Saving QPixmap resutls in corrupt JPG

[Pit]

Hi! When I try to save a QPixmap in Windows, it results in a corrupt JPG file, black or black with white lines. But it doesn't happen in all computers, I tried in four and in two happens, in the other two not.

Saving in PNG doesn't have any errors.

Any ideas???

Thank you.

[wysota]

Please provide a minimal compilable example reproducing the problem.

[Pit]

Hi! Thanks for your reply. Here is the example that fails. Basically it loads a photo into a label and then saves it:

void MainWindow::on_savePushButton_clicked()
{
   [...]
 
    if (!photoLabel->pixmap()->save(QString("fotos/%1.jpg").arg(regLineEdit->displayText()), 0, 100))
      QMessageBox::warning(this, qApp->applicationName(), "Error");
 
   [...]
}
 
void MainWindow::loadPhoto()
{
  QPixmap pixmap;
 
  QString path = QString("fotos/%1.jpg").arg(cardsModel->index(mapper->currentIndex(), ViewCardsTableModel::Registration).data().toString());
 
  if (!pixmap.load(path))
  {
    QMessageBox::critical(this, qApp->applicationName(), QString("Error.").arg(cardsModel->index(mapper->currentIndex(), ViewCardsTableModel::Registration).data().toString()));
    photoLabel->setPixmap(QPixmap());
  }
  else
    photoLabel->setPixmap(pixmap);
}

void MainWindow::on_savePushButton_clicked()
{
   [...]

    if (!photoLabel->pixmap()->save(QString("fotos/%1.jpg").arg(regLineEdit->displayText()), 0, 100))
      QMessageBox::warning(this, qApp->applicationName(), "Error");

   [...]
}

void MainWindow::loadPhoto()
{
  QPixmap pixmap;

  QString path = QString("fotos/%1.jpg").arg(cardsModel->index(mapper->currentIndex(), ViewCardsTableModel::Registration).data().toString());
  
  if (!pixmap.load(path))
  {
    QMessageBox::critical(this, qApp->applicationName(), QString("Error.").arg(cardsModel->index(mapper->currentIndex(), ViewCardsTableModel::Registration).data().toString()));
    photoLabel->setPixmap(QPixmap());
  }
  else
    photoLabel->setPixmap(pixmap);
}

To copy to clipboard, switch view to plain text mode 

[wysota]

This is neither minimal nor compilable.

[Pit]

Sorry, it's too much code. But it's simple, just this:

QPixmap pixmap;
pixmap.load("photo.jpg");
pixmap.save("photo2.jpg");

QPixmap pixmap;
pixmap.load("photo.jpg");
pixmap.save("photo2.jpg");

To copy to clipboard, switch view to plain text mode 

Don't work. I get the attached image.

[wysota]

Does it happen for every jpeg image or this particular one?

[Pit]

For every JPEG, but if I use QImage instead QPixmap it works, so it's not because jpeg plugin (I think).

[wysota]

Well... saving the picture is done by the same code (QImageWriter) regardless if you use QPixmap or QImage as one is converted to the other so if it matters whether you use QPixmap or QImage, I'd say Qt is not to be blamed.

So the following doesn't work?

#include <QtGui>
int main(int argc, char **argv){
  QApplication app(argc, argv);
  QPixmap px;
  px.load("input.jpeg");
  px.save("output.jpeg");
  return 0;
}

#include <QtGui>
int main(int argc, char **argv){
  QApplication app(argc, argv);
  QPixmap px;
  px.load("input.jpeg");
  px.save("output.jpeg");
  return 0;
}

To copy to clipboard, switch view to plain text mode 

And the following does?

#include <QtGui>
int main(int argc, char **argv){
  QApplication app(argc, argv);
  QImage px;
  px.load("input.jpeg");
  px.save("output.jpeg");
  return 0;
}

#include <QtGui>
int main(int argc, char **argv){
  QApplication app(argc, argv);
  QImage px;
  px.load("input.jpeg");
  px.save("output.jpeg");
  return 0;
}

To copy to clipboard, switch view to plain text mode 

Please check both snippets.

