Hello guys I'm drawing 2 lines on top of each other, 1 long and 1 a bit shorter. I draw them on a QGraphicsItem. When I zoom out in my scene, I can see the ovelapping as if the longer line has some opacity, allowing me to see the shorter line below it. But if I zoom back, it looks all good. The color of my line is (180, 180, 180, 255) so no opacity. My line has a width of 1 pixel. My first guess is since 1px at no zoom is 1px, when I zoom out, my 1px becomes less than 1px (problem) but when I zoom in, it becomes more than 1px (so all good). Is there a solution to fix that ? Thx ! :)

Not sure what optimisation flags you are setting on the graphics view but one possibility might be... QGraphicsView::DontAdjustForAntialiasing or render hints set on the graphics view like... QPainter::HighQualityAntialiasing and I guess you read the docs here and here . Remember, all drawing uses the painter in the end unless you are using only OpenGL for your drawing. I hope this helps you find your way to a solution :-)

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painter.drawline() , overlapping opacity problems with 100% opaque pen.

Unsolved General and Desktop

35 Posts 2 Posters 9.6k Views

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kenchan
wrote on last edited by

#18

Great, i will give it a try when I can make a bit of time :-)
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kenchan
wrote on last edited by

#19

BTW, I don't use python so I will be testing it with C++. Someone else might pitch in and try it with python for you.
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kenchan
wrote on last edited by

#20

So which Version of Qt are you using for this? I assume you are using Windows right?
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Goffer
wrote on last edited by Goffer

#21

I'm using Qt5 I think it's 5.6.1 and yes I'm on windows, will give it a try on MacOS
I've tried in Qt 4.8.2 and it's exactly the same
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kenchan
wrote on last edited by kenchan

#22

Hello @Goffer. I made a test app with a QGraphicsView using the same settings as you here are the results so far.
Note: i did not bother to sub class a graphics item yet as I don't think it makes a difference.
I don't see those artifacts at 1:1 zoom scale.
zoom scale 1.0

I do see those artifacts at less than a 1:1 zoom scale. I guess this is because the line thickness of on device pixel is now trying to show line a less than one physical device pixel width?? I think this is what the paint engine does when the device to logical pixels don't match, but I am no expert on that. You need to discuss those details with the developers.
zoom scale 0.5

I used Qt 5.8.0
This is on macOS High Sierra 10.13
I will try it on Windows later and let you know the results.

Hope that helps :-)
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kenchan
wrote on last edited by kenchan

#23

Sorry but my images links are not showing up?? you will have to click on the links to see them I guess :-)
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Goffer
wrote on last edited by

#24

I've tried to make the pen cosmetic by setting the width to 0. That way the line should always have the same width on screen no matter the zoom factor. And the problem still occurs.
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kenchan
wrote on last edited by

#25

hello again @Goffer
I tried my test project on Windows and got the same results. It looks like we are stuck with that behaviour.
You could always submit it as a bug.
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Goffer
wrote on last edited by

#26

I ve sent it to the interest mailing list as you suggested. Will see what happens, how could I submit it as a bug ?
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kenchan
wrote on last edited by

#27

You can find out how to do that and much more here
also the wiki is a good place to look around for information.
The developers mailing list is here.

Good luck :-)
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Goffer
wrote on last edited by Goffer

#28

I've submited the bug ticket.
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kenchan
wrote on last edited by

#29

what is the number of the bug report?
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Goffer
wrote on last edited by

#30

https://bugreports.qt.io/browse/QTBUG-63820
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kenchan
wrote on last edited by

#31

OK, got the link and voted for it.
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Goffer
wrote on last edited by

#32

lets see what happens ;)
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Goffer
replied to Goffer on last edited by

#33

@kenchan can you post your C++ example on the bug report ?
thx
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kenchan
wrote on last edited by

#34

OK. I added my comments and zip file to the bug report.
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Goffer
wrote on last edited by

#35

thx ! hoppefully this will help, what do you think of the current "solution" ?
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