CODEWITHSUNDEEP
c#.netimage-processingtransparencygif
Fred Thomas
Fred Thomas

Reputation: 944

Transparency and GIF in .NET -- Inconsistent behavior

I am trying to edit and save an image using the .net Bitmap class. Some of the pixels are transparent, and they get converted to black under certain circumstances. If I save the same image like this:

image.Save("copy1.png", System.Drawing.Imaging.ImageFormat.Png);
image.Save("copy2.gif", System.Drawing.Imaging.ImageFormat.Gif);
image.Save("copy3.gif");

(Image being originally a gif) the first and third are correct retaining the transparency, but the middle one sets all the transparent pixels to black. I'm not sure what I am doing wrong, AFAIK the last two lines should be equivalent.

Here is a sample program of what I am talking about:

using System.Drawing;
using System.Net;

namespace TestGif
{
    class Program
    {
        static void Main(string[] args)
        {
            Bitmap bitmap = new Bitmap(WebRequest.Create(
                "http://rlis.com/images/column/ie_icon.gif")
                           .GetResponse()
                           .GetResponseStream());

            int width = bitmap.Width;
            int height = bitmap.Height;
            Bitmap copy = new Bitmap(width, height);
            var graphics = Graphics.FromImage(copy);
            graphics.DrawImage(bitmap, new Point(0, 0));
            copy.Save("copy1.png", System.Drawing.Imaging.ImageFormat.Png);
            copy.Save("copy2.gif", System.Drawing.Imaging.ImageFormat.Gif);
            copy.Save("copy3.gif");
        }
    }
}

Upvotes: 5

Views: 2508

Answers (1)

ʞᴉɯ
ʞᴉɯ

Reputation: 5594

Your last line 

copy.Save("copy3.gif");

does not save as gif file, but as png, since the extension is not sufficient to specify the saving format. To make a transparent gif, use something like

Color c = copy.GetPixel(0, 0);
copy.MakeTransparent(c);

Your code is creating a new bitmap, possibly losing the original gif informations.

Upvotes: 3

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