Copy Bits to QtImage in QGLWidget - qt

Question

Maybe someone can help me with the following problem:
I want to draw the content of a QImage in a QGLWidget, but the widget is painted in black.
class QGLCanvas {
public:
QGLCanvas(QWidget* parent) : QGLWidget(parent) {
}
void setImage(const QImage* image) {
img = image;
}
void paintEvent(QPaintEvent*) {
// From Painter Documentation Qt
QPainter p(this);
p.setRenderHint(QPainter::SmoothPixmapTransform, 1);
p.drawImage(this->rect(), *img);
p.end();
}
public slots:
void rgb_data(const void *data) {
memcpy((void *)img->bits(), data, img->byteCount()); // data will be copied (sizes are known)
// img.save("text.png"); // saves right image
this->update(); // calls repaint, but does not draw the image.
}
private:
QImage *img;
}
The Bug:
When the slot is called, the memory is copied to the image. If the image is saved, the content is correct. But the repaint method just draws black content to the widget.
The Fix:
If the memcpy line is implemented outside the slot, the image content is drawn to the widget. This fix increased code complexity a lot. Thus, i've got the following question:
The Question:
Why does the memcpy not work within the slot? Is this a general problem with Qt?

Answer 1

There's nothing special about a slot which would stop your code from working.
What's probably the issue is that when you call update(), a repaint is scheduled but happens asynchronously. From the code you've provided the most likely cause is img being modified between the calls to rbg_data and paintEvent

Answer 2

You want to be sure about the format of the QImage. When you call bits and are expecting it to be RGB, you need to check the format.
if( img->format() != QImage::Format_RGB888 )
{
// convert the image format to RGB888
*img = img->convertToFormat(QImage::Format_RGB888);
}
This way, Qt will know the image format when trying to paint it. If you fill it with RGB data but the QImage is "formatted" as ARGB, you will get some painting errors.