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>

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}
 

src/gui/painting/qpainter.h

@@ -91,7 +91,8 @@ public:
         SmoothPixmapTransform = 0x04,
         HighQualityAntialiasing = 0x08,
         NonCosmeticDefaultPen = 0x10,
-        Qt4CompatiblePainting = 0x20
+        Qt4CompatiblePainting = 0x20,
+        LosslessImageRendering = 0x40,
     };
     Q_FLAG(RenderHint)
 

src/gui/painting/qpdf.cpp

@@ -944,7 +944,8 @@ void QPdfEngine::drawPixmap (const QRectF &rectangle, const QPixmap &pixmap, con
     QPixmap pm = sourceRect != pixmap.rect() ? pixmap.copy(sourceRect) : pixmap;
     QImage image = pm.toImage();
     bool bitmap = true;
-    const int object = d->addImage(image, &bitmap, pm.cacheKey());
+    const bool lossless = painter()->testRenderHint(QPainter::LosslessImageRendering);
+    const int object = d->addImage(image, &bitmap, lossless, pm.cacheKey());
     if (object < 0)
         return;
 
@@ -972,7 +973,8 @@ void QPdfEngine::drawImage(const QRectF &rectangle, const QImage &image, const Q
     QRect sourceRect = sr.toRect();
     QImage im = sourceRect != image.rect() ? image.copy(sourceRect) : image;
     bool bitmap = true;
-    const int object = d->addImage(im, &bitmap, im.cacheKey());
+    const bool lossless = painter()->testRenderHint(QPainter::LosslessImageRendering);
+    const int object = d->addImage(im, &bitmap, lossless, im.cacheKey());
     if (object < 0)
         return;
 
@@ -2578,6 +2580,8 @@ int QPdfEnginePrivate::addConstantAlphaObject(int brushAlpha, int penAlpha)
 
 int QPdfEnginePrivate::addBrushPattern(const QTransform &m, bool *specifyColor, int *gStateObject)
 {
+    Q_Q(QPdfEngine);
+
     int paintType = 2; // Uncolored tiling
     int w = 8;
     int h = 8;
@@ -2607,7 +2611,8 @@ int QPdfEnginePrivate::addBrushPattern(const QTransform &m, bool *specifyColor,
             return 0;
         QImage image = brush.textureImage();
         bool bitmap = true;
-        imageObject = addImage(image, &bitmap, image.cacheKey());
+        const bool lossless = q->painter()->testRenderHint(QPainter::LosslessImageRendering);
+        imageObject = addImage(image, &bitmap, lossless, image.cacheKey());
         if (imageObject != -1) {
             QImage::Format f = image.format();
             if (f != QImage::Format_MonoLSB && f != QImage::Format_Mono) {
@@ -2669,7 +2674,7 @@ static inline bool is_monochrome(const QVector<QRgb> &colorTable)
 /*!
  * Adds an image to the pdf and return the pdf-object id. Returns -1 if adding the image failed.
  */
-int QPdfEnginePrivate::addImage(const QImage &img, bool *bitmap, qint64 serial_no)
+int QPdfEnginePrivate::addImage(const QImage &img, bool *bitmap, bool lossless, qint64 serial_no)
 {
     if (img.isNull())
         return -1;
@@ -2730,7 +2735,7 @@ int QPdfEnginePrivate::addImage(const QImage &img, bool *bitmap, qint64 serial_n
         bool hasAlpha = false;
         bool hasMask = false;
 
-        if (QImageWriter::supportedImageFormats().contains("jpeg") && !grayscale) {
+        if (QImageWriter::supportedImageFormats().contains("jpeg") && !grayscale && !lossless) {
             QBuffer buffer(&imageData);
             QImageWriter writer(&buffer, "jpeg");
             writer.setQuality(94);

src/gui/painting/qpdf_p.h

@@ -241,7 +241,7 @@ public:
     void writeHeader();
     void writeTail();
 
-    int addImage(const QImage &image, bool *bitmap, qint64 serial_no);
+    int addImage(const QImage &image, bool *bitmap, bool lossless, qint64 serial_no);
     int addConstantAlphaObject(int brushAlpha, int penAlpha = 255);
     int addBrushPattern(const QTransform &matrix, bool *specifyColor, int *gStateObject);