FormatSMVTimePix_SU mask out central cross of virtual pixels (#683)

* Always mask out the central cross of pixels, as even if these have
got reasonable intensity values, they do not correctly represent
the reflection profiles. This also means handling between the 4-
panel 512*512 pixel model (which excludes the edge pixels between
the panels) and the 516*516 pixel model are now consistent. Both
versions now exclude these special edge pixels.

newsfragments/683.feature

@@ -0,0 +1 @@
+``FormatSMVTimePix_SU``: Always mask out the central cross of virtual pixels that have been reconstructed from wide edge pixels.

src/dxtbx/format/FormatSMVTimePix_SU.py

@@ -11,7 +11,10 @@
 from iotbx.detectors import SMVImage
 from scitbx import matrix
 
+from dials.array_family import flex
+
 from dxtbx.format.FormatSMV import FormatSMV
+from dxtbx.masking import mask_untrusted_rectangle
 from dxtbx.model.beam import Probe
 from dxtbx.model.detector import Detector
 
@@ -314,7 +317,9 @@ def get_raw_data(self):
 class FormatSMVTimePix_SU_516x516(FormatSMVTimePix_SU):
     """A class for reading SMV format images for the Timepix-based electron
     detector where the wider edge pixels have been split into three normal-sized
-    pixels. The whole 516*516 detector can then be described as a single panel"""
+    pixels. The whole 516*516 detector can then be described as a single panel.
+    We choose to mask out the edge pixels, as they do not correctly represent
+    reflection profiles."""
 
     @staticmethod
     def understand(image_file):
@@ -329,6 +334,15 @@ def understand(image_file):
 
         return True
 
+    def get_static_mask(self):
+        """Return the static mask that excludes the central cross of pixels."""
+
+        mask = flex.bool(flex.grid((516, 516)), True)
+        mask_untrusted_rectangle(mask, 0, 516, 255, 261)
+        mask_untrusted_rectangle(mask, 255, 261, 0, 516)
+
+        return (mask,)
+
     def _goniometer(self):
         """Return a model for a simple single-axis goniometer. For this beamline
         this should be close to the provided values and neither aligned with the