图片旋转修改老外的代码，返回中增加第四个顶点

        public static Bitmap RotateImage(Image image, int iw, int ih, float angle, out Point[] points)
        {
            if (image == null)
                throw new ArgumentNullException("image");

            const double pi2 = Math.PI / 2.0;

            // Why can't C# allow these to be const, or at least readonly
            // *sigh*  I'm starting to talk like Christian Graus :omg:
            double oldWidth = (double)iw;
            double oldHeight = (double)ih;

            // Convert degrees to radians
            double theta = ((double)angle) * Math.PI / 180.0;
            double locked_theta = theta;

            // Ensure theta is now [0, 2pi)
            while (locked_theta < 0.0)
                locked_theta += 2 * Math.PI;

            double newWidth, newHeight;
            int nWidth, nHeight; // The newWidth/newHeight expressed as ints

            double adjacentTop, oppositeTop;
            double adjacentBottom, oppositeBottom;

            // We need to calculate the sides of the triangles based
            // on how much rotation is being done to the bitmap.
            //   Refer to the first paragraph in the explaination above for 
            //   reasons why.
            if ((locked_theta >= 0.0 && locked_theta < pi2) ||
                (locked_theta >= Math.PI && locked_theta < (Math.PI + pi2)))
            {
                adjacentTop = Math.Abs(Math.Cos(locked_theta)) * oldWidth;
                oppositeTop = Math.Abs(Math.Sin(locked_theta)) * oldWidth;

                adjacentBottom = Math.Abs(Math.Cos(locked_theta)) * oldHeight;
                oppositeBottom = Math.Abs(Math.Sin(locked_theta)) * oldHeight;
            }
            else
            {
                adjacentTop = Math.Abs(Math.Sin(locked_theta)) * oldHeight;
                oppositeTop = Math.Abs(Math.Cos(locked_theta)) * oldHeight;

                adjacentBottom = Math.Abs(Math.Sin(locked_theta)) * oldWidth;
                oppositeBottom = Math.Abs(Math.Cos(locked_theta)) * oldWidth;
            }

            newWidth = adjacentTop + oppositeBottom;
            newHeight = adjacentBottom + oppositeTop;

            nWidth = (int)Math.Ceiling(newWidth);
            nHeight = (int)Math.Ceiling(newHeight);

            Bitmap rotatedBmp = new Bitmap(nWidth, nHeight);

            using (Graphics g = Graphics.FromImage(rotatedBmp))
            {
                // This array will be used to pass in the three points that 
                // make up the rotated image
                //Point[] points;

                /*
                 * The values of opposite/adjacentTop/Bottom are referring to 
                 * fixed locations instead of in relation to the
                 * rotating image so I need to change which values are used
                 * based on the how much the image is rotating.
                 * 
                 * For each point, one of the coordinates will always be 0, 
                 * nWidth, or nHeight.  This because the Bitmap we are drawing on
                 * is the bounding box for the rotated bitmap.  If both of the 
                 * corrdinates for any of the given points wasn't in the set above
                 * then the bitmap we are drawing on WOULDN'T be the bounding box
                 * as required.
                 */
                if (locked_theta >= 0.0 && locked_theta < pi2)
                {
                    points = new Point[] { 
											 new Point( (int) oppositeBottom, 0 ), 
											 new Point( nWidth, (int) oppositeTop ),
											 new Point( 0, (int) adjacentBottom),
                                             new Point((int)(nWidth - oppositeBottom), (int)(oppositeTop + adjacentBottom))
										 };

                }
                else if (locked_theta >= pi2 && locked_theta < Math.PI)
                {
                    points = new Point[] { 
											 new Point( nWidth, (int) oppositeTop ),
											 new Point( (int) adjacentTop, nHeight ),
											 new Point( (int) oppositeBottom, 0 ),
                                             new Point((int)(oppositeBottom - (nWidth-adjacentTop)), (int)(nHeight - oppositeTop))					 
										 };
                }
                else if (locked_theta >= Math.PI && locked_theta < (Math.PI + pi2))
                {
                    points = new Point[] { 
											 new Point( (int) adjacentTop, nHeight ), 
											 new Point( 0, (int) adjacentBottom ),
											 new Point( nWidth, (int) oppositeTop ),
                                             new Point((int)(nWidth - adjacentTop), (int)(oppositeTop + adjacentBottom - nHeight))
										 };
                }
                else
                {
                    points = new Point[] { 
											 new Point( 0, (int) adjacentBottom ), 
											 new Point( (int) oppositeBottom, 0 ),
											 new Point( (int) adjacentTop, nHeight ),
                                             new Point((int)(adjacentTop + oppositeBottom), (int)(nHeight - adjacentBottom))		
										 };
                }

                Point[] ps = new Point[] { points[0], points[1], points[2] };
                g.DrawImage(image, ps);
            }

            return rotatedBmp;
        }