Fix performance with incompatible subsets in large images

CHANGES.md

@@ -68,7 +68,7 @@ v0.13.0 (unreleased)
 * EditSubsetMode is now no longer a singleton class and is
   instead instantiated at the Application/Session level. [#1538]
 
-* Improve performance of image viewer. [#1558]
+* Improve performance of image viewer. [#1558, #1562]
 
 v0.12.4 (unreleased)
 --------------------

glue/utils/array.py

@@ -64,7 +64,8 @@ def view_shape(shape, view):
     """
     Return the shape of a view of an array.
 
-    Returns equivalent of ``np.zeros(shape)[view].shape``
+    Returns equivalent of ``np.zeros(shape)[view].shape`` but with minimal
+    memory usage.
 
     Parameters
     ----------
@@ -75,11 +76,8 @@ def view_shape(shape, view):
     """
     if view is None:
         return shape
-    shp = tuple(slice(0, s, 1) for s in shape)
-    xy = np.broadcast_arrays(*np.ogrid[shp])
-    assert xy[0].shape == shape
-
-    return xy[0][view].shape
+    else:
+        return np.broadcast_to(1, shape)[view].shape
 
 
 def stack_view(shape, *views):

glue/viewers/image/composite_array.py

@@ -5,10 +5,13 @@
 
 import numpy as np
 
+from glue.utils import view_shape
+
 from matplotlib.colors import ColorConverter, Colormap
 from astropy.visualization import (LinearStretch, SqrtStretch, AsinhStretch,
                                    LogStretch, ManualInterval, ContrastBiasStretch)
 
+
 __all__ = ['CompositeArray']
 
 COLOR_CONVERTER = ColorConverter()
@@ -156,7 +159,7 @@ def __getitem__(self, view):
             if self.shape is None:
                 return None
             else:
-                img = np.zeros(self.shape + (4,))[view]
+                img = np.zeros(view_shape(self.shape, view) + (4,))
         else:
             img = np.clip(img, 0, 1)
 

glue/viewers/image/layer_artist.py

@@ -280,25 +280,21 @@ def shape(self):
         full_shape = self.layer_state.layer.shape
         return full_shape[y_axis], full_shape[x_axis]
 
-    @property
-    def nan_array(self):
-        return np.ones(self.shape) * np.nan
-
     def __getitem__(self, view=None):
 
         if (self.layer_artist is None or
                 self.layer_state is None or
                 self.viewer_state is None):
-            return self.nan_array
+            return None
 
         if not self.layer_artist._compatible_with_reference_data:
-            return self.nan_array
+            return None
 
         try:
             mask = self.layer_state.get_sliced_data(view=view)
         except IncompatibleAttribute:
             self.layer_artist.disable_incompatible_subset()
-            return self.nan_array
+            return None
         else:
             self.layer_artist.enable()