qt - 如何在QPixmap上使用QPainter

Question

我是Qt / Embedded的新手。我想使用QPainter在QPixmap上绘制内容，该内容将添加到QGraphicsScene中。这是我的代码。但是它没有在像素图上显示图形。它仅显示黑色像素图。

int main(int argc, char **argv) {

  QApplication a(argc, argv);

  QMainWindow *win1 = new QMainWindow();
  win1->resize(500,500);
  win1->show();


  QGraphicsScene *scene = new QGraphicsScene(win1);
  QGraphicsView view(scene, win1);
  view.show();
  view.resize(500,500);

  QPixmap *pix = new QPixmap(500,500);
  scene->addPixmap(*pix);

  QPainter *paint = new QPainter(pix);
  paint->setPen(*(new QColor(255,34,255,255)));
  paint->drawRect(15,15,100,100);

  return a.exec();
}

Answer 1

将位图添加到场景之前，需要在位图上进行绘制。当您将其添加到场景中时，场景将使用它来构造一个QGraphicsPixmapItem对象，该对象也由addPixmap()函数返回。如果要在添加像素图后对其进行更新，则需要调用返回的setPixmap()对象的QGraphicsPixmapItem函数。

所以：

...
QPixmap *pix = new QPixmap(500,500);
QPainter *paint = new QPainter(pix);
paint->setPen(*(new QColor(255,34,255,255)));
paint->drawRect(15,15,100,100);
scene->addPixmap(*pix); // Moved this line
...

要么：

...
QPixmap *pix = new QPixmap(500,500);
QGraphicsPixmapItem* item(scene->addPixmap(*pix)); // Save the returned item
QPainter *paint = new QPainter(pix);
paint->setPen(*(new QColor(255,34,255,255)));
paint->drawRect(15,15,100,100);
item->setPixmap(*pix); // Added this line
...