Add NFFTOperator

nifty5/library/nfft.py

+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+# Copyright(C) 2018-2019 Max-Planck-Society
+#
+# Resolve is being developed at the Max-Planck-Institut fuer Astrophysik.
+
+import numpy as np
+
+import nifty5 as ift
+
+
+class NFFTOperator(ift.LinearOperator):
+    """Performs a non-equidistant Fourier transform, i.e. a Fourier transform
+    followed by a degridding operation.
+
+    Parameters
+    ----------
+    domain : RGSpace
+        Domain of the operator. It has to be two-dimensional and have shape
+        `(2N, 2N)`. The coordinates of the lower left pixel of the dirty image
+        are `(-N,-N)`, and of the upper right pixel `(N-1,N-1)`.
+    uv : numpy.ndarray
+        2D numpy array of type float64 and shape (M,2), where M is the number
+        of measurements. uv[i,0] and uv[i,1] contain the u and v coordinates
+        of measurement #i, respectively. All coordinates must lie in the range
+        `[-0.5; 0,5[`.
+    """
+    def __init__(self, domain, uv):
+        from pynfft.nfft import NFFT
+        npix = domain.shape[0]
+        assert npix == domain.shape[1]
+        assert len(domain.shape) == 2
+        assert type(npix) == int, "npix must be integer"
+        assert npix > 0 and (
+            npix % 2) == 0, "npix must be an even, positive integer"
+        assert isinstance(uv, np.ndarray), "uv must be a Numpy array"
+        assert uv.dtype == np.float64, "uv must be an array of float64"
+        assert uv.ndim == 2, "uv must be a 2D array"
+        assert uv.shape[0] > 0, "at least one point needed"
+        assert uv.shape[1] == 2, "the second dimension of uv must be 2"
+        assert np.all(uv >= -0.5) and np.all(uv <= 0.5),\
+            "all coordinates must lie between -0.5 and 0.5"
+
+        self._domain = ift.DomainTuple.make(domain)
+        self._target = ift.DomainTuple.make(
+            ift.UnstructuredDomain(uv.shape[0]))
+        self._capability = self.TIMES | self.ADJOINT_TIMES
+
+        self.npt = uv.shape[0]
+        self.plan = NFFT(self.domain.shape, self.npt, m=6)
+        self.plan.x = uv
+        self.plan.precompute()
+
+    def apply(self, x, mode):
+        self._check_input(x, mode)
+        if mode == self.TIMES:
+            self.plan.f_hat = x.to_global_data()
+            res = self.plan.trafo().copy()
+        else:
+            self.plan.f = x.to_global_data()
+            res = self.plan.adjoint().copy()
+        return ift.Field.from_global_data(self._tgt(mode), res)