Convert Transparent PNG to JPG with Non-Black Background Color

c# .net image-conversion alpha-transparency
DaveS picture DaveS · Jun 29, 2011 · Viewed 41.6k times · Source

I'm using System.Drawing.Image in .Net to do a simple conversion from png to jpeg. I'm basically just using these two lines of code:

Image img = Image.FromFile(filename);
img.Save(newFilename, System.Drawing.Imaging.ImageFormat.Jpeg);

it works fine except for when the png files contain transparency due to the alpha channel. In which case the converted jpeg has a black background. Is there any way to make the background white instead?

Answer

Ry- picture Ry- · Jun 29, 2011 
// Assumes myImage is the PNG you are converting
using (var b = new Bitmap(myImage.Width, myImage.Height)) {
    b.SetResolution(myImage.HorizontalResolution, myImage.VerticalResolution);

    using (var g = Graphics.FromImage(b)) {
        g.Clear(Color.White);
        g.DrawImageUnscaled(myImage, 0, 0);
    }

    // Now save b as a JPEG like you normally would
}

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