I am new to C++,Qt and Visual Studio and this is my first post on Stack Overflow.
I apologize in advance if this is a repeated question, I tried searching for a similar question but couldn't find one. Let me know if this is a repeated question and I will delete it.
I am trying to create a line plot using QWidget::paintEvent(). The line plot I am drawing is actually a QPainterPath. I want to detect when the mouse hovers over my line plot and so I create a small rectangle where my mouse cursor is and detect when this rectangle intersects with my line plot using bool QPainterPath::intersects() function. The problem is that this function returns true even when my mouse is not exactly over my line plot. In the Image 1 (I am not allowed to embed images yet) my line plot is the thick black curve and the bool QPainterPath::intersects() returns true even when my cursor is over the yellow region. As per the Qt document this is because:
There is an intersection if any of the lines making up the rectangle crosses a part of the path or if any part of the rectangle overlaps with any area enclosed by the path.
There is no way to have a QPainterPath without any enclosed area as Qt only provides two types of fill for QPainterPath: Qt::OddEvenFill or Qt::WindingFill. (To be honest, I find this kind of annoying, since an open path is a series of line segments connected end-to-end, if someone wants to enclose an area they can easily connect the first and last point using either QPainterPath::lineTo() or QPainterPath::moveTo() functions)
Anyway, I decided to get smarter than Qt and drew two extra QPainterPath with pathUp being a few pixels above my line plot and pathDn being a few pixels below my line plot. Image 2 shows these 3 line plots, red one is pathUp, black one is real line plot and green one is pathDn. I thought I coould detect the intersection in the QWidget::mouseMoveEvent() by using the following code:
// cRect: Rectangle at mouse cursor position
if((pathUp.intersects(cRect) && (!pathDn.intersects(cRect))) || ((!pathUp.intersects(cRect)) && pathDn.intersects(cRect)))
{
qDebug() << "Intersects";
}
But this still produces wrong results because now the enclosed area is different, as you can see in Image 3 the green area is an enclosed area of pathDn and red area is the enclosed area of pathUp. The thick black curve is again the line plot that I want to detect my mouse hover on. This enclosed area is not affected by Qt::setFillRule of QPainterPath.
What's even more frustrating is that I tried this technique using QPolygonF instead of QPainterPath on QWidget and the results were exactly the same. I also tried QGraphicsView, there I used QGraphicsPathItem to create my line plot and then used QGraphicsScene::focusItemChanged() signal to detect when I click on my line plot. It again produced the same result of detecting the click when my cursor is over the enclosed area. I do not want to create a custom QGraphicsItem (unless I absolutely have to) just to reimplement it's hoverEnterEvent() and hoverLeaveEvent() method because of the limitations imposed on the boundingRect() of the QGraphicsItem as explained in Qt Docs:
QGraphicsScene expects all items boundingRect() and shape() to remain unchanged unless it is notified. If you want to change an item's geometry in any way, you must first call prepareGeometryChange() to allow QGraphicsScene to update its bookkeeping.
Since I making a plot in real-time the boundingRect() will change quite frequently (> 20 Hz), which will result in an extra computational burden on the software. Is there any way I can solve my problem without creating a custom QGraphicsItem?
P.S. I have been using Stack Overflow for many years whenever I got stuck. I just never made an account here because I never needed to post anything. You guys are the best and I am very happy to be a part of this community!