From 252f4135799027f9ae887e8fd87e3af906187a22 Mon Sep 17 00:00:00 2001
From: Antony Lee <anntzer.lee@gmail.com>
Date: Thu, 27 Jul 2017 22:32:35 -0700
Subject: [PATCH] Let QPaintEvent tell us what region to repaint.

---
 lib/matplotlib/backends/backend_qt5.py      |  2 +
 lib/matplotlib/backends/backend_qt5agg.py   | 54 ++++++++++-----------
 lib/matplotlib/backends/backend_qt5cairo.py |  3 ++
 3 files changed, 30 insertions(+), 29 deletions(-)

diff --git a/lib/matplotlib/backends/backend_qt5.py b/lib/matplotlib/backends/backend_qt5.py
index 0fe2070af3e7..c8f7aaf9434f 100644
--- a/lib/matplotlib/backends/backend_qt5.py
+++ b/lib/matplotlib/backends/backend_qt5.py
@@ -241,6 +241,7 @@ 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
 
@@ -494,6 +495,7 @@ 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
index ab8cbe4994b3..ca2603bca52c 100644
--- a/lib/matplotlib/backends/backend_qt5agg.py
+++ b/lib/matplotlib/backends/backend_qt5agg.py
@@ -15,11 +15,7 @@
 
 class FigureCanvasQTAgg(FigureCanvasAgg, FigureCanvasQT):
 
-    def __init__(self, figure):
-        super().__init__(figure=figure)
-        self._bbox_queue = []
-
-    def paintEvent(self, e):
+    def paintEvent(self, event):
         """Copy the image from the Agg canvas to the qt.drawable.
 
         In Qt, all drawing should be done inside of here when a widget is
@@ -37,29 +33,31 @@ def paintEvent(self, e):
 
         painter = QtGui.QPainter(self)
 
-        if self._bbox_queue:
-            bbox_queue = self._bbox_queue
-        else:
+        if self._erase_before_paint:
             painter.eraseRect(self.rect())
-            bbox_queue = [
-                Bbox([[0, 0], [self.renderer.width, self.renderer.height]])]
-        self._bbox_queue = []
-        for bbox in bbox_queue:
-            l, b, r, t = map(int, bbox.extents)
-            w = r - l
-            h = t - b
-            reg = self.copy_from_bbox(bbox)
-            buf = reg.to_string_argb()
-            qimage = QtGui.QImage(buf, w, h, QtGui.QImage.Format_ARGB32)
-            # Adjust the buf reference count to work around a memory leak bug
-            # in QImage under PySide on Python 3.
-            if QT_API == 'PySide':
-                ctypes.c_long.from_address(id(buf)).value = 1
-            if hasattr(qimage, 'setDevicePixelRatio'):
-                # Not available on Qt4 or some older Qt5.
-                qimage.setDevicePixelRatio(self._dpi_ratio)
-            origin = QtCore.QPoint(l, self.renderer.height - t)
-            painter.drawImage(origin / self._dpi_ratio, qimage)
+            self._erase_before_paint = False
+
+        rect = event.rect()
+        left = rect.left()
+        top = rect.top()
+        width = rect.width()
+        height = rect.height()
+        # See documentation of QRect: bottom() and right() are off by 1, so use
+        # left() + width() and top() + height().
+        bbox = Bbox([[left, self.renderer.height - (top + height)],
+                     [left + width, self.renderer.height - top]])
+        reg = self.copy_from_bbox(bbox)
+        buf = reg.to_string_argb()
+        qimage = QtGui.QImage(buf, width, height, QtGui.QImage.Format_ARGB32)
+        if hasattr(qimage, 'setDevicePixelRatio'):
+            # Not available on Qt4 or some older Qt5.
+            qimage.setDevicePixelRatio(self._dpi_ratio)
+        origin = QtCore.QPoint(left, top)
+        painter.drawImage(origin / self._dpi_ratio, qimage)
+        # Adjust the buf reference count to work around a memory
+        # leak bug in QImage under PySide on Python 3.
+        if QT_API == 'PySide':
+            ctypes.c_long.from_address(id(buf)).value = 1
 
         self._draw_rect_callback(painter)
 
@@ -73,8 +71,6 @@ def blit(self, bbox=None):
         if bbox is None and self.figure:
             bbox = self.figure.bbox
 
-        self._bbox_queue.append(bbox)
-
         # repaint uses logical pixels, not physical pixels like the renderer.
         l, b, w, h = [pt / self._dpi_ratio for pt in bbox.bounds]
         t = b + h
diff --git a/lib/matplotlib/backends/backend_qt5cairo.py b/lib/matplotlib/backends/backend_qt5cairo.py
index 2544fb203e55..bea4f6069c33 100644
--- a/lib/matplotlib/backends/backend_qt5cairo.py
+++ b/lib/matplotlib/backends/backend_qt5cairo.py
@@ -37,6 +37,9 @@ 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.drawImage(0, 0, qimage)
         self._draw_rect_callback(painter)
         painter.end()