FIX: always eraseRect in Qt widget (#13050)

tacaswell · timhoffm · commit 879b7f034243 · 2018-12-30T16:29:27.000+01:00
Re-focusing the figure window will call the draw method. If the facecolor of the figure and axes are transparent and we do not erase the widget we are effectively compositing the figure on to its self which results in artifacts anywhere there is alpha or anti-aliasing. Closes #13012
diff --git a/lib/matplotlib/backends/backend_qt5.py b/lib/matplotlib/backends/backend_qt5.py
@@ -240,7 +240,6 @@ def __init__(self, figure):
         self._dpi_ratio_prev = None
 
         self._draw_pending = False
-        self._erase_before_paint = False
         self._is_drawing = False
         self._draw_rect_callback = lambda painter: None
 
@@ -491,7 +490,6 @@ def draw(self):
             return
         with cbook._setattr_cm(self, _is_drawing=True):
             super().draw()
-        self._erase_before_paint = True
         self.update()
 
     def draw_idle(self):
diff --git a/lib/matplotlib/backends/backend_qt5agg.py b/lib/matplotlib/backends/backend_qt5agg.py
@@ -38,10 +38,6 @@ def paintEvent(self, event):
 
         painter = QtGui.QPainter(self)
 
-        if self._erase_before_paint:
-            painter.eraseRect(self.rect())
-            self._erase_before_paint = False
-
         rect = event.rect()
         left = rect.left()
         top = rect.top()
@@ -55,6 +51,10 @@ def paintEvent(self, event):
         reg = self.copy_from_bbox(bbox)
         buf = cbook._unmultiplied_rgba8888_to_premultiplied_argb32(
             memoryview(reg))
+
+        # clear the widget canvas
+        painter.eraseRect(rect)
+
         qimage = QtGui.QImage(buf, buf.shape[1], buf.shape[0],
                               QtGui.QImage.Format_ARGB32_Premultiplied)
         if hasattr(qimage, 'setDevicePixelRatio'):
diff --git a/lib/matplotlib/backends/backend_qt5cairo.py b/lib/matplotlib/backends/backend_qt5cairo.py
@@ -37,9 +37,7 @@ def paintEvent(self, event):
             # Not available on Qt4 or some older Qt5.
             qimage.setDevicePixelRatio(dpi_ratio)
         painter = QtGui.QPainter(self)
-        if self._erase_before_paint:
-            painter.eraseRect(self.rect())
-            self._erase_before_paint = False
+        painter.eraseRect(event.rect())
         painter.drawImage(0, 0, qimage)
         self._draw_rect_callback(painter)
         painter.end()
