Fix several bugs in the background-painting logic

qpageview/export.py

@@ -243,11 +243,25 @@ def export(self):
         res = self.resolution
         if self.oversample != 1:
             res *= self.oversample
-        i = self.page().image(self._rect, res, res, self.paperColor)
+        if self.grayscale:
+            # convertToFormat() does weird things with the alpha channel,
+            # so we always render grayscale images with a solid background
+            # and handle transparent areas manually if needed
+            paperColor = self.paperColor or Qt.GlobalColor.white
+        else:
+            paperColor = self.paperColor
+        i = self.page().image(self._rect, res, res, paperColor)
         if self.oversample != 1:
             i = i.scaled(i.size() / self.oversample, transformMode=Qt.TransformationMode.SmoothTransformation)
         if self.grayscale:
             i = i.convertToFormat(QImage.Format.Format_Grayscale8)
+            if self.paperColor is None:
+                # restore the original transparent background
+                # since our alpha map here is just this image's negative,
+                # we save memory by doing this instead of making a copy()
+                i.invertPixels(QImage.InvertMode.InvertRgb)
+                i.setAlphaChannel(i)
+                i.invertPixels(QImage.InvertMode.InvertRgb)
         if self.autocrop:
             i = i.copy(util.autoCropRect(i))
         # needed for correct resolution metadata; see issue #44

qpageview/pdf.py

@@ -303,10 +303,6 @@ def draw(self, page, painter, key, tile, paperColor=None):
                 Qt.AspectRatioMode.IgnoreAspectRatio,
                 Qt.TransformationMode.SmoothTransformation)
 
-        # Erase the target area and draw the image
-        painter.eraseRect(target)
-        if paperColor:
-            painter.fillRect(target, paperColor)
         painter.drawImage(target, image, QRectF(image.rect()))
 
 

qpageview/render.py

@@ -219,17 +219,15 @@ def render(self, page, key, tile, paperColor=None):
         The default implementation prepares the image, a painter and then
         calls draw() to actually draw the contents.
 
-        If the paperColor is not specified, it will be read from the Page's
-        paperColor attribute (if not None) or else from the renderer's
-        paperColor attribute.
+        If the paperColor is not specified, the returned image will have a
+        transparent background, and the caller is responsible to paint or
+        erase the background as needed before calling this method.
 
         """
-        if paperColor is None:
-            paperColor = page.paperColor or self.paperColor
-
         i = QImage(tile.w, tile.h, self.imageFormat)
-        if paperColor:
-            i.fill(paperColor)
+        # leave the background transparent if no paperColor is specified,
+        # since we don't necessarily want to fill it e.g. when printing
+        i.fill(paperColor or Qt.GlobalColor.transparent)
         painter = QPainter(i)
 
         # rotate the painter accordingly
@@ -371,6 +369,8 @@ def paint(self, page, painter, rect, callback=None):
         for (r, image, source) in reversed(images):
             # scale the target rect back to the paint device
             target = QRectF(r.x() / info.ratio, r.y() / info.ratio, r.width() / info.ratio, r.height() / info.ratio)
+            # job() already filled the background when rendering the page
+            # so we don't have to do that separately, which can cause flicker
             painter.drawImage(target, image, source)
 
     def schedule(self, page, key, tiles, callback):
@@ -398,7 +398,9 @@ def job(self, page, key, tile):
         exception = []
         def work():
             try:
-                return self.render(page, key, tile)
+                # filling the background here is an optimization for paint()
+                return self.render(page, key, tile,
+                                   page.paperColor or self.paperColor)
             except Exception:
                 exception.extend(sys.exc_info())
                 return QImage()