diff --git a/fsl/transform/nonlinear.py b/fsl/transform/nonlinear.py
index 19f41c0345c8ab461b8cfbbad82847d1016c550c..9e903912edcae73fcd997e43702d401befe574b2 100644
--- a/fsl/transform/nonlinear.py
+++ b/fsl/transform/nonlinear.py
@@ -26,10 +26,13 @@ class NonLinearTransform(fslimage.Image):
some mapping between a source image coordinate system and a reference image
coordinate system.
+
In FSL, non-linear transformations are defined in the same space as the
reference image. At a given location in the reference image space, the
non-linear mapping at that location can be used to calculate the
- corresponding location in the source image space.
+ corresponding location in the source image space. Therefore, these
+ non-linear transformation effectively encode a transformation *from* the
+ reference image *to* the source image.
"""
@@ -46,7 +49,7 @@ class NonLinearTransform(fslimage.Image):
or a :mod:`numpy` array, or a :mod:`nibabel` image
object.
- :arg src: :class:`.Nifti` representing the sourceimage
+ :arg src: :class:`.Nifti` representing the source image.
:arg ref: :class:`.Nifti` representing the reference image.
If not provided, it is assumed that this
@@ -66,11 +69,6 @@ class NonLinearTransform(fslimage.Image):
if srcSpace is None: srcSpace = 'fsl'
if refSpace is None: refSpace = 'fsl'
- if not (isinstance(src, (fslimage.Nifti, type(None))) and
- isinstance(ref, fslimage.Nifti)):
- raise ValueError('Invalid source/reference: {} -> {}'.format(
- src, ref))
-
if srcSpace not in ('fsl', 'voxel', 'world') or \
refSpace not in ('fsl', 'voxel', 'world'):
raise ValueError('Invalid source/reference space: {} -> {}'.format(
@@ -78,6 +76,12 @@ class NonLinearTransform(fslimage.Image):
fslimage.Image.__init__(self, image, **kwargs)
+ # Displacement fields must be
+ # defined in the same space
+ # as the reference image
+ if not self.sameSpace(ref):
+ raise ValueError('Invalid reference image: {}'.format(ref))
+
self.__src = fslimage.Nifti(src.header.copy())
self.__ref = fslimage.Nifti(ref.header.copy())
self.__srcSpace = srcSpace
@@ -172,7 +176,8 @@ class DisplacementField(NonLinearTransform):
def transform(self, coords, from_=None, to=None):
- """Transform the given XYZ coordinates from the reference to the source.
+ """Transform the given XYZ coordinates from the reference image space
+ to the source image space.
:arg coords: A sequence of XYZ coordinates, or ``numpy`` array of shape
``(n, 3)`` containing ``n`` sets of coordinates in the
@@ -192,22 +197,21 @@ class DisplacementField(NonLinearTransform):
# same reference image space as the
# displacements
if from_ != self.refSpace:
- coords = affine.transform(
- coords, self.ref.getAffine(from_, self.refSpace))
+ xform = self.ref.getAffine(from_, self.refSpace)
+ coords = affine.transform(coords, xform)
# We also need to make sure that the
# coordinates are in voxels, so we
# can look up the displacements
if self.refSpace != 'voxel':
- voxels = affine.transform(
- coords, self.ref.getAffine(self.refSpace, 'voxel'))
+ xform = self.ref.getAffine(self.refSpace, 'voxel')
+ voxels = affine.transform(coords, xform)
else:
voxels = coords
- voxels = np.round(voxels).astype(np.int)
-
# Mask out the coordinates
# that are out of bounds
+ voxels = np.round(voxels).astype(np.int)
voxmask = (voxels >= [0, 0, 0]) & (voxels < self.shape[:3])
voxmask = voxmask.all(axis=1)
voxels = voxels[voxmask]
@@ -221,8 +225,8 @@ class DisplacementField(NonLinearTransform):
# are in the requested
# source image space
if to != self.srcSpace:
- disps = affine.transform(
- disps, self.src.getAffine(self.srcSpace, to))
+ xform = self.src.getAffine(self.srcSpace, to)
+ disps = affine.transform(disps, xform)
# Nans for input coordinates
# which were outside of the
@@ -276,12 +280,13 @@ def convertDisplacementType(field, dispType=None):
