GDI+: how do you render a Graphics object to a bitmap on a background thread?

Question

I\'d like to use GDI+ to render an image on a background thread. I found this example on how to rotate an image using GDI+, which is the operation I\'d like to do.

Answer 1

To draw on a bitmap you don't want to create a Graphics object for an UI control. You create a Graphics object for the bitmap using the FromImage method:

Graphics g = Graphics.FromImage(theImage);

A Graphics object doesn't contain the graphics that you draw to it, instead it's just a tool to draw on another canvas, which is usually the screen, but it can also be a Bitmap object.

So, you don't draw first and then extract the bitmap, you create the bitmap first, then create the Graphics object to draw on it:

Bitmap destination = new Bitmap(200, 200);
using (Graphics g = Graphics.FromImage(destination)) {
   Matrix rotation = new Matrix();
   rotation.Rotate(30);
   g.Transform = rotation;
   g.DrawImage(source, 0, 0, 200, 200);
}