some new facts.
Actually QGraphicsScene::render works fine in separate thread.

Problem happens when I try to call QPixmap::toImage() method. There is much time taken and finally program got crashed.

So, some problems in QImage.

Same situation happen when call QGraphicsScene::render to QImage based QPainter.

I have got some workaround, but this is strange that GUI thread is required. No painting to widgets was requested...