What exactly are the positioning rules for text in Qt? - c++

Question

Here's a very basic piece of code which:
Measures the size a piece of text would take.
Draws the rectangle which corresponds to this size at coordinates (100, 25).
Displays text at coordinates (100, 25).
auto str = "Hello, World!";
auto metrix = window->fontMetrics();
auto text = scene->addText(str);
text->setPos(100, 25);
text->setDefaultTextColor(Qt::white);
auto r = metrix.boundingRect(str);
int x, y, w, h;
r.getRect(&x, &y, &w, &h);
scene->addRect(100, 25, w, h, QPen(Qt::white));
The scene in code is a QGraphicsScene with no specific customizations, with the exception of a border set to zero.
I would expect the text to be exactly inside the rectangle. The text is however shifted by a few pixels to the left and a few more pixels to the bottom. Why?

Answer 1

Solution
Setting the document margins to 0, as #NgocMinhNguyen suggested, might seem to work, but it is not a real solution, because you lose the margins. It would be better, if you could get the actual geometry, including margins etc. For that purpose you can use QGraphicsTextItem::boundingRect() instead of QFontMetrics::boundingRect.
Example
Here is a minimal and complete example I have written for you, in order to demonstrate the proposed solution:
#include <QApplication>
#include <QGraphicsView>
#include <QGraphicsItem>
#include <QBoxLayout>
struct MainWindow : public QWidget
{
MainWindow(QWidget *parent = nullptr) : QWidget(parent) {
QPointF p(100, 25);
auto *l = new QVBoxLayout(this);
auto *view = new QGraphicsView(this);
auto *textItem = new QGraphicsTextItem(tr("HHHHHHHH"));
auto *rectItem = new QGraphicsRectItem(textItem->boundingRect()
.adjusted(0, 0, -1, -1));
textItem->setPos(p);
rectItem->setPos(p);
view->setScene(new QGraphicsScene(this));
view->scene()->addItem(textItem);
view->scene()->addItem(rectItem);
l->addWidget(view);
resize(300, 300);
}
};
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.show();
return a.exec();
}
Note: Please note how I create the rectangle. There is a difference between
auto *item = new QGraphicsRectItem(100, 25, w, h);
and
auto *item = new QGraphicsRectItem(0, 0, w, h);
item->setPos(100, 25);
Result
This example produces the following result:

Answer 2

QGraphicsTextItem is held by QTextDocument, which can have a margin.
Setting the margin to 0 and the rectangle will be correctly drawn.
text->document()->setDocumentMargin(0);