Graphics glitch in QLabel (showing old image data when calling setText())

bepaald

Hi! I have an application that shows images on a QLabel. Every once in a while, no image is available, and the label clears and displays some text ('no image'). I noticed that whenever this happens, a previous image appears on the label for a fraction of a second between the current image and the new text contents.

I tried to setUpdatesEnabled(false) before the setText() call, and reenabling updates after, but the results are the same. I also tried to QPixmapCache::clear(), but I don't think that does what I thought it does and doesn't work anyway.

I recorded a little video of it here: https://www.youtube.com/watch?v=LRZi4hHD29o

The code used in the video is the following (to run this, you would need to add two images, 1.jpg and 2.jpg, to the application directory).

// main.cc
#include "button.h"

int main(int argc, char *argv[])
{
  QApplication app(argc, argv);

  QMainWindow mw;

  QLabel *l = new QLabel(&mw);
  l->setFixedSize(300, 300);
  l->setText("test");

  Button *b = new Button(l, &mw);

  QObject::connect(b, SIGNAL(clicked()), b, SLOT(next()));

  QGridLayout *layout = new QGridLayout(&mw);
  layout->addWidget(l);
  layout->addWidget(b);

  QWidget *w = new QWidget(&mw);
  w->setLayout(layout);

  mw.setCentralWidget(w);

  mw.show();

  return app.exec();
}

//button.h
#include <QtGui>

class Button : public QPushButton
{
  Q_OBJECT;

  int count;
  QLabel *label;
  QPixmap pm;

 public:
  Button(QLabel *l, QWidget *parent = 0)
    : QPushButton("NEXT", parent), count(0), label(l)
  {};

 public slots:
  void next()
  {
    if (count % 3 == 0)
    {
      if (pm.load("1.jpg"))
        label->setPixmap(pm.scaled(label->size(), Qt::KeepAspectRatio));
    }
    else if (count % 3 == 1)
    {
      if (pm.load("2.jpg"))
        label->setPixmap(pm.scaled(label->size(), Qt::KeepAspectRatio));
    }
    else if (count % 3 == 2)
    {
      label->setText("NO IMAGE");
    }
    ++count;
  };
};

Does anyone have an idea how to solve this? The issue was observed under X11 by the way, I don't know if it happens on other platforms.

Thanks!

? Offline

Hi. Can't access the video, it's private.

bepaald

@Wieland Hi! Sorry, that was my first time on youtube. It should be fixed now. Thanks!

? Offline

I see. That's obviously a bug. The question remains if it's a bug in Qt or somewhere else in the graphics stack. Maybe this bug is already in our bug database. See https://bugreports.qt.io. What graphics driver do you use? The Nvidia drivers caused a lot of problems in the past.

As a workaround, have you tried to assign a null pixmap?

else if (count % 3 == 2)
    {
       pm = QPixmap();
       label->setPixmap(pm);
       label->setText("NO IMAGE");
    }

bepaald

Thanks for the reply. This is using intel graphics, and unfortunately your workaround does not help. After your message, I've tested the same code from a newer live image and also on my more up-to-date laptop (which has similar hardware) and I can not reproduce the issue in either of these cases. On the affected machine, I tried updating the intel driver and mesa, but this did not solve it, so I guess it probably needs the kernel or qt packages updated as well. Unfortunately, this is not an option at this point.

I've worked around the issue by re-implementing paintEvent() to do something like the following

if (no_pixmap_set)
{
  pos = calculateTextPosition()
  QPainter::drawText(pos, text);
  return;
}
QLabel::paintEvent(event);

Which seems to work just fine. In fact, re-implementing the paintEvent has given me the chance to animate the image changes which looks pretty nice, and it will be easy to simplify the code a bit when all my machines are fully updated and the glitch is gone.

Thanks again!