Generate transparent PNG c#

Question

I have the function below to generate a sample logo. What I want to do is to return a transparent png or gif instead of a white background.

How can I do that?

private Bitmap CreateLogo(string subdomain)
{
    Bitmap objBmpImage = new Bitmap(1, 1);
    int intWidth  = 0;
    int intHeight = 0;
    Font objFont = new Font(
        "Arial", 
        13, 
        System.Drawing.FontStyle.Bold, 
        System.Drawing.GraphicsUnit.Pixel);

    Graphics objGraphics = Graphics.FromImage(objBmpImage);
    intWidth  = (int)objGraphics.MeasureString(subdomain, objFont).Width;
    intHeight = (int)objGraphics.MeasureString(subdomain, objFont).Height;

    objBmpImage = new Bitmap(objBmpImage, new Size(intWidth, intHeight));
    objGraphics = Graphics.FromImage(objBmpImage);
    objGraphics.Clear(Color.White);
    objGraphics.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;
    objGraphics.TextRenderingHint = System.Drawing.Text.TextRenderingHint.AntiAlias;
    objGraphics.DrawString(
        subdomain, objFont, 
        new SolidBrush(Color.FromArgb(102, 102, 102)), 0, 0);

    objGraphics.Flush();
    return (objBmpImage);
}

Here is the end result:

context.Response.ContentType = "image/png";
using (MemoryStream memStream = new MemoryStream()) 
{ 
    CreateLogo(_subdname).Save(memStream, ImageFormat.Png); 
    memStream.WriteTo(context.Response.OutputStream); 
}

In the CreateLogo function:

• objGraphics.Clear(Color.White) was changed to objGraphics.Clear(Color.Transparent)
• new SolidBrush(Color.FromArgb(102, 102, 102)) changed to new SolidBrush(Color.FromArgb(255, 255, 255))

Answer 1

You can do something like this:

Bitmap bmp = new Bitmap(300, 300);
Graphics g = Graphics.FromImage(bmp);

g.Clear(Color.Transparent);
g.FillRectangle(Brushes.Red, 100, 100, 100, 100);

g.Flush();
bmp.Save("test.png", System.Drawing.Imaging.ImageFormat.Png);

Answer 2

看看你能用 C# 制作一个 alpha 透明的 PNG 吗？