Animated GIF in QGraphicsScene/QGraphicsView

artwaw

Good morning,
I have QGraphicsScene in QGraphicsView to display and manipulate images. It works well, no issues here. It is the first time I tried QGraphicsScene instead if QLabel and I am very fond of how neatly it all works.
However I stumbled upon the edge case where animated gif is selected for display - as expected, only the first frame is displayed.
I tried usual way of having QMovie assigned to QLabel etc. - added that to the scene with addWidget() method but label is not visible.
This project is built using Qt5.15.2 (macOS clang/win mingw8)

So the question: is there any sane way of making it work without me having to rewrite half of the Qt ;) or should I just let go the idea of displaying animated gifs in the scene?

As usual, many thanks in advance.
Artur

JonB

@artwaw
I would search for what others have done to get this working. For example
https://forum.qt.io/topic/15658/solved-how-to-play-gif-animation-in-qgraphicsview-widget
https://www.qtcentre.org/threads/61528-How-to-place-animated-GIF-on-QGraphicsScene
https://stackoverflow.com/a/21083803/489865

I think the last one at least is doing it via QMovie/QLabel/QGraphicsScene::addWidget(). So don't know why you seem to say you can't.

artwaw

@JonB Thanks. This didn't popup (first result seems the way to go) when I used search in the forum O_O

Chris Kawa

You could certainly use a label, but proxy widgets are a bit heavy. All you really need to do is paint a frame of QMovie, so you can very easily make a custom graphics item that does that. You just need to implement 3 simple methods and you should be good to go, e.g.

class GraphicsMovieItem : public QGraphicsItem
{
public:
    using QGraphicsItem::QGraphicsItem;

    void setMovie(QMovie* movie)
    {
        prepareGeometryChange();
        QObject::disconnect(mConnection); // disconnect old object
        mMovie = movie;
        if (mMovie)
            mConnection = QObject::connect(mMovie, &QMovie::frameChanged, [=]{ update(); });
    }

    QRectF boundingRect() const override
    {
        if (mMovie)
            return mMovie->frameRect();
        else
            return QRectF();
    }

    void paint(QPainter* painter, const QStyleOptionGraphicsItem* option, QWidget* widget) override
    {
        if (mMovie)
            painter->drawPixmap(mMovie->frameRect(), mMovie->currentPixmap(), mMovie->frameRect());
    }

private:
    QPointer<QMovie> mMovie;
    QMetaObject::Connection mConnection;
};

artwaw

@Chris-Kawa That's smart! I'll definitely try this approach out, thank you!

artwaw

@Chris-Kawa I am sorry to bother you but example code provided raises some issues during the compilation:

/Users/arturwawrowski/cpp/build-TNImageViewer-Desktop_Qt_5_15_2_clang_64bit-Debug/moc_qgraphicsmovieitem.cpp:67: error: no member named 'staticMetaObject' in 'QGraphicsItem'; did you mean simply 'staticMetaObject'?
moc_qgraphicsmovieitem.cpp:67:34: error: no member named 'staticMetaObject' in 'QGraphicsItem'; did you mean simply 'staticMetaObject'?
    QMetaObject::SuperData::link<QGraphicsItem::staticMetaObject>(),
                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                 staticMetaObject
moc_qgraphicsmovieitem.cpp:66:58: note: 'staticMetaObject' declared here
QT_INIT_METAOBJECT const QMetaObject QGraphicsMovieItem::staticMetaObject = { {
                                                         ^

and several of:

/Users/arturwawrowski/cpp/build-TNImageViewer-Desktop_Qt_5_15_2_clang_64bit-Debug/moc_qgraphicsmovieitem.cpp:78: error: invalid use of non-static data member 'd_ptr'
moc_qgraphicsmovieitem.cpp:78:21: error: invalid use of non-static data member 'd_ptr'
    return QObject::d_ptr->metaObject ? QObject::d_ptr->dynamicMetaObject() : &staticMetaObject;
           ~~~~~~~~~^~~~~

intertwined with:

/Users/arturwawrowski/cpp/build-TNImageViewer-Desktop_Qt_5_15_2_clang_64bit-Debug/moc_qgraphicsmovieitem.cpp:78: error: 'd_ptr' is a protected member of 'QObject'
moc_qgraphicsmovieitem.cpp:78:21: error: 'd_ptr' is a protected member of 'QObject'
../../Qt/5.15.2/clang_64/lib/QtCore.framework/Headers/qobject.h:450:33: note: declared protected here
    QScopedPointer<QObjectData> d_ptr;
                                ^

Code:
header

#ifndef QGRAPHICSMOVIEITEM_H
#define QGRAPHICSMOVIEITEM_H

#include <QGraphicsItem>
#include <QObject>
#include <QMovie>
#include <QPainter>

class QGraphicsMovieItem : public QGraphicsItem
{
    Q_OBJECT
public:
    QGraphicsMovieItem(QGraphicsItem *parent = nullptr);
    void setMovie(QMovie* movie);
    QRectF boundingRect() const override;
    void paint(QPainter* painter, const QStyleOptionGraphicsItem* option, QWidget* widget) override;

private:
    QPointer<QMovie> mMovie;
    QMetaObject::Connection mConnection;
};

#endif // QGRAPHICSMOVIEITEM_H

implementation:

#include "qgraphicsmovieitem.h"

QGraphicsMovieItem::QGraphicsMovieItem(QGraphicsItem *parent) : QGraphicsItem(parent) {}

void QGraphicsMovieItem::setMovie(QMovie* movie) {
    prepareGeometryChange();
    QObject::disconnect(mConnection);
    mMovie = movie;
    if (mMovie) {
        mConnection = QObject::connect(mMovie, &QMovie::frameChanged, [=]{ update(); });
    }
}

QRectF QGraphicsMovieItem::boundingRect() const {
    if (mMovie) { return mMovie->frameRect(); }
    else { return QRectF(); }
}

void QGraphicsMovieItem::paint(QPainter* painter, const QStyleOptionGraphicsItem* option, QWidget* widget) {
    Q_UNUSED(option)
    Q_UNUSED(widget)
    if (mMovie) {
        painter->drawPixmap(mMovie->frameRect(), mMovie->currentPixmap(), mMovie->frameRect());

    }
}

What I did wrong this time?

Chris Kawa

QGraphicsItem does not inherit QObject so don't use Q_OBJECT for your class.

Btw. modern C++ has this wonderful thing called inherited constructors, so you don't have to write the boilerplate empty constructors that just pass parameters to base classes. Instead of writing QGraphicsMovieItem(QGraphicsItem *parent = nullptr); and implementing it as an empty method you can just write using QGraphicsItem::QGraphicsItem; and it does exactly the same thing. Super useful when base class has multiple constructors with different parameters as you can inherit them all in just that one line.

artwaw

@Chris-Kawa Noted thank you. Now it compiles without errors. I think I need to read a bit about newer c++...