Re: [Interest] Performance of QGraphics[View Scene] changes with zoom

[Lisandro Damián Nicanor Pérez Meyer]

On Friday 20 May 2016 07:14:15 Uwe Rathmann wrote:
> On Wed, 18 May 2016 15:57:04 -0300, Lisandro Damián Nicanor Pérez Meyer
> 
> wrote:
> > Now if I add a lot of straight lines (QGraphicsLineItem instances)
> > performance (number of movements seen on screen per second) varies
> > according the zoom. If I zoom in I get to a point in which everything is
> > too slow.
> 
> Qt does the polygon clipping very late, so a huge amount of pointless
> caclulations is going on before it detects that it is not needed. IIRC
> the Graphics View framework isn't any better than QPainter - but I havn't
> checked the code quite some time.
> 
> If this is still the case you need to implement your own polygon clipping
> before entering Qt rendering code.
> 
> At least this is what I do it in the Qwt project ( qwt.sf.net ). If you
> need an implementation for polygon clipping you find something there too:
> see https://sourceforge.net/p/qwt/code/HEAD/tree/trunk/qwt/src/
> qwt_clipper.cpp

That sound svery plausible indeed! For now I switched to staright lines and 
"works for the moment", but if I need to speed up things or go back to other 
kind of lines I'll surely give it a look.

Thanks a lot!!

-- 
If you have an apple and I have an apple and we exchange these apples then you
and I will still each have one apple. But if you have an idea and I have an
idea and we exchange these ideas, then each of us will have two ideas.
 George Bernard Shaw

Lisandro Damián Nicanor Pérez Meyer
http://perezmeyer.com.ar/
http://perezmeyer.blogspot.com/


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Re: [Interest] Performance of QGraphics[View Scene] changes with zoom

[Uwe Rathmann]

On Wed, 18 May 2016 15:57:04 -0300, Lisandro Damián Nicanor Pérez Meyer
wrote:

> Now if I add a lot of straight lines (QGraphicsLineItem instances)
> performance (number of movements seen on screen per second) varies
> according the zoom. If I zoom in I get to a point in which everything is
> too slow.

Qt does the polygon clipping very late, so a huge amount of pointless 
caclulations is going on before it detects that it is not needed. IIRC 
the Graphics View framework isn't any better than QPainter - but I havn't 
checked the code quite some time.

If this is still the case you need to implement your own polygon clipping 
before entering Qt rendering code.

At least this is what I do it in the Qwt project ( qwt.sf.net ). If you 
need an implementation for polygon clipping you find something there too:
see https://sourceforge.net/p/qwt/code/HEAD/tree/trunk/qwt/src/
qwt_clipper.cpp

HTH,
Uwe

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Re: [Interest] Performance of QGraphics[View Scene] changes with zoom

[Lisandro Damián Nicanor Pérez Meyer]

On 18 May 2016 at 15:57, Lisandro Damián Nicanor Pérez
 wrote:
> Before I try to code a stripped-down version of my issue I would like to
> describe it in case I'm missing something obvious.
>
> I have an app with a QGraphicsScene that holds a vehicle which is constantly
> moving at a fixed rate (~10 times per second). Every time it moves it draws
> some polygons behind (the path it went trough). Up to that point, everything
> is ok.
>
> Now if I add a lot of straight lines (QGraphicsLineItem instances) performance
> (number of movements seen on screen per second) varies according the zoom. If
> I zoom in I get to a point in which everything is too slow. If I zoom out
> things go back to normal. Note that once the lines are generated they are not
> changed in any way.
>
> I understand that performance might decrease if those lines are near 45º due
> to bounding boxes, but I fail to understand why it varies with zoom.

I minimized my code to simple lines and drawing the bounding box of
the vehicle, and found
out that the problems starts if I use anything different than
Qt::SolidLine for drawing the lines.

As soon as I zoom out enough (the transformation matrix becoming
0.1*Identity) the problem
"goes away", so I suspect a rasterization issue.


-- 
Lisandro Damián Nicanor Pérez Meyer
http://perezmeyer.com.ar/
http://perezmeyer.blogspot.com/
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[Interest] Performance of QGraphics[View Scene] changes with zoom

[Lisandro Damián Nicanor Pérez Meyer]

Before I try to code a stripped-down version of my issue I would like to 
describe it in case I'm missing something obvious.

I have an app with a QGraphicsScene that holds a vehicle which is constantly 
moving at a fixed rate (~10 times per second). Every time it moves it draws 
some polygons behind (the path it went trough). Up to that point, everything 
is ok.

Now if I add a lot of straight lines (QGraphicsLineItem instances) performance 
(number of movements seen on screen per second) varies according the zoom. If 
I zoom in I get to a point in which everything is too slow. If I zoom out 
things go back to normal. Note that once the lines are generated they are not 
changed in any way.

I understand that performance might decrease if those lines are near 45º due 
to bounding boxes, but I fail to understand why it varies with zoom.

I'm using Qt 5.5 on Linux.

Thanks in advance, Lisandro.

-- 
Lisandro Damián Nicanor Pérez Meyer
http://perezmeyer.com.ar/
http://perezmeyer.blogspot.com/


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