code: public static void hessianNaive(GrayF32 integral, int skip , int size ,
  									GrayF32 intensity)
  	{
  		final int w = intensity.width;
  		final int h = intensity.height;
  
  		// get convolution kernels for the second order derivatives
  		IntegralKernel kerXX = DerivativeIntegralImage.kernelDerivXX(size,null);
  		IntegralKernel kerYY = DerivativeIntegralImage.kernelDerivYY(size,null);
  		IntegralKernel kerXY = DerivativeIntegralImage.kernelDerivXY(size,null);
  
  		float norm = 1.0f/(size*size);
  
  		for( int y = 0; y < h; y++ ) {
  			for( int x = 0; x < w; x++ ) {
  
  				int xx = x*skip;
  				int yy = y*skip;
  
  				computeHessian(integral, intensity, kerXX, kerYY, kerXY, norm, y, yy, x, xx);
  			}
  		}
  	}
docs: Brute force approach which is easy to validate through visual inspection.