Scrolling Background Terrain Drawing

gistfile1.cs

            int vpw = graphics.GraphicsDevice.Viewport.Width;
            int vph = graphics.GraphicsDevice.Viewport.Height;

            Rectangle rect = new Rectangle(0, 0, vpw, vph);

            spriteBatch.Begin();
            
            //construct a rectangle to select an area of background to draw
            //position(upper left of) rectangle is camera position minus halh size of viewport
            Rectangle terrainRect=new Rectangle(
                    (int)(camera.Position.X-vpw/2), 
                    (int)(camera.Position.Y-vph/2),
                    graphics.GraphicsDevice.Viewport.Width, 
                    graphics.GraphicsDevice.Viewport.Height);
            spriteBatch.Draw(terrain, Vector2.Zero, 

                terrainRect, Color.White);
            spriteBatch.End();