When QgsImageCache is used to render pdf content, respect the target DPI

For the PDF QImage driver we need to pass the desired target size onto the image reader so that it can correctly render the (vector) pdf content at the desired dpi. Otherwise it returns a very low resolution image (the driver assumes points == pixels!)
qgis · Sep 27, 2021 · 902ad77 · 902ad77
1 parent 51cb63f
commit 902ad77
Showing 1 changed file with 44 additions and 1 deletion.
diff --git a/src/core/qgsimagecache.cpp b/src/core/qgsimagecache.cpp
@@ -199,7 +199,29 @@ QImage QgsImageCache::renderImage( const QString &path, QSize size, const bool k
   // direct read if path is a file -- maybe more efficient than going the bytearray route? (untested!)
   if ( !path.startsWith( QLatin1String( "base64:" ) ) && QFile::exists( path ) )
   {
-    im = QImage( path );
+    QImageReader reader( path );
+
+    if ( reader.format() == "pdf" )
+    {
+      if ( !size.isEmpty() )
+      {
+        // special handling for this format -- we need to pass the desired target size onto the image reader
+        // so that it can correctly render the (vector) pdf content at the desired dpi. Otherwise it returns
+        // a very low resolution image (the driver assumes points == pixels!)
+        // For other image formats, we read the original image size only and defer resampling to later in this
+        // function. That gives us more control over the resampling method used.
+        reader.setScaledSize( size );
+      }
+      else
+      {
+        // driver assumes points == pixels, so driver image size is reported assuming 72 dpi.
+        const QSize sizeAt72Dpi = reader.size();
+        const QSize sizeAtTargetDpi = sizeAt72Dpi * targetDpi / 72;
+        reader.setScaledSize( sizeAtTargetDpi );
+      }
+    }
+
+    im = reader.read();
   }
   else
   {
@@ -256,6 +278,27 @@ QImage QgsImageCache::renderImage( const QString &path, QSize size, const bool k
       buffer.open( QIODevice::ReadOnly );
 
       QImageReader reader( &buffer );
+
+      if ( reader.format() == "pdf" )
+      {
+        if ( !size.isEmpty() )
+        {
+          // special handling for this format -- we need to pass the desired target size onto the image reader
+          // so that it can correctly render the (vector) pdf content at the desired dpi. Otherwise it returns
+          // a very low resolution image (the driver assumes points == pixels!)
+          // For other image formats, we read the original image size only and defer resampling to later in this
+          // function. That gives us more control over the resampling method used.
+          reader.setScaledSize( size );
+        }
+        else
+        {
+          // driver assumes points == pixels, so driver image size is reported assuming 72 dpi.
+          const QSize sizeAt72Dpi = reader.size();
+          const QSize sizeAtTargetDpi = sizeAt72Dpi * targetDpi / 72;
+          reader.setScaledSize( sizeAtTargetDpi );
+        }
+      }
+
       im = reader.read();
     }
   }