Image rotation with bilinear interpolation and no clipping

const int cnSizeBuffer = 20;
// 30 deg = PI/6 rad
// rotating clockwise, so it's negative relative to Cartesian quadrants
const double cnAngle = -0.52359877559829887307710723054658;
// use whatever image you fancy
Bitmap bm = new Bitmap("slantedtreetopsky.jpg");
// general iterators
int i, j;
// calculated indices in Cartesian coordinates
int x, y;
double fDistance, fPolarAngle;
// for use in neighbouring indices in Cartesian coordinates
int iFloorX, iCeilingX, iFloorY, iCeilingY;
// calculated indices in Cartesian coordinates with trailing decimals
double fTrueX, fTrueY;
// for interpolation
double fDeltaX, fDeltaY;
// pixel colours
Color clrTopLeft, clrTopRight, clrBottomLeft, clrBottomRight;
// interpolated "top" pixels
double fTopRed, fTopGreen, fTopBlue;
// interpolated "bottom" pixels
double fBottomRed, fBottomGreen, fBottomBlue;
// final interpolated colour components
int iRed, iGreen, iBlue;
int iCentreX, iCentreY;
int iDestCentre;
int iWidth, iHeight;
int iDiagonal;
iWidth = bm.Width;
iHeight = bm.Height;
iDiagonal = (int)(Math.Ceiling(Math.Sqrt((double)(bm.Width * bm.Width + bm.Height * bm.Height)))) + cnSizeBuffer;

iCentreX = iWidth / 2;
iCentreY = iHeight / 2;
iDestCentre = iDiagonal / 2;

Bitmap bmBilinearInterpolation = new Bitmap(iDiagonal, iDiagonal);

for (i = 0; i < iDiagonal; ++i)
{
	for (j = 0; j < iDiagonal; ++j)
	{
		bmBilinearInterpolation.SetPixel(j, i, Color.Black);
	}
}

// assigning pixels of destination image from source image
// with bilinear interpolation
for (i = 0; i < iDiagonal; ++i)
{
	for (j = 0; j < iDiagonal; ++j)
	{
		// convert raster to Cartesian
		x = j - iDestCentre;
		y = iDestCentre - i;

		// convert Cartesian to polar
		fDistance = Math.Sqrt(x * x + y * y);
		fPolarAngle = 0.0;
		if (x == 0)
		{
			if (y == 0)
			{
				// centre of image, no rotation needed
				bmBilinearInterpolation.SetPixel(j, i, bm.GetPixel(iCentreX, iCentreY));
				continue;
			}
			else if (y < 0)
			{
				fPolarAngle = 1.5 * Math.PI;
			}
			else
			{
				fPolarAngle = 0.5 * Math.PI;
			}
		}
		else
		{
			fPolarAngle = Math.Atan2((double)y, (double)x);
		}

		// the crucial rotation part
		// "reverse" rotate, so minus instead of plus
		fPolarAngle -= cnAngle;

		// convert polar to Cartesian
		fTrueX = fDistance * Math.Cos(fPolarAngle);
		fTrueY = fDistance * Math.Sin(fPolarAngle);

		// convert Cartesian to raster
		fTrueX = fTrueX + (double)iCentreX;
		fTrueY = (double)iCentreY - fTrueY;

		iFloorX = (int)(Math.Floor(fTrueX));
		iFloorY = (int)(Math.Floor(fTrueY));
		iCeilingX = (int)(Math.Ceiling(fTrueX));
		iCeilingY = (int)(Math.Ceiling(fTrueY));

		// check bounds
		if (iFloorX < 0 || iCeilingX < 0 || iFloorX >= iWidth || iCeilingX >= iWidth || iFloorY < 0 || iCeilingY < 0 || iFloorY >= iHeight || iCeilingY >= iHeight) continue;

		fDeltaX = fTrueX - (double)iFloorX;
		fDeltaY = fTrueY - (double)iFloorY;

		clrTopLeft = bm.GetPixel(iFloorX, iFloorY);
		clrTopRight = bm.GetPixel(iCeilingX, iFloorY);
		clrBottomLeft = bm.GetPixel(iFloorX, iCeilingY);
		clrBottomRight = bm.GetPixel(iCeilingX, iCeilingY);

		// linearly interpolate horizontally between top neighbours
		fTopRed = (1 - fDeltaX) * clrTopLeft.R + fDeltaX * clrTopRight.R;
		fTopGreen = (1 - fDeltaX) * clrTopLeft.G + fDeltaX * clrTopRight.G;
		fTopBlue = (1 - fDeltaX) * clrTopLeft.B + fDeltaX * clrTopRight.B;

		// linearly interpolate horizontally between bottom neighbours
		fBottomRed = (1 - fDeltaX) * clrBottomLeft.R + fDeltaX * clrBottomRight.R;
		fBottomGreen = (1 - fDeltaX) * clrBottomLeft.G + fDeltaX * clrBottomRight.G;
		fBottomBlue = (1 - fDeltaX) * clrBottomLeft.B + fDeltaX * clrBottomRight.B;

		// linearly interpolate vertically between top and bottom interpolated results
		iRed = (int)(Math.Round((1 - fDeltaY) * fTopRed + fDeltaY * fBottomRed));
		iGreen = (int)(Math.Round((1 - fDeltaY) * fTopGreen + fDeltaY * fBottomGreen));
		iBlue = (int)(Math.Round((1 - fDeltaY) * fTopBlue + fDeltaY * fBottomBlue));

		// make sure colour values are valid
		if (iRed < 0) iRed = 0;
		if (iRed > 255) iRed = 255;
		if (iGreen < 0) iGreen = 0;
		if (iGreen > 255) iGreen = 255;
		if (iBlue < 0) iBlue = 0;
		if (iBlue > 255) iBlue = 255;

		bmBilinearInterpolation.SetPixel(j, i, Color.FromArgb(iRed, iGreen, iBlue));
	}
}
bmBilinearInterpolation.Save("rotatedslantedtreetopsky.jpg", System.Drawing.Imaging.ImageFormat.Jpeg);