allow sigma/strength of 0

nifty/library/nonlinear_power_energy.py

@@ -41,9 +41,8 @@ class NonlinearPowerEnergy(Energy):
         self.D = D
         self.d = d
         self.N = N
-        self.sigma = sigma
         self.T = SmoothnessOperator(domain=self.position.domain[0],
-                                    strength=self.sigma, logarithmic=True)
+                                    strength=sigma, logarithmic=True)
         self.FFT = FFT
         self.Instrument = Instrument
         self.nonlinearity = nonlinearity
@@ -63,7 +62,7 @@ class NonlinearPowerEnergy(Energy):
     def at(self, position):
         return self.__class__(position, self.d, self.N, self.m, self.D,
                               self.FFT, self.Instrument, self.nonlinearity,
-                              self.Projection, sigma=self.sigma,
+                              self.Projection, sigma=self.T.strength,
                               samples=len(self.sample_list),
                               sample_list=self.sample_list,
                               inverter=self.inverter)

nifty/operators/smoothness_operator.py

@@ -30,8 +30,8 @@ class SmoothnessOperator(EndomorphicOperator):
         self._laplace = LaplaceOperator(domain, logarithmic=logarithmic,
                                         space=space)
 
-        if strength <= 0:
-            raise ValueError("ERROR: invalid sigma.")
+        if strength < 0:
+            raise ValueError("ERROR: strength must be >=0.")
         self._strength = strength
 
     @property
@@ -47,6 +47,8 @@ class SmoothnessOperator(EndomorphicOperator):
         return False
 
     def _times(self, x):
+        if self._strength == 0.:
+            return x.zeros_like(x)
         result = self._laplace.adjoint_times(self._laplace(x))
         result *= self._strength**2
         return result