'QMessageBox::critical' : none of the 4 overloads could convert all the argument type

[ChrisW67]

GUI objects in threads aside, the original error message:

QMessageBox::critical' : none of the 4 overloads could convert all the argument

is caused by:

QMessageBox::critical(this,tr("ERROR"),tr("Large change in illumination.\nPlease re-capture reference image.\n"));

QMessageBox::critical(this,tr("ERROR"),tr("Large change in illumination.\nPlease re-capture reference image.\n"));

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because the first argument, this, is not a pointer to a QWidget and cannot be converted to one.