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I'm using pyqt to try to create an application which display images and curves.

To draw bezier curves, I found the QPainterPath class and specifically QpainterPath.cubicTo. However, I don't understand how this class must be used. In which widget should I draw my curves?

I see there are Qpainter and QGraphicsView/QGraphicsScene, but I don't know how to use my QPainterPath with them.

Do you have any example of the use of QPainterPath with pyqt/pyside? (for example, a simple window which displays a cubic bezier curve)

Nejat
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arthropode
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2 Answers2

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QPainter is a pretty low-level class. For simple applications you can ignore it. Add a QGraphicsView widget and do something like this:

# Prepare the QGraphicsView widget
scene = QtGui.QGraphicsScene(graphicsView)
graphicsView.setScene(scene)
graphicsView.setRenderHint(QtGui.QPainter.Antialiasing)

# Draw a line
path = QtGui.QPainterPath()
path.moveTo(0, 0)
path.cubicTo(100, -20, 40, 90, 20, 20)
scene.addPath(path)
pdw
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    I think that a line is missing at the beginning : `graphicsView = QtGui.QGraphicsView()`. With this, it works. – arthropode Aug 11 '14 at 12:55
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You can draw on a custom widget. The drawing is done within the paintEvent() method :

import sys
from PyQt4 import QtGui, QtCore

class Example(QtGui.QWidget):

def __init__(self):
    super(Example, self).__init__()

    self.initUI()

def initUI(self):      

    self.setGeometry(300, 300, 280, 170)
    self.setWindowTitle('Draw Bezier')
    self.show()

def paintEvent(self, event):


    startPoint = QtCore.QPointF(0, 0)
    controlPoint1 = QtCore.QPointF(100, 50)
    controlPoint2 = QtCore.QPointF(200, 100)
    endPoint = QtCore.QPointF(300, 300)

    cubicPath = QtGui.QPainterPath(startPoint)
    cubicPath.cubicTo(controlPoint1, controlPoint2, endPoint)


    painter = QtGui.QPainter(self)
    painter.begin(self)
    painter.drawPath(cubicPath);
    painter.end()

This adds a cubic Bezier curve between startPoint and endPoint using the control points specified by controlPoint1, and controlPoint2.

Nejat
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  • What is the difference between Qpainter and QGraphicsScene used by pdw ? – arthropode Aug 11 '14 at 12:59
  • The `QPainter` class performs low-level painting on widgets and other paint devices. You can use it for painting in `QWidget` or in a `QGraphicsItem`. `QGraphicsItem` could be added on scene and displayed on a view. The `addPath` in the mentioned solution creates and adds a path item to the scene. – Nejat Aug 11 '14 at 13:24