如何在 C# WPF 中使图像（徽标）的背景透明？

Question

I've an image ( logo ) and I would like to pass it as an argument and make the background transparent and then save it.

How can I achieve this?

This what I've tried:

public string SaveImage(Bitmap bmp,string SavePath,string Name)
    {
        string path = "";
        var format = ImageFormat.Jpeg;
        using (var m = new MemoryStream())
        {
            bmp.Save(m, format);

            var img = System.Drawing.Image.FromStream(m);


            using (var g = Graphics.FromImage(img)) // This is what I've used for making the background transparent 
            {
                g.Clear(System.Drawing.Color.Transparent);
                g.Save();
            }
            var ImgPath = Path.Combine(SavePath, Path.ChangeExtension(Name,"png"));

            img.Save(ImgPath);
            path = ImgPath;

        }

        return path;
    }

After testing this function I'm getting a black image as an output ( save image )

Answer 1

Some remarks for your code:

• You use System.Drawing types wrapping native GDI+ components, not WPF.
• You save the Bitmap with JPEG encoder, which does not support transparency. Even if the logo had a transparent background it is removed by the JPEG encoder
• Now you reload the saved image and clear the complete content with transparent color (which will not work either as the reloaded JPEG image will have 24-bit RGB pixel format without alpha support so the whole image will be black)

Possible solutions:

• Use PNG encoder instead, which supports transparency. If the logo had a transparent background you don't even need anything else.
• If the logo was not transparent originally, use the MakeTransparent method instead of Clear . It will replace a single color so it is not able to restore possible anti-aliased alpha blending. It will also convert the pixel format so can be used also on the reloaded JPEG.
• To do the same in WPF you can use this library (disclaimer: written by me). You can use the GetReadWriteBitmapData extension method on a WriteableBitmap to access the same MakeTransparent functionality as above along with a sort of other features . But to restore a nicely anti-aliased alpha background you might want to use the TransformColors method instead if you know how to separate the blended colors.