How to rotate a bitmap cleanly in GDI+ without a string of images appearing?

Question

I have drawn a bitmap image using GDI+ in C# on a windows form and would like to rotate the image while the up arrow key is pressed.

The effect I would like to get is to see the image move in a circular motion while it rotates. The rotation angle is incremented by 5 degrees each time the key pressed event handler executes.

The problem is that while the image does move in a circular path, a string of previously rotated images appear along the rotation path instead of just the latest rotated image.

i.e. If the maximum angle is set to 20 degrees, 4 images will be on the screen with the first rotated at 5 degrees, the next at 10 degrees and so on.

How do I just show a single bitmap image rotating instead of a string of previous images showing up along the rotation path?

The code is shown below:

[code]

(in the form1 load event)
Graphics g = CreateGraphics();
granade = new Bitmap("granade.jpg");

(in the form1 paint event)
g.DrawImage(granade,0,0,20,20);

(in the key down event)
Matrix rotMatrix = new Matrix();
rotateAngle -= 5;

rotMatrix.RotateAt(rotateAngle);
g.Transform = rotMatrix;

[/code]

Answer 1

Use Invalidate to clear the drawing surface.

If you only want a portion of the the surface invalidated you can specify the rectangle.

For example, if you are drawing on a form you would call myForm.Invalidate();

Answer 2

I haven't used GDI in a while, but I think you're missing a call to a function that clears the drawing area before you call Transform(...). I forget what the function is called though. Look through the MSDN

Without calling it, your newly rendered image will just go on top of the previous ones.

Answer 3

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