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.

private void RotationMenu_Click(object sender, System.EventArgs e)
{
    Graphics g = this.CreateGraphics();
    g.Clear(this.BackColor);
    Bitmap curBitmap = new Bitmap(@"roses.jpg"); 
    g.DrawImage(curBitmap, 0, 0, 200, 200);  

    // Create a Matrix object, call its Rotate method,
    // and set it as Graphics.Transform
    Matrix X = new Matrix();
    X.Rotate(30);
    g.Transform = X;  

    // Draw image
    g.DrawImage(curBitmap, 
    new Rectangle(205, 0, 200, 200), 
        0, 0, curBitmap.Width, 
        curBitmap.Height, 
        GraphicsUnit.Pixel);  

    // Dispose of objects
    curBitmap.Dispose();
    g.Dispose(); 
} 

My question has two parts:

1. How would you accomplish this.CreateGraphics() on a background thread? Is it possible? My understanding is that a UI object is this in this example. So if I’m doing this processing on a background thread, how would I create a graphics object?

2. How would I then extract a bitmap from the Graphics object I’m using once I’m done processing? I haven’t been able to find a good example of how to do that.

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Also: when formatting a code sample, how do I add newlines? If someone could leave me a comment explaining that I’d really appreciate it. Thanks!

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);
}