Add tiling support to the QtPDF backend

qpageview/pdf.py

@@ -220,16 +220,7 @@ def document(self):
 
 class PdfRenderer(render.AbstractRenderer):
     oversampleThreshold = 96    # DPI of a standard PC screen
-
-    def tiles(self, width, height):
-        """Yield four-tuples Tile(x, y, w, h) describing the tiles to render.
-
-        For the QtPdf backend, this always returns a single tile covering
-        the entire page because QtPdf does not support selectively rendering
-        a smaller area.
-
-        """
-        yield render.Tile(0, 0, width, height)
+    renderFullPages = True      # QtPDF doesn't support partial page rendering
 
     def draw(self, page, painter, key, tile, paperColor=None):
         """Draw a tile on the painter.
@@ -303,10 +294,6 @@ 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

@@ -112,6 +112,13 @@ class AbstractRenderer:
     # antialias True by default (not all renderers may support this)
     antialiasing = True
 
+    # qpageview was designed to render pages tile-by-tile. However, some
+    # backends like QtPDF only support full-page rendering, so for these we
+    # render the page first and divide it into tiles later. This is less
+    # efficient, but still provides certain performance advantages of using
+    # tiles like reduced flicker when repainting the widget.
+    renderFullPages = False
+
     def __init__(self, cache=None):
         if cache:
             self.cache = cache
@@ -219,17 +226,15 @@ 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 +376,8 @@ 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):
@@ -380,6 +387,14 @@ def schedule(self, page, key, tiles, callback):
         pending job.
 
         """
+        if self.renderFullPages:
+            # ignore the caller's request and always render the full page;
+            # job() will do the actual division into tiles later
+            tiles = [Tile(0, 0, key.width, key.height)]
+            # ensure the cache can always hold at least two full pages to
+            # avoid excessive re-rendering at higher zoom levels
+            pageBytes = key.width * key.height * 4  # 32 bits == 4 bytes
+            self.cache.maxsize = max(cache.ImageCache.maxsize, pageBytes * 2)
         for tile in tiles:
             try:
                 job = _jobs[(key, tile)]
@@ -398,12 +413,19 @@ 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()
         def finalize(image):
-            self.cache.addtile(key, tile, image)
+            if self.renderFullPages:
+                for subtile in self.tiles(key.width, key.height):
+                    self.cache.addtile(key, subtile,
+                                       image.copy(QRect(*map(int, subtile))))
+            else:
+                self.cache.addtile(key, tile, image)
             for cb in callbacks:
                 cb(page)
             del _jobs[(key, tile)]