drawText() on a QImage crashes program

Question

I have an image in uint8_t buffer and I am trying to use QImage as a wrapper to write text on the image. I have used drawLine() with no issues, but drawText() crashes the program. The below code is part of a boost thread in which I want to write text unto each image as it iterates through the function. Are there any bugs in Qt I am unaware of?

uint8_t *frameBuffer; // this contains image pixels
QImage img(frameBuffer, sizeX, m_sizeY, QImage::Format_RGB888);

QPainter p(&img);

p.setPen(QPen(Qt::green));
p.setFont(QFont("Times", 10, QFont::Bold));
p.drawLine(img.rect().bottomLeft().x(), img.rect().bottomLeft().y()-10,
           img.rect().bottomRight().x(), img.rect().bottomRight().y()-10);  //works!

p.drawText(img.rect(), Qt::AlignCenter, "Help");  //crashes program

Yes I have. When I comment out the drawText line, the drawline command works as expected. — JonnyCplusplus, Jun 04 '12 at 16:19
1, Is this part of a full Qt app? do you have a QApplication at the start? 2, do you call p.end(), it might actually be crashing when something goes out of scope and the error just points at the last op. — Martin Beckett, Jun 04 '12 at 16:48
It was a QCoreApplication (I had no GUI). Changing it to QApplication did the trick! — JonnyCplusplus, Jun 04 '12 at 19:08

score 5 · Accepted Answer · answered Jun 04 '12 at 19:08

5

My project was set to a QCoreApplication (I had no GUI). Changing it to QApplication did the trick!

answered Jun 04 '12 at 19:08

JonnyCplusplus

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score 1 · Answer 2 · answered Jun 04 '12 at 17:01

Just a guess... (I've never seen this error before, but have had other font issues on threads.)

Font rendering on background threads can be a little flaky in Qt, depending on how it was compiled. Check the value of QFontDatabase::supportsThreadedFontRendering on your system.

Note the documentation:

Returns true if font rendering is supported outside the GUI thread, false otherwise. In other words, a return value of false means that all QPainter::drawText() calls outside the GUI thread will not produce readable output.

I have a single thread and I got that error, so which thread would not be a problem in my situation... — Alexis Wilke, May 22 '18 at 00:37

drawText() on a QImage crashes program

2 Answers2