How can I remove transparency from an animated GIF?

Question

I can't believe this hasn't been asked yet, but...

How can I replace the transparent background of a GIF image to white, without breaking the animation?

I am way out of my field here, so I'm pretty much lost (I really don't know what to do).

This is what I tried without luck:

public byte[] RemoveTransparency(byte[] gifBytes)
{
    using(MemoryStream gifStream = new MemoryStream(gifBytes))
    using(Image gifImage = Image.FromStream(gifStream))
    using(Image newImage = new Bitmap(gifImage.Width, gifImage.Height, PixelFormat.Format32bppPArgb))
    using(MemoryStream newImageStream = new MemoryStream())
    {
        Color background = Color.White;

        newImage.SetResolution(gifImage.HorizontalResolution, gifImage.VerticalResolution);
        using (Graphics graphics = Graphics.FromImage(newImage))
        {
            graphics.Clear(background );
            graphics.DrawImage(gifImage, 0, 0, gifImage.Width, gifImage.Height);
        }
        newImage.Save(newImageStream);
        return newImageStream.ToArray();
    }
}

Maybe this could help you: http://stackoverflow.com/questions/618259/remove-transparency-in-images-with-c-sharp — Erik Auranaune, Sep 23 '15 at 18:49
If your GIF has animation, you could atleast try to loop through every frame of your GIF, remove the transparency with the solution above, then save each frame to disk, then use some other program to make it a GIF again (or C# if you find out how) (https://stackoverflow.com/questions/11856210/trying-to-iterate-over-the-frames-in-a-system-drawing-image-throws-invalid-para) — Maximilian Gerhardt, Sep 23 '15 at 18:56
You'll probably have to do something like your example for each frame in the gif http://stackoverflow.com/questions/540701/access-gif-frames-with-c-sharp — John Boker, Sep 23 '15 at 18:56
Wouldn't I break the animation? I mean, when I am creating a `new Bitmap(...)`, am I not losing my original animation headers? — Matias Cicero, Sep 23 '15 at 19:02
On a sidenote, how did you get the animated gif to animate in the first place? — Sophie Coyne, Sep 23 '15 at 19:10

score 1 · Answer 1 · answered Sep 23 '15 at 18:57

You can use a bitmap.

using(var ms = new MemoryStream(gifBytes))
    using(var b = new Bitmap(Image.FromStream(ms)))
    {
        for(var y = 0; y < b.Height; ++y)
            for(var x = 0; x < b.Width; ++x)
                if(b.GetPixel(x, y).A == 0) //Completely opaque
                    b.SetPixel(x, y, Color.White); //Sets white
    }
//Note: b goes out of scope here and will be deleted eventually.

Do note that this algorithm is extremely slow on large images, a typical approach to get around this is to access the internal buffer directly which requires unsafe context. See MSDN Bitmap reference or this: https://msdn.microsoft.com/en-us/library/5ey6h79d(v=vs.110).aspx

If the bitmap breaks the animation, load each frame as described in the comments of your post and then write the final buffer back into main image memory. Comment if you want me to edit for this context.

How can I remove transparency from an animated GIF?

1 Answers1