Free Image Transformation

/**
 * 
 * Linear interpolation between two points.
 * Return interpolated color Y at distance l.
 * 
 * @param A ARGB for point A.
 * @param B ARGB for point B.
 * @param l Distance Y from A.
 * @param L Distance between A and B.
 * @return Interpolated color Y.
 */
public int linearInterpolate(int A, int B, int l, int L) {
    // extract r, g, b information
    // A and B is a ARGB-packed int so we use bit operation to extract
    int Ar = (A >> 16) & 0xff ;
    int Ag = (A >> 8) & 0xff ;
    int Ab = A & 0xff ;
    int Br = (B >> 16) & 0xff ;
    int Bg = (B >> 8) & 0xff ;
    int Bb = B & 0xff ;
    // now calculate Y. convert float to avoid early rounding
    // There are better ways but this is for clarity's sake
    int Yr = (int)(Ar + l*(Br - Ar)/(float)L) ;
    int Yg = (int)(Ag + l*(Bg - Ag)/(float)L) ;
    int Yb = (int)(Ab + l*(Bb - Ab)/(float)L) ;
    // pack ARGB with hardcoded alpha
    return 0xff000000 | // alpha
            ((Yr << 16) & 0xff0000) |
            ((Yg << 8) & 0xff00) |
            (Yb & 0xff) ;
}

        protected override void OnPaint(PaintEventArgs e)
        {
            Point ptTopLeft = new Point(320, 97);
            Point ptTopRight = new Point(653, 24);
            Point ptBottomRight = new Point(653, 553);
            Point ptBottomLeft = new Point(320, 481);
 

            YLScsDrawing.Imaging.Filters.FreeTransform filter = new YLScsDrawing.Imaging.Filters.FreeTransform();
            filter.Bitmap = (Bitmap)((System.Drawing.Image)(global::ChartThumbnailDesign.Properties.Resources.Castle));
 
            // assign FourCorners (the four X/Y coords) of the new perspective shape
            filter.FourCorners = new System.Drawing.PointF[] { (PointF)ptTopLeft, (PointF)ptTopRight, (PointF)ptBottomRight, (PointF)ptBottomLeft };
            filter.IsBilinearInterpolation = true; // optional for higher quality

            using (System.Drawing.Bitmap perspectiveImg = filter.Bitmap)
            {
                e.Graphics.DrawImage(perspectiveImg, ptTopLeft);
 
                using (GraphicsPath path = new GraphicsPath())
                {
                    path.AddPolygon(new Point[] { ptTopLeft, ptTopRight, ptBottomRight, ptBottomLeft });
                    e.Graphics.DrawPath(new Pen(Color.Blue, 4.0f), path);
                }
            }
        }