QGraphicsView

//当前项矩阵
void transform();
//转换至视口坐标矩阵
void viewportTransform();

//根据视口坐标提取项
void itemAt();
void items();
//视口坐标 -> 场景坐标
void mapToScene();
//场景坐标 -> 视口坐标
void mapFromScene();

QGraphicsScene

//根据场景坐标提取项
void itemAt();
void items();

QGraphicsItem

//当前项矩阵
void transform();
//转换至场景矩阵
void sceneTransform();
//转换至其它项矩阵
void itemTransform();

//外部视口坐标 -> 内部视口坐标
void mapFromItem();
//父类视口坐标 -> 内部视口坐标
void mapFromParent();
//场景坐标 -> 视口坐标
void mapFromScene();

//内部视口坐标 -> 外部视口坐标
void mapToItem();
//内部视口坐标 -> 父类视口坐标
void mapToParent();
//视口坐标 -> 场景坐标
void mapToScene();

测试

#include "mainwindow.h"
#include <QApplication>
#include <QtWidgets>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QGraphicsView view;
    QGraphicsScene* scene=new QGraphicsScene();
    view.setScene(scene);

    scene->addLine(-200,0,200,0);
    scene->addLine(0,-200,0,200);

    QGraphicsRectItem* rect1=new QGraphicsRectItem();
    rect1->setPos(-25,-25);
    rect1->setRect(QRect(0,0,100,100));
    scene->addItem(rect1);

    QGraphicsRectItem* rect2=new QGraphicsRectItem();
    rect2->setPos(25,25);
    rect2->setRect(QRect(0,0,100,100));
    scene->addItem(rect2);

    qDebug()<<rect1->mapFromItem(rect2,QPointF(0,0));
    qDebug()<<rect1->parentItem()<<rect1->mapFromParent(QPointF(0,0));
    qDebug()<<rect1->mapFromScene(QPointF(0,0));

    qDebug()<<rect1->mapToItem(rect2,QPointF());
    qDebug()<<rect1->parentItem()<<rect1->mapToParent(QPointF(0,0));
    qDebug()<<rect1->mapToScene(QPointF(0,0));

    view.show();

    return a.exec();
}

运行结果

QPointF(50,50)
QGraphicsItem(0) QPointF(25,25)
QPointF(25,25)
QPointF(-50,-50)
QGraphicsItem(0) QPointF(-25,-25)
QPointF(-25,-25)

QGraphicsView坐标系统