JP-3781: Use center=False for compute_s_region_imaging (spacetelescop…

…e#8897)

Co-authored-by: Nadia Dencheva <nadia.astropy@gmail.com>

changes/8897.assign_wcs.rst

@@ -0,0 +1 @@
+Use pixel vertices to define s_region instead of pixel centers."

jwst/assign_wcs/tests/test_nircam.py

@@ -10,6 +10,7 @@
 """
 import numpy as np
 import pytest
+import re
 
 
 from astropy.io import fits
@@ -244,3 +245,31 @@ def test_wfss_sip():
     util.wfss_imaging_wcs(wfss_model, nircam.imaging, bbox=((1, 1024), (1, 1024)))
     for key in ['a_order', 'b_order', 'crpix1', 'crpix2', 'crval1', 'crval2', 'cd1_1']:
         assert key in wfss_model.meta.wcsinfo.instance
+
+
+def _sregion_to_footprint(s_region):
+    """
+    Parameters
+    ----------
+    s_region : str
+        The S_REGION header keyword
+
+    Returns
+    -------
+    footprint : np.array
+        A 2D array of the footprint of the region, shape (N, 2)
+    """
+    no_prefix = re.sub(r"[a-zA-Z]", "", s_region)
+    return np.array(no_prefix.split(), dtype=float).reshape(-1, 2)
+
+
+def test_update_s_region_imaging():
+    """Ensure the s_region keyword matches output of wcs.footprint()"""
+    model = ImageModel(create_hdul())
+    model.meta.wcs = create_imaging_wcs()
+    util.update_s_region_imaging(model)
+
+    s_region = model.meta.wcsinfo.s_region
+    footprint = model.meta.wcs.footprint().flatten()
+    footprint_sregion = _sregion_to_footprint(s_region).flatten()
+    assert np.allclose(footprint, footprint_sregion, rtol=1e-9)

jwst/assign_wcs/util.py

@@ -795,7 +795,7 @@ def update_s_region_imaging(model):
     """
     Update the ``S_REGION`` keyword using ``WCS.footprint``.
     """
-    s_region = compute_s_region_imaging(model.meta.wcs, shape=model.data.shape)
+    s_region = compute_s_region_imaging(model.meta.wcs, shape=model.data.shape, center=False)
     if s_region is not None:
         model.meta.wcsinfo.s_region = s_region