What steps are necessary to enable antialiasing when using a QPainter on a QGLWidget?

Question

I am trying to draw basic shapes on a QGLWidget. I am trying to enable antialiasing to smooth out the lines, but it is not working.

This is what I am trying at the moment:

QGLWidget *widget = ui->renderWidget;

QPainter painter;

widget->makeCurrent();
glEnable(GL_MULTISAMPLE);
glEnable(GL_LINE_SMOOTH);

painter.setRenderHint(QPainter::Antialiasing);
painter.setRenderHint(QPainter::HighQualityAntialiasing);

painter.begin(widget);

However, anything drawn with this painter still has jagged edges. What else do I need to do?

Yes, it's windows 7, and I've gotten it to work on another Qt project that used QGLWidget without a QPainter, by just enabling GL_MULTISAMPLE. — Elliott, Jun 10 '12 at 20:19
The fact that it's Windows 7 is irrelevant, it's whether or not your GPU and drivers support them - but if another project _on the same machine_ worked correctly then it must be OK. Your example seems to be hinting that you're trying to draw on the widget outside of a `paintEvent(QPaintEvent* event)`, or is it just confusing pseudo-code!? — cmannett85, Jun 11 '12 at 06:41

Elliott · Accepted Answer · 2012-12-26T20:13:54.380

I found the solution. When debugging a different issue, I found messages in my debug output to the effect that you can't set renderhints before the call to begin().

The following works:

QGLWidget *widget = ui->renderWidget;

QPainter painter;

widget->makeCurrent();
glEnable(GL_MULTISAMPLE);
glEnable(GL_LINE_SMOOTH);

painter.begin(widget);

painter.setRenderHint(QPainter::Antialiasing);
painter.setRenderHint(QPainter::HighQualityAntialiasing);

score 10 · Answer 2 · answered Jun 11 '12 at 13:22

10

You can try to enable the antialiasing on the complete Widget :

QGLWidget::setFormat(QGLFormat(QGL::SampleBuffers));

answered Jun 11 '12 at 13:22

Marc Plano-Lesay

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2

Works perfectly. Note, you still need `setRenderHint(QPainter::Antialiasing)`. – Timmmm May 17 '17 at 09:08

score 6 · Answer 3 · answered May 22 '17 at 10:49

This question is quite old but I still found it on Google. You shouldn't use QGLWidget any more. Use the newer QOpenGLWidget. This renders the scene off-screen rather than creating a native OpenGL window which causes all sorts of issues with resizing layouts. This code works for me. Put it in your QGraphicsView constructor:

QOpenGLWidget* gl = new QOpenGLWidget;
QSurfaceFormat fmt;
fmt.setSamples(8);
gl->setFormat(fmt);
setViewport(gl);
setRenderHint(QPainter::Antialiasing);

score 1 · Answer 4 · answered Sep 23 '19 at 16:50

If you work on PyQt5, you will typically subclass QOpenGLWidget() to build your own GPU-powered widget. To turn on anti-aliasing, have a look at the code snippet below:

from PyQt5.QtWidgets import *
from PyQt5.QtGui import *
from PyQt5.QtCore import *

class MyFigureClass(QOpenGLWidget):
    def __init__(self, parent):
        super().__init__(parent)
        fmt = QSurfaceFormat()    # -╷
        fmt.setSamples(8)         #  > anti-aliasing
        self.setFormat(fmt)       # -╵
        [...]

    def paintEvent(self, event):
        qp = QPainter()
        qp.begin(self)
        qp.setRenderHint(QPainter.Antialiasing)
        [...]
        qp.end()

_{Note: Thank you @Timmmm for your answer. I found the PyQt5 solution when looking at your C++ code snippets.}

What steps are necessary to enable antialiasing when using a QPainter on a QGLWidget?

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