make random inverse affine matrix (#1621)

Signed-off-by: Richard Brown <33289025+rijobro@users.noreply.github.com>

tests/utils.py

@@ -155,6 +155,19 @@ def make_nifti_image(array, affine=None):
     return image_name
 
 
+def make_rand_affine(ndim: int = 3, random_state: Optional[np.random.RandomState] = None):
+    """Create random affine transformation (with values == -1, 0 or 1)."""
+    rs = np.random if random_state is None else random_state
+
+    vals = rs.choice([-1, 1], size=ndim)
+    positions = rs.choice(range(ndim), size=ndim, replace=False)
+    af = np.zeros([ndim + 1, ndim + 1])
+    af[ndim, ndim] = 1
+    for i, (v, p) in enumerate(zip(vals, positions)):
+        af[i, p] = v
+    return af
+
+
 class DistTestCase(unittest.TestCase):
     """
     testcase without _outcome, so that it's picklable.