for GIF transparencies. saveFormat will vary depending on the format of the input file. If the input file does not support alpha channel, then it's safe to assume the image will not contain alpha channel pixels. — Jonathan Wood
Thanks for a quick reply. In the meantime I've found an article about GIF transparency. Take a look at it: support.microsoft.com/kb/319061. I suppose that basically you just have to get the image's palette and set the color saved in the first index to a transparent one. — Lukasz M

Hans Passant Hans Passant · Accepted Answer · 2012-05-19T00:28:04

It is pretty unclear, there's a lot you didn't say. The biggest issue with transparency is that you can't see it. You skipped a couple of steps, you didn't explicitly specify the pixel format of your new bitmap, you didn't initialize it at all and you didn't say what output format you use. Some don't support transparency. So let's make a version that makes it crystal clear. From a PNG image that looks like this in paint.net:

enter image description here

Using this code

        using (var src = new Bitmap("c:/temp/trans.png"))
        using (var bmp = new Bitmap(100, 100, PixelFormat.Format32bppPArgb)) 
        using (var gr = Graphics.FromImage(bmp)) {
            gr.Clear(Color.Blue);
            gr.DrawImage(src, new Rectangle(0, 0, bmp.Width, bmp.Height));
            bmp.Save("c:/temp/result.png", ImageFormat.Png);
        }

Produces this image:

enter image description here

You can clearly see the blue background so the transparency worked.

Using Graphics.DrawImage() to Draw Image with Transparency/Alpha Channel

2 Answers