Add support for generating PDFs with losslessly encoded images

This patch adds the QPainter::LosslessImageRendering flag to
QPainter::RenderHints and allows QPdfEngine to recognize it and encode
images using a lossless data compression algorithm provided by zlib
instead of using lossy JPEG compression, which can produce artifacts
with some images.

Task-number: QTBUG-54809
Change-Id: If9e2bc64da3bf3afcd5d6c74f0ee82336ef05a19
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

1 files changed, 7 insertions, 0 deletions

diff --git a/src/gui/painting/qpainter.cpp b/src/gui/painting/qpainter.cpp
index b70b29e54e..df2f5e11d3 100644
--- a/src/gui/painting/qpainter.cpp
+++ b/src/gui/painting/qpainter.cpp
@@ -1437,6 +1437,13 @@ void QPainterPrivate::updateState(QPainterState *newState)
     by slightly less than half a pixel. Also will treat default constructed pens
     as cosmetic. Potentially useful when porting a Qt 4 application to Qt 5.
 
+    \value LosslessImageRendering Use a lossless image rendering, whenever possible.
+    Currently, this hint is only used when QPainter is employed to output a PDF
+    file through QPrinter or QPdfWriter, where drawImage()/drawPixmap() calls
+    will encode images using a lossless compression algorithm instead of lossy
+    JPEG compression.
+    This value was added in Qt 5.13.
+
     \sa renderHints(), setRenderHint(), {QPainter#Rendering
     Quality}{Rendering Quality}, {Concentric Circles Example}