C++ 如何在 Qt 中绘制半透明矩形？
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How do I draw a semi-transparent rectangle in Qt?
提问by Managu
I'm trying to draw a semi-transparent rectangle on top of an image to act as a highlight. Unfortunately, nothing I try seems to be able to perform the transparency effect I want. Instead I just get solid filled rectangles, with no transparency.
我正在尝试在图像顶部绘制一个半透明矩形作为高光。不幸的是，我尝试的任何东西似乎都无法实现我想要的透明效果。相反，我只是得到实心填充的矩形，没有透明度。
Here's what I'm doing right now:
这是我现在正在做的事情：
void PageView::paintEvent(QPaintEvent *event)
{
QPainter painter(this);
QImage img=...;
painter.drawImage(0, 0, img);
...
// draw a light blue, transparent rectangle to highlight
QRect rect=...;
painter.fillRect(rect, QColor(128, 128, 255, 128));
...
}
Unfortunately, for me, this draws a solidblue rectangle, instead of the semi-transparent one I expect due to giving the QBrushan alpha value.
不幸的是，对我来说，这绘制了一个纯蓝色矩形，而不是我期望的半透明矩形，因为它给出了QBrush一个 alpha 值。
I've also tried drawing to an intermediate QImageor QPixMap, playing around with painter.setCompositionMode(...). No luck so far.
我还尝试绘制到中间QImage或QPixMap，玩弄painter.setCompositionMode(...). 到目前为止没有运气。
Thus my question: How can I convince Qt to draw a semi-transparent rectangle to my PageView?
因此我的问题是：如何说服 Qt 为我的PageView.
EDIT: If it's relevant, I'm building this under Qt 4.8.1 on Windows.
编辑：如果相关，我将在 Windows 上的 Qt 4.8.1 下构建它。
回答by Arnold Spence
The code works for me with a slight modification as it does not compile as you have it:
代码对我有用，稍加修改，因为它不会像你那样编译：
painter.fillRect(rect, QBrush(QColor(128, 128, 255, 128)));
NOTE:
笔记：
The OP was painting the semi transparent rectangle in a loop causing the same area to be painted multiple times. This will result in an additive effect which will eventually cause that area to look the same as a solid fill.
OP 在循环中绘制半透明矩形，导致多次绘制同一区域。这将导致附加效果，最终导致该区域看起来与实体填充相同。
