1. Routine code

void Widget::paintEvent(QPaintEvent *event)
{
    
    QPainter painter;
    
    // draw image
    QImage image(100,100,QImage::Format_ARGB32);
    painter.begin(&image);
    painter.setPen(QPen(Qt::red,3));
    painter.setBrush(Qt::yellow);
    painter.drawRect(10,10,60,60);
    painter.drawText(10,10,60,60,Qt::AlignCenter,tr("QImage"));
    painter.setBrush(QColor(0,0,0,100));
    painter.drawRect(50,50,40,40);
    painter.end();
    
    // draw pixmap
    QPixmap pix(100,100);
    painter.begin(&pix);
    painter.setPen(QPen(Qt::red,3));
    painter.setBrush(Qt::yellow);
    painter.drawRect(10,10,60,60);
    painter.drawText(10,10,60,60,Qt::AlignCenter,tr("QPixmap"));
    painter.setBrush(QColor(0,0,0,100));
    painter.drawRect(50,50,40,40);
    painter.end();

    // draw bitmap
    QBitmap bit(100,100);
    painter.begin(&bit);
    painter.setPen(QPen(Qt::red,3));
    painter.setBrush(Qt::yellow);
    painter.drawRect(10,10,60,60);
    painter.drawText(10,10,60,60,Qt::AlignCenter,tr("QBitmap"));
    painter.setBrush(QColor(0,0,0,100));
    painter.drawRect(50,50,40,40);
    painter.end();

    // draw picture
    QPicture picture;
    painter.begin(&picture);
    painter.setPen(QPen(Qt::red,3));
    painter.setBrush(Qt::yellow);
    painter.drawRect(10,10,60,60);
    painter.drawText(10,10,60,60,Qt::AlignCenter,tr("QPicture"));
    painter.setBrush(QColor(0,0,0,100));
    painter.drawRect(50,50,40,40);
    painter.end();

    // stay widget Draw on the part 
    painter.begin(this);
    painter.drawImage(50,20,image);
    painter.drawPixmap(200,20,pix);
    painter.drawPixmap(50,170,bit);
    painter.drawPicture(200,170,picture);
}

2. Four common drawing devices

2.1 QImage

• QImage Mainly used for I/O Handle , It's right I/O Processing operations are optimized , And it can be used to directly access and manipulate pixels ;

2.2 QPixmap

• QPixmap It's mainly used to display images on the screen , It optimizes which image is displayed on the screen ;

2.3 QBitmap

• QBitmap yes QPixmap Subclasses of , It's used to process colors with a depth of 1 Image , That is, only black and white can be displayed ;

2.4 QPicture

• QPicture Used to record and replay QPainter command .

3. Compound mode

void Widget::paintEvent(QPaintEvent *event)
{
    
    QPainter painter;
    QImage image(400,300,QImage::Format_ARGB32_Premultiplied);
    painter.begin(&image);
    painter.setBrush(Qt::green);
    painter.drawRect(100,50,200,200);
    painter.setBrush(QColor(0,0,255,15));
    painter.drawRect(50,0,100,100);
    painter.setCompositionMode(QPainter::CompositionMode_SourceIn);
    painter.drawRect(250,0,100,100);
    painter.setCompositionMode(QPainter::CompositionMode_DestinationOver);
    painter.drawRect(50,200,100,100);
    painter.setCompositionMode(QPainter::CompositionMode_Xor);
    painter.drawRect(250,200,100,100);
    painter.end();
    painter.begin(this);
    painter.drawImage(0,0,image);
}