# File narray_ext.rb, line 353
  def convol(a1,a2)
    n1x,n1y = a1.shape
    n2x,n2y = a2.shape
    raise "arrays must have same shape" if n1x!=n2x || n1y!=n2y
    (FFTW.fftw( FFTW.fftw(a1,-1) * FFTW.fftw(a2,-1), 1).real) / (n1x*n1y)
  end