Class differences of QT processing image data (qpixmap, qimage, qpicture)

1. brief introduction

Qt Provides classes for processing image data :QImage, QPixmap and QPicture.

• QImage Is for I/O And direct pixel access and operation
• QPixmap It is designed and optimized for displaying images on the screen .
• QPicture Class is a record and playback QPainter Command's drawing device .

2.QImage

QImage Class provides a hardware independent image representation , It allows direct access to pixel data , And can be used as a drawing device ; Suitable for loading large pictures ; You can draw in a thread , Can be optimized UI response time . The effect of drawing pictures has nothing to do with the platform ,QImage Drawing on is fast , But showing it is slower .

Drawing pictures ：

QImage image;
image.load("./test.png");

QPainter painter(this);
painter.drawImage(0,0,image);

Draw a big picture ： It can be used QImage Load the picture first , Change size , Turn into QPixmap.

QImage image;
image.load("./big.png");

QPixmap pixmap = QPixmap::fromImage(image.scaled(size(),Qt::KeepAspectRatio));

QPainter painter(this);
painter.drawPixmap(0,0,pixmap);;

2.QPixmap

QPixmap Drawing depends on hardware ,QPixmap Is designed to Speed up the display , use paint For drawing QPixmap It will be much better than other classes . Generally, small pictures are used QPixmap load , Platform related （ The effect may be different on different platforms ）.QPixmap Drawing on is slow , But showing it is faster .

QLabel Loading pictures ：

ui->lbImg->setPixmap(QPixmap("test.jpg"));

QPixmap And QImage Interconversion

QImage image = pixmap.toImage();
QPixmap pixmap = QPixmap::fromImage(image);

3.QPicture

Record and playback QPainter Drawing instructions for

• begin()  Start in QPicture Draw on  
• end()  It ends in QPicture Draw on  
• save()  Save as .pic file  
• load()  load .pic file

Example ： A circle , Save the picture

 QPicture picture;
 QPainter painter;
 painter.begin(&picture);           // paint in picture
 painter.drawEllipse(10,20, 80,70); // draw an ellipse
 painter.end();                     // painting done
 picture.save("drawing.pic");       // save picture

Example ： Loading pictures

QPicture picture;
QPainter painter;
picture.load("drawing.pic");
painter.begin(this);
painter.drawPicture(0,0,picture);
painter.end();