Don't read full cube into mem for histogramming

dstansby · dstansby · commit f94ccf0ef99a · 2023-07-21T09:37:29.000+01:00
diff --git a/src/napari_matplotlib/histogram.py b/src/napari_matplotlib/histogram.py
@@ -33,7 +33,6 @@ def draw(self) -> None:
         Clear the axes and histogram the currently selected layer/slice.
         """
         layer = self.layers[0]
-        bins = np.linspace(np.min(layer.data), np.max(layer.data), 100)
 
         if layer.data.ndim - layer.rgb == 3:
             # 3D data, can be single channel or RGB
@@ -42,6 +41,10 @@ def draw(self) -> None:
         else:
             data = layer.data
 
+        # Important to calculate bins after slicing 3D data, to avoid reading
+        # whole cube into memory.
+        bins = np.linspace(np.min(layer.data), np.max(layer.data), 100)
+
         if layer.rgb:
             # Histogram RGB channels independently
             for i, c in enumerate("rgb"):
