rotate bitmap windows phone 8 c#

        private double CalculateConstraintScale(double rotation, int pixelWidth, int pixelHeight)
        {
            double PiDiv180 = Math.PI / 180;
            // Convert angle to radians for the math lib
            double rotationRadians = rotation * PiDiv180;

            // Centre is half the width and height
            double width = pixelWidth / 2.0;
            double height = pixelHeight / 2.0;
            double radius = Math.Sqrt(width * width + height * height);

            // Convert BR corner into polar coordinates
            double angle = Math.Atan(height / width);

            // Now create the matching BL corner in polar coordinates
            double angle2 = Math.Atan(height / -width);

            // Apply the rotation to the points
            angle += rotationRadians;
            angle2 += rotationRadians;

            // Convert back to rectangular coordinate
            double x = Math.Abs(radius * Math.Cos(angle));
            double y = Math.Abs(radius * Math.Sin(angle));
            double x2 = Math.Abs(radius * Math.Cos(angle2));
            double y2 = Math.Abs(radius * Math.Sin(angle2));

            // Find the largest extents in X & Y
            x = Math.Max(x, x2);
            y = Math.Max(y, y2);

            // Find the largest change (pixel, not ratio)
            double deltaX = x - width;
            double deltaY = y - height;

            // Return the ratio that will bring the largest change into the region
            return (deltaX > deltaY) ? width / x : height / y;
        }

        private WriteableBitmap GenerateConstrainedBitmap(BitmapImage sourceImage, int pixelWidth, int pixelHeight, double rotation)
        {
            double scale = CalculateConstraintScale(rotation, pixelWidth, pixelHeight);

            // Create a transform to render the image rotated and scaled
            var transform = new TransformGroup();
            var rt = new RotateTransform()
            {
                Angle = rotation,
                CenterX = (pixelWidth / 2.0),
                CenterY = (pixelHeight / 2.0)
            };
            transform.Children.Add(rt);
            var st = new ScaleTransform()
            {
                ScaleX = scale,
                ScaleY = scale,
                CenterX = (pixelWidth / 2.0),
                CenterY = (pixelHeight / 2.0)
            };
            transform.Children.Add(st);

            // Resize to specified target size
            var tempImage = new Image()
            {
                Stretch = Stretch.Fill,
                Width = pixelWidth,
                Height = pixelHeight,
                Source = sourceImage,
            };
            tempImage.UpdateLayout();

            // Render to a writeable bitmap
            var writeableBitmap = new WriteableBitmap(pixelWidth, pixelHeight);
            writeableBitmap.Render(tempImage, transform);
            writeableBitmap.Invalidate();
            return writeableBitmap;
        }