Pixmap updating QLabel

[Matt]

When I run that, replacing the image obviously, it displays the image I want it to.

If I had a null pixmap in one of the lower functions (onPrev, etc), would it display a null or just continue to show the same thing. If it continues to show the same thing, that is my problem.

Edit: A little testing with and pixmap.isNull() has determined that my image isn't null in any of my functions. It just refuses to update to the new filepath.

[Urthas]

If I had a null pixmap in one of the lower functions (onPrev, etc), would it display a null or just continue to show the same thing.

It would display absolutely nothing at all, or in the case of my example code, "Null Pixmap!" (tested). A null pixmap has no content, but QLabel::setPixMap() wipes out the label's previous pixmap.

[Matt]

QString number;
  QString png = ".png";
  QString pics = "pics/";
  number = number.setNum(val);
  number = number.prepend(pics);
  number = number.append(png);
  QPixmap *original=new QPixmap(QString(number));
  QPixmap image= (original->scaled ( 1300, 600, Qt::KeepAspectRatio, Qt::SmoothTransformation ));
  label = new QLabel(this);
  label->setPixmap(image);

QString number;
  QString png = ".png";
  QString pics = "pics/";
  number = number.setNum(val);
  number = number.prepend(pics);
  number = number.append(png);
  QPixmap *original=new QPixmap(QString(number));
  QPixmap image= (original->scaled ( 1300, 600, Qt::KeepAspectRatio, Qt::SmoothTransformation ));
  label = new QLabel(this);
  label->setPixmap(image);

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So in this code, label should be cleared and replaced with the new value of image, which is clearly not happening. Could the problem be in QPixmap *original? Removing the * from that variable makes it refuse to compile, but could the * be preventing the value of original from updating? If so, is there a way to get rid of the * safely without giving errors about converting QPixmap* to QPixmap?

[Urthas]

How about I just show you what worked for me?

First, here is the header file:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
 
#include <QtGui/QWidget>
 
class QLabel;
class QPushButton;
 
class MainWindow : public QWidget
{
    Q_OBJECT
 
public:
    MainWindow(QWidget *parent = 0);
    ~MainWindow();
 
public slots:
    void updatePixmap();
 
private:
    QPixmap pmap1, pmap2;
    QLabel *label;
    QPushButton *button;
    int numUpdates;
};
 
#endif // MAINWINDOW_H

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QtGui/QWidget>

class QLabel;
class QPushButton;

class MainWindow : public QWidget
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = 0);
    ~MainWindow();

public slots:
    void updatePixmap();

private:
    QPixmap pmap1, pmap2;
    QLabel *label;
    QPushButton *button;
    int numUpdates;
};

#endif // MAINWINDOW_H

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Now, here is the .cpp file:

#include "mainwindow.h"
#include <QLabel>
#include <QPushButton>
#include <QVBoxLayout>
 
MainWindow::MainWindow(QWidget *parent)
    : QWidget(parent), numUpdates(0)
{
    pmap1 = QPixmap("C:\\path\\to\\image1.jpg");
    pmap2 = QPixmap("C:\\path\\to\\image2.jpg");
    label = new QLabel();
    label->setPixmap(pmap1);
 
    button = new QPushButton("Update!");
    connect(button, SIGNAL(clicked()), this, SLOT(updatePixmap()));
 
    QVBoxLayout *layout = new QVBoxLayout(this);
    layout->addWidget(label);
    layout->addWidget(button);
    layout->setSizeConstraint(QLayout::SetFixedSize);
    setLayout(layout);
}
 
void MainWindow::updatePixmap()
{
    ++numUpdates % 2 ? label->setPixmap(pmap2) : label->setPixmap(pmap1);
}
 
MainWindow::~MainWindow()
{
 
}

#include "mainwindow.h"
#include <QLabel>
#include <QPushButton>
#include <QVBoxLayout>

MainWindow::MainWindow(QWidget *parent)
    : QWidget(parent), numUpdates(0)
{
    pmap1 = QPixmap("C:\\path\\to\\image1.jpg");
    pmap2 = QPixmap("C:\\path\\to\\image2.jpg");
    label = new QLabel();
    label->setPixmap(pmap1);

    button = new QPushButton("Update!");
    connect(button, SIGNAL(clicked()), this, SLOT(updatePixmap()));

    QVBoxLayout *layout = new QVBoxLayout(this);
    layout->addWidget(label);
    layout->addWidget(button);
    layout->setSizeConstraint(QLayout::SetFixedSize);
    setLayout(layout);
}

void MainWindow::updatePixmap()
{
    ++numUpdates % 2 ? label->setPixmap(pmap2) : label->setPixmap(pmap1);
}

MainWindow::~MainWindow()
{

}

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update: images now toggle back and forth instead of being a one-shot deal

[Urthas]

label should be cleared and replaced with the new value of image, which is clearly not happening.

But but but you are destroying the previous label entirely...

What happens if you comment out line 9?

[Matt]

If I comment out line 9, I get an error. But that line is only for resizing the window, it doesn't affect anything else.

I am reading over your first post trying to integrate it into my system.

Edit: Thank you very much, that did it. When I removed the code that destroyed label, it solved the problem. I will definitely remember this in the future.