Text rendered as blobs on mono images in qt5 - image

Question

I am trying to "print" items that contain text, onto a mono image.
I have items that either subclass QGraphicsTextItem, or some other class that uses painter->drawText().
I can do it in Qt4 - but in Qt5, all I get are some blobs. I have tried to experiment with a small sample program, but I can't figure out what is going on...
Here is a simple self contained program:
#include <QApplication>
#include <QGraphicsView>
#include <QGraphicsTextItem>
class TextItem : public QGraphicsItem
{
public:
TextItem(){}
virtual void paint(QPainter* painter, const QStyleOptionGraphicsItem*, QWidget* = NULL)
{
QPen p(Qt::red);
painter->setPen(p);
painter->setBrush(Qt::NoBrush);
//painter->setFont(QFont("Arial", 30));
painter->drawText(boundingRect(), Qt::AlignCenter, "abcd");
}
virtual QRectF boundingRect() const { return QRectF(0, 0, 30, 20); }
};
void processScene(QGraphicsScene* s) {
QGraphicsTextItem* t = new QGraphicsTextItem();
t->setPlainText("abcd");
// t->setDefaultTextColor(Qt::red); // won't make a difference
t->setPos(0, 0);
s->addItem(t);
//t->setFont(QFont("Arial", 30));
//t->setTransform(t->transform().fromScale(3,3));
TextItem* t1 = new TextItem();
t1->setPos(0, 20);
s->addItem(t1);
QImage image = QImage(s->sceneRect().size().toSize(),
QImage::Format_Mono);
//QImage::Format_RGB32); // this works in qt5
image.fill(QColor(Qt::color0).rgb());
QPainter painter;
painter.begin(&image);
s->render(&painter);
painter.end();
image.save(QString("../test.bmp"));
}
int main(int argc, char *argv[]) {
QApplication app(argc, argv);
QGraphicsScene s;
s.setSceneRect(0, 0, 50, 50);
QGraphicsView view(&s);
view.show();
processScene(&s);
return app.exec();
}
Running it in Qt4 it gives what is expected: outputs a mono image containing text (in both versions, using QGraphicsTextItem or paint->drawText() (left image).
The same in Qt5 only shows some blobs in place of text (right image):
I have tried all kinds of experiments... Setting the font doesn't make a difference (except, for QGraphicsTextItem, setting the font to something big AND scaling to something big - but that is not useful in a practical application).
The other thing that worked in Qt5 is rendering to a color image instead of mono - but that is very bad for my application, where I need to store small mono images. My thought would be then, for Qt5, to render text to a color bitmap and then convert it to mono... But really !
I have also tried to save the items themselves to a bitmap, in a function implemented inside their class - it did the same thing.
I have tried to set point size on the QFont before applying it.
Is there something else I can set on QPainter in order to draw text ?
I found a related question: QT5 Text rendering issue also asked again in Qt forums: QT5 Text rendering issue. But that question refers to all rendering, and to a Qt5 in Alpha on a MIPS based platform... It implies that other platforms don't see that issue...
I'm using 5.5.0... I am also running under windows... And rendering on color image works... And...
I simply do not understand the "solution":
For time being, when we tried forcefully to enable Glyph cache, then
we are not seeing this issue.
Not sure what that means and how to do it.
Why is text painting blobs on mono images (but fine on color images) in qt5 and what can I do to fix it ?
Using Qt 5.5.0 32 bit, in Windows 7
Edit:
1) I tested the code in 5.4.2 and it renders text correctly.
I tried to compare some of the relevant code in qpainter.cpp, qtextlayout.cpp, qpaintengine_raster.cpp, qtextengine.cpp
- but with my limited understanding, the changes between Qt 5.4.2 and Qt 5.5.0 look logical... and would not affect the behavior...
2) I did paint my text on a color QImage, then painted it on mono - it worked fine on my own text items, but a third party software that creates very small text gives me very bad resolution when I try the same (illegible).