QPainterPath from list of points

Joel Bodenmann

I have a class named SchematicWire that inherits from QGraphiscItem.The class contains a list of points: QList<QPoint> points. Inside the SchematicWire::paint() function I use QPainter::drawPolyline() to draw the line in the scene. SchematicWire::boundingRect() has been implemented to return a rectangle that contains all points of the line. Everything works fine so far.
Now, I would like to implement SchematicWire::shape(). I could return a QPainterPath that just is a line consisting of the points stored in the list. However, I would like to add some padding. Therefore, I must create a QPainterPath that's a loop around the actual line with a given padding.
Can anyone tell me how I would create that QPainterPath the easiest way?

Here's a picture of a SchematicWireand the boundingRect() around it (there are a few pixels of padding): http://paste.ugfx.org/sores/7505d64a54e0/da1b18bc89fc.jpg

mrjj

I assume that
void QPainterPath::addPolygon(const QPolygonF & polygon)
is no use as u want to have it offset for each line segment?

Joel Bodenmann

Thanks for the quick reply.
Well, the problem is that I'd have to build the polygon first. The real question is exactly how to make that polygon.

For some clarification, this is what I want: http://paste.ugfx.org/sores/7505d64a54e0/264c3e10630e.jpg
The green line is the QPainterPath that should be returned by SchematicWire::shape().

mrjj

@Joel-Bodenmann
Ahh you mean the actual outline math
Well its a well defined problem
http://www.cescg.org/CESCG99/SKrivograd/

Not sure there is an easy(er) way.
Since you must offset each line segments to each side of it. for that u need at least
some angle.

mrjj

oh
have u tested with void
QPainter::strokePath(const QPainterPath & path, const QPen & pen)
I mean , if u build Path with points and then stroke it?

Joel Bodenmann

Thanks for linking the article - that might be handy in case of I really have to implement this manually.
QPainter::strokePath won't work because I really need the resulting polygon to be returned by my shape() function.

The reason I want to do this is because QGraphcisItem::shape() is used to determine when a hover event occurs over an item.
I want to show up little 'grab handles' (small squares) on the edges of my line in order to allow the user to move around the line/wire segments. Therefore, I need to know whether the mouse is currently hovering above the item.
Using QGraphicsItem::boundingRect() won't work because you'd get hover events even when you are not above the line (because the rectangle contains a lot of free space).

Edit: The shape in form of the polygon is also needed in order to know when the line was selected in order to eg. delete it.

mrjj

Ok. i see.
So it is not enough to actually test for mouse click on line
http://stackoverflow.com/questions/26849632/see-if-a-point-lies-on-a-linevector

I have no good suggestions for easy way. so let's wait and see
if someone has a nice trick before resorting calculating offsets for each line segments :)

Joel Bodenmann

Thanks for your help, very appreciated!

I hope that somebody has a better, easier way to get this done :/

mrjj

@Joel-Bodenmann
well if no neat tricks comes, here is easy to understand
implementation of click a line which would be easy to adapt to
trigger when near a line as it uses vectors.
https://msdn.microsoft.com/en-us/library/ms969920.aspx

Joel Bodenmann

@mrjj I don't think that that's going to help because the mouse-click-detection should be handled by the QGraphicsView/QGraphicsScene and for that they use QGraphcisItem::shape().

mrjj

@Joel-Bodenmann
ok, so you must have an actual polygon to return
for hit-testing/handles to work.

kshegunov

@Joel-Bodenmann
~~Let me first clarify something, you want to get an envelope around your path, that is larger (respectful to the lines) by some fixed pixel offset? Or you're talking about enveloping a polygon?~~

Right, I read your post again and understand what you mean now. I believe you should implement that yourself, at least I don't know any easy way. I assume your points are ordered, so it's a simple linear algebra problem. You take the points by pairs, and from that pair you can easily calculate the alongside and normal vector for each line. From those two vectors (after normalization) you can build up your envelope. I hope that helps.

Kind regards.

Joel Bodenmann

@kshegunov You are correct, I need an envelope around the path that has a few pixels of padding on each side of the line. Like the green line in this crude drawing that I made earlier: http://paste.ugfx.org/sores/7505d64a54e0/264c3e10630e.jpg

kshegunov

@Joel-Bodenmann
Sorry for updating a previous post, I didn't realize you were writing at that same time. Look up my suggestion, and if there's something unclear I'll try to expand.

Kind regards.

Joel Bodenmann

@kshegunov
Thanks for your answer. I guess I'll have to figure out how to properly implement this in the next couple of days then. I'll definitely publish results here so people don't have to do that in the future themselves :)

kshegunov

@Joel-Bodenmann
Also QPainterPathStroker::createStroke coupled with QPainterPath::toFillPolygon might be the easiest way to get what you're after.

Joel Bodenmann

@kshegunov
Can you elaborate? I still need to get the outline of the stroked polygon somehow, no?

kshegunov

@Joel-Bodenmann
Hello,
Surely, note however that I've not tried this. Set the pen width with your desired offset. Create the stroke, with the createStroke method. The outline of the stroke is supposed to be (as per the documentation) the painter path you get from it. Convert the path to a polygon with the toFillPolygon method. If it doesn't work, you can always revert to implementing it yourself, but if it does, well, it looks simpler. :)

Kind regards.

Joel Bodenmann

@kshegunov
Thanks for the explanation. The trouble I am having is getting the QPainterPath (that needs to be passed to QPainterPathStroker::createStroke() from my list of lines. The only reasonable option I see is QPainterPath::addPolygon() but that I can't use as I don't have a polygon, I only have a polyline (like a non-closed polygon).

Any thoughts?

kshegunov

@Joel-Bodenmann
Hello,
One thing is to create the path from a polygon (no one says it has to be a closed one ;)). You could also perhaps get it by constructing the path from your points with QPainterPath::moveTo and QPainterPath::lineTo. The latter would be my choice if I were supposed to compose a painter path.

Kind regards.