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

  • 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 ! :)

  • Is it due to antialiasing effects perhaps?

  • from the view ?

  • Not sure what optimisation flags you are setting on the graphics view but one possibility might be...

    or render hints set on the graphics view like...

    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 :-)

  • So I ve tried to turn off all the anitialiasing setting that I have + added the "DontAdjustForAntialiasing"

    self.setRenderHint(QtGui.QPainter.Antialiasing, False)
    self.setRenderHint(QtGui.QPainter.TextAntialiasing, True)
    self.setRenderHint(QtGui.QPainter.HighQualityAntialiasing, False)
    self.setRenderHint(QtGui.QPainter.SmoothPixmapTransform, True)
    self.setRenderHint(QtGui.QPainter.NonCosmeticDefaultPen, True)

    I ve also tried

    self.setRenderHint(QtGui.QPainter.NonCosmeticDefaultPen, False)

    But I still have the same issue ... looks like it might not be aliasing related.

  • hmm... It might be helpful to see a screen shot of the results you are seeing.

  • 0_1508028436519_bug2.jpg

    As you can see we see it on the small one but not on the zoomed in one.

  • Yes, i can see what you mean about the smaller scale image. Just as a matter of interest, what does the smaller one look like when you zoom the pixels of your screen to the extent that you can see each pixel clearly?
    As opposed to zooming it with the graphics view.

  • What do you mean ? When it's at scale 1:1 ?
    The problem occurs as soon as on my screen it becomes less than 1 px

  • Sorry, I guessed you might not get what I meant.
    What I meant was it would be nice to see an enlarged screen shot of your 1:1 display of it. i.e. can you apply a magnifying glass to the small image on your screen so one can see what the actual pixels look like?

  • 0_1508029843148_Untitled-1.jpg

    this one is 400%

  • Thank you.
    Well, I can see evidence of antialiasing on the line ends and the circles even though you have turned the antialiasing features off. I expect to see them on the text because you left the text antialiasing settings on. Horizontal lines look darker (more transparent?) and corners look lighter because they are drawn twice. Am I seeing this correctly?

    So, either your antialias settings are getting changed at the last moment or your display device has a hardware antialiasing capability or it is actually getting that way due to windows scaling??

    Other than that I am out of ideas. I use the graphics view in a similar fashion with the same settings as you and do not see that kind of thing. However, I do see exactly that kind of thing when I turn antialiasing on (the user can switch it on and off in my application).

  • My bad, this screenshot has the antialiasing on. Will do it again tomorrow with antialiasing off.

    They can turn it too in mine but i feel like it shouldn't happen even with antialiasing on

  • OK.
    If you have issues with how the antialiasing is implemented I suggest you send your questions to the interest mailing list here and try to catch the eye of a developer who knows about the details.

  • self.setOptimizationFlag(QtWidgets.QGraphicsView.DontAdjustForAntialiasing)
    self.setRenderHint(QtGui.QPainter.Antialiasing, False)
    self.setRenderHint(QtGui.QPainter.TextAntialiasing, False)
    self.setRenderHint(QtGui.QPainter.HighQualityAntialiasing, False)
    self.setRenderHint(QtGui.QPainter.SmoothPixmapTransform, True)
    self.setRenderHint(QtGui.QPainter.NonCosmeticDefaultPen, False)

    even with those settings, that's what I get


    This is not antialiasing related. Somehow lines get some opacity.

  • Well that is strange isn't it.
    Maybe you could put the code that draws that into a minimal example app so we can all try to reproduce it.

  • Okay, I wrote a quick example with the same view settings.
    It looks fine at scale 1 and same problem when zooming out.

    from PySide2 import QtGui, QtCore, QtWidgets
    class TestView(QtWidgets.QGraphicsView):
        def __init__(self, parent=None):
            super(TestView, self).__init__(parent)
            self.setRenderHint(QtGui.QPainter.Antialiasing, False)
            self.setRenderHint(QtGui.QPainter.HighQualityAntialiasing, False)
            self.setRenderHint(QtGui.QPainter.NonCosmeticDefaultPen, False)
        def wheelEvent(self, event):
            inFactor = 1.15
            outFactor = 1 / inFactor
            if > 0:
                zoomFactor = inFactor
                zoomFactor = outFactor
            self.scale(zoomFactor, zoomFactor)
    class TestItem(QtWidgets.QGraphicsItem):
        def __init__(self):
            super(TestItem, self).__init__()
        def boundingRect(self):
            return QtCore.QRect(0, 0, 200, 400)
        def shape(self):
            path = QtGui.QPainterPath()
            return path
        def paint(self, painter, option, widget):
            brush = QtGui.QBrush(QtGui.QColor(70, 70, 70, 255))
            painter.drawRoundedRect(0, 0, 200, 400, 20, 20)
            pen = QtGui.QPen(QtGui.QColor(200, 200, 200, 255))
            painter.drawLine(50, 50, 50, 300)
            painter.drawLine(50, 80, 50, 200)
            painter.drawLine(50, 80, 150, 80)
    view = TestView()
    scene = QtWidgets.QGraphicsScene()
    item = TestItem()

  • Great, i will give it a try when I can make a bit of time :-)

  • 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.

  • So which Version of Qt are you using for this? I assume you are using Windows right?

  • 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

  • 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 :-)

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

  • 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.

  • 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.

  • I ve sent it to the interest mailing list as you suggested. Will see what happens, how could I submit it as a bug ?

  • 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 :-)

  • I've submited the bug ticket.

  • what is the number of the bug report?

  • OK, got the link and voted for it.

  • lets see what happens ;)

  • @kenchan can you post your C++ example on the bug report ?

  • OK. I added my comments and zip file to the bug report.

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

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