diff --git a/python/_project.pyx b/python/_project.pyx
index 317f001..0906474 100644
--- a/python/_project.pyx
+++ b/python/_project.pyx
@@ -82,6 +82,76 @@ cdef int interp3d_INTERNAL_periodic(DTYPE_t x, DTYPE_t y,
 
     return 0
 
+@cython.boundscheck(False)
+@cython.cdivision(True)
+cdef int ngp3d_INTERNAL_periodic(DTYPE_t x, DTYPE_t y, 
+                                 DTYPE_t z, 
+                                 DTYPE_t[:,:,:] d, DTYPE_t Lbox, DTYPE_t *retval) nogil:
+
+    cdef int Ngrid = d.shape[0]
+    cdef DTYPE_t inv_delta = Ngrid/Lbox
+    cdef int ix, iy, iz
+    cdef DTYPE_t f[2][2][2]
+    cdef DTYPE_t rx, ry, rz
+    cdef int jx, jy, jz
+
+    rx = (inv_delta*x)
+    ry = (inv_delta*y)
+    rz = (inv_delta*z)
+
+    ix = int(floor(rx))
+    iy = int(floor(ry))
+    iz = int(floor(rz))
+
+
+    ix = ix%Ngrid
+    iy = iy%Ngrid
+    iz = iz%Ngrid
+
+    if (ix < 0)  or (jx >= Ngrid):
+      return -1
+    if (iy < 0)  or (jy >= Ngrid):
+      return -2
+    if (iz < 0)  or (jz >= Ngrid):
+      return -3
+
+    retval[0] = d[ix ,iy ,iz ] 
+
+    return 0
+
+
+@cython.boundscheck(False)
+@cython.cdivision(True)
+cdef int ngp3d_INTERNAL(DTYPE_t x, DTYPE_t y, 
+                         DTYPE_t z, 
+                         DTYPE_t[:,:,:] d, DTYPE_t Lbox, DTYPE_t *retval) nogil:
+
+    cdef int Ngrid = d.shape[0]
+    cdef DTYPE_t inv_delta = Ngrid/Lbox
+    cdef int ix, iy, iz
+    cdef DTYPE_t f[2][2][2]
+    cdef DTYPE_t rx, ry, rz
+    cdef int jx, jy, jz
+
+    rx = (inv_delta*x)
+    ry = (inv_delta*y)
+    rz = (inv_delta*z)
+
+    ix = int(floor(rx))
+    iy = int(floor(ry))
+    iz = int(floor(rz))
+
+    if (ix < 0)  or (jx >= Ngrid):
+      return -1
+    if (iy < 0)  or (jy >= Ngrid):
+      return -2
+    if (iz < 0)  or (jz >= Ngrid):
+      return -3
+
+    retval[0] = d[ix ,iy ,iz ] 
+
+    return 0
+
 
 @cython.boundscheck(False)
 @cython.cdivision(True)
@@ -145,8 +215,8 @@ cdef int interp3d_INTERNAL(DTYPE_t x, DTYPE_t y,
 def interp3d(x not None, y not None, 
              z not None,
              npx.ndarray[DTYPE_t, ndim=3] d not None, DTYPE_t Lbox,
-             bool periodic=False, bool centered=True):
-    """ interp3d(x,y,z,d,Lbox,periodic=False) -> interpolated values
+             bool periodic=False, bool centered=True, bool ngp=False):
+    """ interp3d(x,y,z,d,Lbox,periodic=False,centered=True,ngp=False) -> interpolated values
     
     Compute the tri-linear interpolation of the given field (d) at the given position (x,y,z). It assumes that they are box-centered coordinates. So (x,y,z) == (0,0,0) is equivalent to the pixel at (Nx/2,Ny/2,Nz/2) with Nx,Ny,Nz = d.shape. If periodic is set, it assumes the box is periodic 
 """
@@ -157,10 +227,11 @@ def interp3d(x not None, y not None,
     cdef DTYPE_t retval
     cdef long i
     cdef long Nelt
-    cdef int myperiodic
+    cdef int myperiodic, myngp
     cdef DTYPE_t shifter
 
     myperiodic = periodic
+    myngp = ngp
 
     if centered:
       shifter = Lbox/2
@@ -186,17 +257,28 @@ def interp3d(x not None, y not None,
         in_slice = d
         Nelt = ax.size
         with nogil:
-          if myperiodic:
-            for i in prange(Nelt):
-              if interp3d_INTERNAL_periodic(shifter+ax[i], shifter+ay[i], shifter+az[i], in_slice, Lbox, &out_slice[i]) < 0:
-                with gil:
-                  raise ierror 
+          if myngp:
+            if myperiodic:
+              for i in prange(Nelt):
+                if interp3d_INTERNAL_periodic(shifter+ax[i], shifter+ay[i], shifter+az[i], in_slice, Lbox, &out_slice[i]) < 0:
+                  with gil:
+                    raise ierror 
+            else:
+              for i in prange(Nelt):
+                if interp3d_INTERNAL(shifter+ax[i], shifter+ay[i], shifter+az[i], in_slice, Lbox, &out_slice[i]) < 0:
+                  with gil:
+                    raise ierror
           else:
-            for i in prange(Nelt):
-              if interp3d_INTERNAL(shifter+ax[i], shifter+ay[i], shifter+az[i], in_slice, Lbox, &out_slice[i]) < 0:
-                with gil:
-                  raise ierror
-
+            if myperiodic:
+              for i in prange(Nelt):
+                if ngp3d_INTERNAL_periodic(shifter+ax[i], shifter+ay[i], shifter+az[i], in_slice, Lbox, &out_slice[i]) < 0:
+                  with gil:
+                    raise ierror 
+            else:
+              for i in prange(Nelt):
+                if ngp3d_INTERNAL(shifter+ax[i], shifter+ay[i], shifter+az[i], in_slice, Lbox, &out_slice[i]) < 0:
+                  with gil:
+                    raise ierror
         return out
     else:
       if periodic: