image: modify alpha premultiply to work with modified to_rgba

efiring · efiring · commit 95e6fcda1cec · 2012-01-07T10:24:19.000-10:00
The earlier version of the premultiplication step did not work
with numpy 2, and required additional dtype conversion steps in
the common case of a colormapped image.
diff --git a/lib/matplotlib/image.py b/lib/matplotlib/image.py
@@ -199,9 +199,14 @@ def _get_unsampled_image(self, A, image_extents, viewlim):
                 im.is_grayscale = False
             else:
                 if self._rgbacache is None:
-                    x = self.to_rgba(self._A, bytes=True)
+                    x = self.to_rgba(self._A, bytes=False)
+                    # Avoid side effects: to_rgba can return its argument
+                    # unchanged.
+                    if np.may_share_memory(x, self._A):
+                        x = x.copy()
                     # premultiply the colors
-                    x[...,0:3] *= x[...,3:4] / 255.0
+                    x[...,0:3] *= x[...,3:4]
+                    x = (x * 255).astype(np.uint8)
                     self._rgbacache = x
                 else:
                     x = self._rgbacache
