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Friday, December 30, 2016

Making Transparent PNGs in C#

This bit of code shows how to make a PNG with transparency totally from scratch in code.  This code will save the image back as a file, but it could as easily be streamed back to a web request.  This is a complete console app, if you want to compile it, you'll need to add a reference to System.Drawing.



using System.Drawing;
using System.Drawing.Drawing2D;
using System.Drawing.Imaging;

namespace TransparentPNGTests
{
    class Program
    {
        static void Main(string[] args)
        {
            using (var bmp = new System.Drawing.Bitmap(100, 100, PixelFormat.Format32bppArgb))
            using (Graphics g = Graphics.FromImage(bmp))
            using (Font f = new Font("Univers", 14f, FontStyle.Regular, GraphicsUnit.Pixel))
            {
                //set up bitmap
                g.Clear(Color.Transparent);
                g.SmoothingMode = SmoothingMode.AntiAlias;
                g.TextRenderingHint = System.Drawing.Text.TextRenderingHint.AntiAlias;

                //draw text                
                Brush b = Brushes.Black;
                Rectangle textrect = new Rectangle(5, 5, 90, 90);  
                
                //using the rectangle instead of a point is a quick way to do word wrapping on the graphic
                g.DrawString("This is a bunch of text, for testing.", f, b, textrect);

                //save from the bitmap
                g.Flush(FlushIntention.Flush);   //this seems to be wise, but unnecessary.
                bmp.Save("bmpsave.png", ImageFormat.Png);                

                //save from the image - loses transparency
                //Image image = Image.FromHbitmap(bmp.GetHbitmap());
                //image.Save("image.png");
            }
        }
    }

}


Note that the last 2 lines:

Image image = Image.FromHbitmap(bmp.GetHbitmap());
image.Save("image.png");
Are the wrong way.   This is the way you'll find if you Google for "How do I save a Graphic object to a file or stream".  Somehow, the GetHbitmap function manages to mangle the formatting out of the image.
Here are the resulting images from this code.
Bitmap.Save()

Image Save()

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