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

[Goffer]

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 ! :)

[kenchan]

Is it due to antialiasing effects perhaps?

[Goffer]

from the view ?

[kenchan]

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 :-)

[Goffer]

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

self.setOptimizationFlag(QtWidgets.QGraphicsView.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)
self.setViewportUpdateMode(QtWidgets.QGraphicsView.FullViewportUpdate)

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.

[kenchan]

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

[Goffer]

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

[kenchan]

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.

[Goffer]

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

[kenchan]

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?

[Goffer]

this one is 400%

[kenchan]

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).

[Goffer]

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

[kenchan]

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.

[Goffer]

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)
self.setViewportUpdateMode(QtWidgets.QGraphicsView.FullViewportUpdate)

even with those settings, that's what I get

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

[kenchan]

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.

[Goffer]

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.setOptimizationFlag(QtWidgets.QGraphicsView.DontAdjustForAntialiasing)
        self.setRenderHint(QtGui.QPainter.Antialiasing, False)
        self.setRenderHint(QtGui.QPainter.HighQualityAntialiasing, False)
        self.setRenderHint(QtGui.QPainter.NonCosmeticDefaultPen, False)
        self.setViewportUpdateMode(QtWidgets.QGraphicsView.FullViewportUpdate)
        self.setTransformationAnchor(QtWidgets.QGraphicsView.AnchorUnderMouse)
        self.setHorizontalScrollBarPolicy(QtCore.Qt.ScrollBarAlwaysOff)
        self.setVerticalScrollBarPolicy(QtCore.Qt.ScrollBarAlwaysOff)

    def wheelEvent(self, event):
        self.setTransformationAnchor(QtWidgets.QGraphicsView.AnchorUnderMouse)

        inFactor = 1.15
        outFactor = 1 / inFactor

        if event.delta() > 0:
            zoomFactor = inFactor
        else:
            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()
        path.addRect(self.boundingRect())
        return path

    def paint(self, painter, option, widget):
        brush = QtGui.QBrush(QtGui.QColor(70, 70, 70, 255))
        painter.setBrush(brush)

        painter.drawRoundedRect(0, 0, 200, 400, 20, 20)

        pen = QtGui.QPen(QtGui.QColor(200, 200, 200, 255))
        pen.setWidth(1)
        painter.setPen(pen)

        painter.drawLine(50, 50, 50, 300)
        painter.drawLine(50, 80, 50, 200)
        painter.drawLine(50, 80, 150, 80)


view = TestView()
scene = QtWidgets.QGraphicsScene()
view.setScene(scene)

item = TestItem()
scene.addItem(item)
    
view.show()

[kenchan]

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

[kenchan]

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.

[kenchan]

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