Question

I’m using C# and I would like to draw some polygons on a Form, then to save the graphics in a Bitmap.

Following this question answers I wrote a method in my Form class:

  private void draw_pol()
  {
      Graphics d = this.CreateGraphics();

      // drawing stuff

      Bitmap bmp = new Bitmap(this.Width, this.Height, d);
      bmp.Save("image.bmp");
  }

In this way the Form displays correctly the graphics and the Bitmap file named “image.bmp” is created, but that file is a white image.

Why isn’t the bmp file showing any image? What I’m doing wrong?

Thank you very much.

Answer 1

The graphics parameter you are passing to your bitmap is only used to specify the resolution of the bitmap. It does not in any way paint to the bitmap.

from MSDN:

The new Bitmap that this method creates takes its horizontal and vertical resolution from the DpiX and DpiY properties of g, respectively.

Instead, use Graphics.FromImage() to get a Graphics object you can use. Moreover, you should Dispose the Graphics object after painting. This is an ideal usage for the using statement.

Bitmap bmp = new Bitmap(this.Width, this.Height);
using (Graphics g = Graphics.FromImage(bmp))
{
    //paint stuff
}
bmp.Save(yourFile);

If you also need to paint this to the form, you can easily just draw the bitmap you created:

Graphics g = this.CreateGraphics();
g.DrawImage(bmp, 0, 0);