[Pit]

That's right, I checked it and the first doesn't work, the second does.

[wysota]

That's right, I checked it and the first doesn't work, the second does.

This exact same code, right? What platform are you using and which version of Qt (you might want to update your QtCentre profile to avoid such questions in future)?

[Pit]

Yes, this exact same code. I'm using Windows XP SP3 and Qt 4.6.3 (thanks for the advice ). This code works in another computer with XP SP3, but it doesn't in the computer with QT installed, neither in another with XP SP3 too.

[Pit]

I tried this, but it failed too:

#include <QtGui>
int main(int argc, char **argv){
QApplication app(argc, argv);
QImage img;
QPixmap px;
img.load("input.jpeg");
px = QPixmap::fromImage(img);
px.save("output.jpeg");
return 0;
}

#include <QtGui>
int main(int argc, char **argv){
QApplication app(argc, argv);
QImage img;
QPixmap px;
img.load("input.jpeg");
px = QPixmap::fromImage(img);
px.save("output.jpeg");
return 0;
}

To copy to clipboard, switch view to plain text mode 

[wysota]

One more try:

#include <QtGui>
 
int main(int argc, char **argv){
  QApplication app(argc, argv);
  QImage img = QImageReader("input.jpeg").read();  // this is almost exactly
  QPixmap px = QPixmap::fromImage(img); // what QPixmap::load() does
  QLabel l;
  l.setPixmap(px);
  l.show();
  px.save("output.jpeg");
  return app.exec();
}

#include <QtGui>

int main(int argc, char **argv){
  QApplication app(argc, argv);
  QImage img = QImageReader("input.jpeg").read();  // this is almost exactly
  QPixmap px = QPixmap::fromImage(img); // what QPixmap::load() does
  QLabel l;
  l.setPixmap(px);
  l.show();
  px.save("output.jpeg");
  return app.exec();
}

To copy to clipboard, switch view to plain text mode 

Is the label correct? Is output.jpeg corrupted?

[Pit]

The label is correct, and output.jpeg is corrupted. That happens in my program too, I have a label and I put the QPixmap there, but when I save it the jpeg is corrupted. The problem appears to be the save function

[wysota]

Save essentially does this:

QPixmap px(...);
QImage img = px.toImage();
QImageWriter writer(fiileName);
writer.write(img);

QPixmap px(...);
QImage img = px.toImage();
QImageWriter writer(fiileName);
writer.write(img);

To copy to clipboard, switch view to plain text mode 

Try this piece of code:

int main(...) {
  QApplication app(argc, argv);
  QLabel l;
  QImage img("input.jpeg");
  QPixmap px = QPixmap::fromImage(img);
  img = px.toImage();
  px = QPixmap::fromImage(img);
  l.setPixmap(px);
  l.show();
  img.save("output_image.jpeg", "JPEG");
  px.save("output_pixmap.jpeg", "JPEG");
  return app.exec();
}

int main(...) {
  QApplication app(argc, argv);
  QLabel l;
  QImage img("input.jpeg");
  QPixmap px = QPixmap::fromImage(img);
  img = px.toImage();
  px = QPixmap::fromImage(img);
  l.setPixmap(px);
  l.show();
  img.save("output_image.jpeg", "JPEG");
  px.save("output_pixmap.jpeg", "JPEG");
  return app.exec();
}

To copy to clipboard, switch view to plain text mode 

Basically Qt loads and saves pixmaps and images using the same code, so essentially either both or none of them should work so if what you experience is different, there might be something wrong with your installation.

[Pit]

Same result. Label shows correctly, but both output_image and output_pixmap are corrupted. I will go to install QT in the computer that works, and compile from them and see the result. I'll post the results soon. Thank you

[Pit]

Hmmmm... I installed QT 4.6.3 in the computer where the program works. After install, QPixmap fails again

[wysota]

I think there is some software installed on your computer which causes malfunction of libjpeg or something like that.

[Pit]

Well... I don't know what is wrong. I tried in many computers, some of them works, some don't, so finally I'm saving in PNG format... Thanks for all anyway