class Viewer : public QWidget{
...
public slots:
void zoomIn();
protected:
QImage *image;
QImage *t_image;
QPainter painter;
void paintEvent(QPaintEvent * event);
qreal zoom_value; // коэф. масштабирования
};
--------------------------------
Viewer::Viewer(QWidget *parent) : QWidget(parent){
zoom_value = 1.0;
image = new QImage();
t_image = new QImage();
if(image->load("...jpg")){
t_image->operator = (*image);
if(!t_image->isNull()){
setGeometry(QRect(0,0,t_image->width(),t_image->height()));
}
}
}
void Viewer::paintEvent(QPaintEvent *event){
qDebug()<<"paint:";
if(!t_image->isNull()){
qDebug()<<" draw image";
painter.begin(this);
painter.drawImage(0,0,*t_image);
painter.end();
}
else
qDebug()<<" image is null";
}
void Viewer::zoomIn(){
if(!t_image->isNull()){
QMatrix matrix;
zoom_value*=1.3;
t_image->operator = (image->transformed(matrix.scale(zoom_value,zoom_value)));
while(t_image->isNull()){
zoom_value/=1.3;
qDebug()<<" image is null, zoom value = "<<zoom_value;
t_image->operator =(image->transformed(matrix.scale(zoom_value,zoom_value)));
}
qDebug()<<"transform image["<<image->size()<<"] to ["<<t_image->size()<<"]";
qDebug()<<" image is OK,  zoom value = "<<zoom_value;
setGeometry(QRect(0,0,t_image->width(),t_image->height()));
update();
}
}

transform image[ QSize(800, 600) ] to [ QSize(1040, 780) ]
 image is OK,  zoom value =  1.3
paint:
 draw image
// изображение увеличелось

...

transform image[ QSize(800, 600) ] to [ QSize(11029, 8272) ]
 image is OK,  zoom value =  13.7858
paint:
 draw image
// изображение увеличелось
QImage: out of memory, returning null image
 image is null, zoom value =  13.7858

...

QImage: out of memory, returning null image
 image is null, zoom value =  0.0725382
transform image[ QSize(800, 600) ] to [ QSize(14337, 10753) ]
 image is OK,  zoom value =  0.0725382
paint:
 draw image
// изображение увеличелось!!!

transform image[ QSize(800, 600) ] to [ QSize(75, 57) ]
 image is OK,  zoom value =  0.0942996
paint:
 draw image
// изображение уменьшилось от исходного в zoom value =  0.0942996 раз
 
transform image[ QSize(800, 600) ] to [ QSize(98, 74) ]
 image is OK,  zoom value =  0.122589
paint:
 draw image
// изображение увеличелось
итд

painter.begin(this);
 painter.scale(zoom_value,zoom_value)
 painter.drawImage(0,0,*t_image);
 painter.end();