QPainter DrawImage CenterAligned

Question

有没有办法在QPainter上画一张居中对齐的图片？我看到QPainter::drawText给了我们这个条款，但是drawImage没有。我有一个源rect，目标rect和一个图像。当源大小较小时，图像将绘制在页面的左侧。我想把它打印出来居中对齐。

Answer 1

画笔没有尺寸，但它所画的device()有尺寸。您可以使用QRect(painter.device()->width(), painter.device()->height())作为要在其中居中显示图像的矩形。

然后将图像居中绘制，如下所示：

QImage source;
QPainter painter(...);
...
QRect rect(source.rect());
QRect devRect(0, 0, painter.device()->width(), painter.device()->height());
rect.moveCenter(devRect.center());
painter.drawImage(rect.topLeft(), source);

Answer 2

我将尝试执行以下操作(请遵循源代码注释)：

应绘制的示例图像

// The image to draw - blue rectangle 100x100.
QImage img(100, 100, QImage::Format_ARGB32);
img.fill(Qt::blue);

在paint事件处理程序中

[..]
QRect source(0, 0, 100, 100);
QRect target(0, 0, 400, 400);

// Calculate the point, where the image should be displayed.
// The center of source rect. should be in the center of target rect.
int deltaX = target.width() - source.width();
int deltaY = target.height() - source.height();

// Just apply coordinates transformation to draw where we need.
painter.translate(deltaX / 2, deltaY / 2);

painter.drawImage(source, img);

当然，在应用此方法之前，您应该检查源矩形是否小于目标矩形。为了简单起见，我省略了这些代码，只是为了演示如何将图像居中。

Answer 3

我想要展示一个更完整的示例，它具有可变的图像大小，并保持在所提供的区域范围内，以添加到其他很好的答案中。

void ImageView::paintEvent(QPaintEvent*)
{
  if (this->imageBuffer.empty()){ return; }

  double widgetWidth = this->width();
  double widgetHeight = this->height();
  QRectF target(0, 0, widgetWidth, widgetHeight);

  QImage tempQImage = *this->imageBuffer.at(this->imageBuffer.count()-1);
  tempQImage = tempQImage.scaled(rect().size(), Qt::KeepAspectRatio, Qt::SmoothTransformation);

  double imageSizeWidth = static_cast<double>(tempQImage.width());
  double imageSizeHeight = static_cast<double>(tempQImage.height());
  QRectF source(0.0, 0.0, imageSizeWidth, imageSizeHeight);

  int deltaX = 0;
  int deltaY = 0;
  if(source.width() < target.width())
      deltaX = target.width() - source.width();
  else
      deltaX = source.width() - target.width();

  if(source.height() < target.height())
      deltaY = target.height() - source.height();
  else
      deltaY = source.height() - target.height();

  QPainter painter(this);
  painter.translate(deltaX / 2, deltaY / 2);

  painter.drawImage(source, tempQImage);
}