Qt Gui Chapter drawing classes

Double buffering

void Plotter::refreshPixmap()
{
    pixmap = QPixmap(size());
    pixmap.fill(this, 0, 0);

    QPainter painter(&pixmap);
    painter.initFrom(this);
    drawGrid(&painter);
    drawCurves(&painter);
    update();
}

QPixmap stored in the graphics memory, it is optimized for a particular screen, and therefore, it is the device (graphics engine's operating system) is closely related to the actual underlying display.

When the picture is small, the load directly with QPixmap; big picture when the time is preferably loaded through the QImage, then drawn again QPixmap user. E.g:

QImage image;
image.load(":/pics/earth.png" );
QPainter painter(this);
QPixmap pixmapToShow = QPixmap::fromImage( image.scaled(size(), Qt::KeepAspectRatio) );
painter.drawPixmap(0,0, pixmapToShow);
 
// attachment QPixmap and method of mutual conversion QImage 
QImage QPixmap :: The toImage () const ;
 static QPixmap QPixmap :: fromImage ( const QImage & Image ,,,);
 // QPicture not convert from these two types, only from IO device path or file name to load. 
BOOL Load (a QIODevice dev *, const  char * = the format Q_NULLPTR);
 BOOL Load ( const QString & fileName, const  char * = Q_NULLPTR the format);

The reference csdn's blog: https://blog.csdn.net/qq_33266987/article/details/73187140