[C#] Rotate bitmap with a special angle

public unsafe void Rotate(Bitmap bmp, float angle)
{
  int width=bmp.Width;
  int height=bmp.Height;
 /*
  * right, down = positive
  * p1------------p2
  * |            | 
  * p4------------p3
  * 
  * p1(0,0), p2(width-1,0), p3(width-1,height-1), p4(0,height-1)
  * 
  * In this coordinate system(left-handed coordinate system), clockwise rotation matrix (theta): (cos -sin 
  *                                                                                               sin  cos)
  * 
  */
 //Calculate new vertex
 Point p1 = RotatePoint(new Point(0, 0), angle);
 Point p2 = RotatePoint(new Point(width - 1, 0), angle);
 Point p3 = RotatePoint(new Point(width - 1, height - 1), angle);
 Point p4 = RotatePoint(new Point(0, height - 1), angle);
 //Calculate new size
 int dstWidth = Math.Max(Math.Abs(p3.X - p1.X) + 1, Math.Abs(p4.X - p2.X) + 1);
 int dstHeight = Math.Max(Math.Abs(p3.Y - p1.X) + 1, Math.Abs(p4.Y - p2.Y) + 1);
 /*
  * Calculate offset between old and new coordinate system
  * left-top point in new coordiante system -> (0,0)
  * 
  */
 int offsetX = -new int[4] { p1.X, p2.X, p3.X, p4.X }.Min();
 int offsetY = -new int[4] { p1.Y, p2.Y, p3.Y, p4.Y }.Min();
 //create bmp
 Bitmap dstBitmap = new Bitmap(dstWidth, dstHeight, PixelFormat.Format32bppArgb);
 Rectangle srcRect = new Rectangle(0, 0, width, height);
 Rectangle dstRect = new Rectangle(0, 0, dstWidth, dstHeight);
 BitmapData srcBmpData = bmp.LockBits(srcRect, ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb);
 BitmapData dstBmpData = dstBitmap.LockBits(dstRect, ImageLockMode.WriteOnly, PixelFormat.Format32bppArgb);
 //define sin and cos
 double sin = Math.Sin(angle * Math.PI / 180);
 double cos = Math.Cos(angle * Math.PI / 180);
 int srcStride = srcBmpData.Stride;
 int dstStride = dstBmpData.Stride;
 //define pointer
 byte* srcP = (byte*)srcBmpData.Scan0.ToPointer();
 byte* dstP = (byte*)dstBmpData.Scan0.ToPointer();
 Parallel.For(0,dstHeight,i=>
 {
  Parallel.For(0, dstWidth, j =>
  {
   int k = 4 * j + i * dstStride;
   //Calculate corresponding point in old coordinate system
   Point oldPoint = RotatePoint(new Point(j - offsetX, i - offsetY), -angle);
   if (oldPoint.X >= 0 && oldPoint.X < width && oldPoint.Y >= 0 && oldPoint.Y < height)
   {
    dstP[k] = srcP[4 * oldPoint.X + srcStride * oldPoint.Y];
    dstP[k + 1] = srcP[4 * oldPoint.X + srcStride * oldPoint.Y + 1];
    dstP[k + 2] = srcP[4 * oldPoint.X + srcStride * oldPoint.Y + 2];
    dstP[k + 3] = srcP[4 * oldPoint.X + srcStride * oldPoint.Y + 3];
   }
   else
   {
    dstP[k] = dstP[k + 1] = dstP[k + 2] = 0xff;
    dstP[k + 3] = 0x0;
   }
  });
 });
 bmp.UnlockBits(srcBmpData);
 dstBitmap.UnlockBits(dstBmpData);
 bmp = (Bitmap)dstBitmap.Clone();
 dstBitmap.Dispose();
 width = bmp.Width;
 height = bmp.Height;
}