# we need the coordinates of every voxel
# in the reference FSL coordinate system.
dx, dy, dz = field.shape[:3]
- v2fsl = field.getAffine('voxel', field.srcSpace)
+ xform = field.getAffine('voxel', field.refSpace)
+
coords = np.meshgrid(np.arange(dx),
np.arange(dy),
np.arange(dz), indexing='ij')
coords = np.array(coords).transpose((1, 2, 3, 0))
- coords = affine.transform(coords.reshape((-1, 3)), v2fsl)
+ coords = affine.transform(coords.reshape((-1, 3)), xform)
coords = coords.reshape((dx, dy, dz, 3))
# If converting from relative to absolute,
@@ -299,47 +304,58 @@ def convertDisplacementSpace(field, from_, to):
the ``from_`` and ``to`` arguments.
:arg field: A :class:`DisplacementField` instance
- :arg from_: New source image coordinate system
- :arg to: New reference image coordinate system
+ :arg from_: New reference image coordinate system
+ :arg to: New source image coordinate system
:returns: A new :class:`DisplacementField` which transforms between
- the source ``from_`` coordinate system and the reference ``to``
+ the reference ``from_`` coordinate system and the source ``to``
coordinate system.
"""
- # Get the field in absolute
- # coordinates if necessary
+ # Get the field in absolute coordinates
+ # if necessary - these are our source
+ # coordinates in the original "to" space.
fieldcoords = field.data
if field.relative: srccoords = convertDisplacementType(field)
else: srccoords = fieldcoords
- # Now transform those source
- # coordinates from the original
- # source space to the source
- # space specified by "from_"
- srcmat = field.src.getAffine(field.srcSpace, from_)
srccoords = srccoords.reshape((-1, 3))
- srccoords = affine.transform(srccoords, srcmat)
+
+ # Now transform those source coordinates
+ # from the original source space to the
+ # source space specified by "to"
+ if to != field.srcSpace:
+
+ srcmat = field.src.getAffine(field.srcSpace, to)
+ srccoords = affine.transform(srccoords, srcmat)
# If we have been asked to return
# an absolute displacement, the
- # reference "to" coordinate system
- # is irrelevant - we're done.
+ # reference "from_" coordinate
+ # system is irrelevant - we're done.
if field.absolute:
fieldcoords = srccoords
# Otherwise our displacement field
# will contain relative displacements
- # between the reference image "to"
+ # between the reference image "from_"
# coordinate system and the source
- # image "from_" coordinate system.
- # We need to re-calculate the relative
- # displacements between source "from_"
- # space and reference "to" space.
+ # image "to" coordinate system. We
+ # need to re-calculate the relative
+ # displacements between the new
+ # reference "from_" space and source
+ # "to" space.
else:
- refmat = field.ref.getAffine(field.refSpace, to)
- refcoords = fieldcoords.reshape((-1, 3))
- refcoords = affine.transform(refcoords, refmat)
+ refcoords = np.meshgrid(np.arange(field.shape[0]),
+ np.arange(field.shape[1]),
+ np.arange(field.shape[2]), indexing='ij')
+ refcoords = np.array(refcoords)
+ refcoords = refcoords.transpose((1, 2, 3, 0)).reshape((-1, 3))
+
+ if from_ != 'voxel':
+ refmat = field.ref.getAffine('voxel', from_)
+ refcoords = affine.transform(refcoords, refmat)
+
fieldcoords = srccoords - refcoords
return DisplacementField(
@@ -347,6 +363,6 @@ def convertDisplacementSpace(field, from_, to):
header=field.header,
src=field.src,
ref=field.ref,
- srcSpace=from_,
- refSpace=to,
+ srcSpace=to,
+ refSpace=from_,
dispType=field.displacementType)