Solved QPainter line defects when panning and zooming in QGraphicsView

[johnratius]

I am drawing a grid using QPainter and pen strokes.

Here is the loop for the X-axis, the same thing is used for the Y-axis.

for (int xi = int(left); xi <= int(right); ++xi)
	{
		if (xi % 8 == 0)
			painter->setPen(QPen(QColor::fromRgb(0, 0, 0, 100), 2, Qt::DashLine));
		else if (xi % 4 == 0)
			painter->setPen(QPen(QColor::fromRgb(0, 0, 0, 100), 1, Qt::DashLine));
		else
			painter->setPen(QPen(QColor::fromRgb(0, 0, 0, 100), 1, Qt::DotLine));

		painter->drawLine(xi * step, bottom * step,
								xi * step, top * step);
	}

My problem is when I pan around the scene, I begin to see small defects in the grid. Specifically, from the Qt::DotLine pen (It does happen with lines that use Qt::DashLine as well). Some of the dots look closer together, or disappear completely, or look spread out. I thought this was a problem with my program, until I resized the window, or selected another open window on my system. The defects reverted back to normal and it looked great after. I am assuming it's a problem with QPainter or QGraphicsView.

My question is why is this happening, and if this is just a problem with QPainter? If it is a problem with QPainter and not my program, is there a workaround to keep the defects from appearing? I thought about calling a resize function of the window or something similar whenever the mouse moves or gets pressed, but I fear it will affect my performance greatly. Please comment if you have any suggestions I should try.

(I can't show a picture because when I select the Window Snipping tool, my grid reverts back to normal)

[gde23]

It could be a problem with aliasing.
Have you set the RennderHint to do anti aliasing
(see here: https://doc.qt.io/archives/qt-4.8/qpainter.html#RenderHint-enum)?

Edit: You mention that resizing it solves the problem. So have you tried to manually update the widget after panning around? It might be a timing issu there.

You can try add following at the end of your pan function:

QTimer::singleShot(0, your_graphics_widget, SLOT(repaint()));

[gde23]

It could be a problem with aliasing.
Have you set the RennderHint to do anti aliasing
(see here: https://doc.qt.io/archives/qt-4.8/qpainter.html#RenderHint-enum)?

Edit: You mention that resizing it solves the problem. So have you tried to manually update the widget after panning around? It might be a timing issu there.

You can try add following at the end of your pan function:

QTimer::singleShot(0, your_graphics_widget, SLOT(repaint()));

[johnratius]

@gde23 Good idea, I tried adding the last line but it didn't do anything. I still got some issues when zooming in and out and panning. When I resized the window, it cleared up again. I used the QTimer as such: QTimer::singleShot(0, this, SLOT(repaint())); and I also put it in the zoom function. I'll try the aliasing.

Edit: Turns out, it was the aliasing. I set
painter->setRenderHints(QPainter::Antialiasing); at the top of my drawBackground() function and it seems to work well now.