Как работает QPainter?

Question

Где его надо объявлять и производить рисование ? У меня он работает через раз, если наследовать класс и там переопределить paintEvent, то все нормально.

Часто выходит такая ошибка:

QWidget::paintEngine: Should no longer be called
QPainter::begin: Paint device returned engine == 0, type: 1
QPainter::end: Painter not active, aborted

Пример кода где проявляется проблема:

from PySide2.QtWidgets import *
from PySide2.QtGui import *
from PySide2.QtMultimediaWidgets import QCameraViewfinder
from PySide2.QtMultimedia import QCamera, QCameraImageCapture, QCameraInfo


class form(QWidget):
    def __init__(self):
        super().__init__()
        self.initUI()

    def initUI(self):
        self.cameraViewfinder = QCameraViewfinder ()
        self.pushButton = QPushButton()
        self.pushButton.clicked.connect(self.func)
        self.layout=QHBoxLayout()
        self.layout.addWidget(self.cameraViewfinder)
        self.layout.addWidget(self.pushButton)
        self.setLayout(self.layout)

    def func (self):
        print()

    def paintEvent(self,event):
        qp = QPainter ()
        qp.begin (self.cameraViewfinder)
        qp.drawLine (5, 5, 10, 10)
        qp.end ()


if __name__ == "__main__":
    import sys
    app = QApplication(sys.argv)
    wform = form ()
    wform.show()
    sys.exit(app.exec_())

Тут если использовать унаследованный класс, камера на форме не отображается. Если использовать QCameraViewfinder, то отображается. В режиме реального времени на камере нужно рисовать.

from PySide2.QtWidgets import *
from PySide2.QtGui import *
from PySide2.QtMultimediaWidgets import QCameraViewfinder
from PySide2.QtCore import *
from PySide2.QtMultimedia import QCamera, QCameraImageCapture, QCameraInfo
#from PyQt5.Qt import *

class cameraViewfinder2(QCameraViewfinder):
    def __init__(self):
        super ().__init__ ()
        self.flag = False

    def paintEvent(self, QPaintEvent):
        painter = QPainter (self)
        pen = QPen ()
        pen.setColor (Qt.green)
        painter.setPen (pen)
        painter.drawLine (100, 100, 300, 100)
        if wform.flag:
            pen.setColor (Qt.red)
            painter.setPen (pen)
            # painter.drawLine(5, 20, 100, 20)3

            w, h = wform.size ().width (), wform.size ().height ()
            painter.drawLine (wform.cameraMoi.pos (),
                              wform.cameraMoi.pos () + QPoint (w, h) - wform.cameraMoi.pos () * 2)

            wform.flag = False  # попробуйте изменить размер окна


class form (QWidget):
    def __init__(self):
        super ().__init__ ()
        self.flag = False
        self.initUI ()

    def initUI(self):
        self.pushButton = QPushButton ()
        self.pushButton.clicked.connect (self.func)

        self.layout = QHBoxLayout ()
        self.setLayout (self.layout)
        self.cameraView = cameraViewfinder2 ()
        self.cameraInfo = QCameraInfo.defaultCamera ()
        self.camera = QCamera (self.cameraInfo)
        self.camera.setViewfinder (self.cameraView)

        self.imageCapture = QCameraImageCapture (self.camera)
        self.camera.setCaptureMode (QCamera.CaptureStillImage)

        self.layout.addWidget (self.pushButton)
        self.layout.addWidget (self.cameraView)

        if self.camera.status () != QCamera.UnavailableStatus:
            self.camera.start ()

    def func(self):
        self.flag = True
        self.cameraView.update ()


if __name__ == "__main__":
    import sys
    app = QApplication (sys.argv)
    wform = form ()
    wform.resize (400, 300)
    wform.show ()
    sys.exit (app.exec_ ())

Answer 1

попробуйте так:

'''
from PySide2.QtWidgets import *
from PySide2.QtGui import *
from PySide2.QtMultimediaWidgets import QCameraViewfinder
from PySide2.QtMultimedia import QCamera, QCameraImageCapture, QCameraInfo
'''
import sys
from PyQt5.Qt import *


class form(QWidget):
    def __init__(self):
        super().__init__()
        self.flag = False       # +
        
        self.initUI()
        
    def initUI(self):
        self.cameraViewfinder = QCameraViewfinder()
        self.pushButton = QPushButton()
        self.pushButton.clicked.connect(self.func)
        self.layout = QHBoxLayout()
        self.layout.addWidget(self.cameraViewfinder)
        self.layout.addWidget(self.pushButton)
        self.setLayout(self.layout)

    def func(self):
        self.flag = True
        self.update()
        
    def paintEvent(self, QPaintEvent):
        painter = QPainter(self)      
        pen = QPen()            
        pen.setColor(Qt.green)
        painter.setPen(pen)
        painter.drawLine(100, 100, 300, 100)   
        
        if self.flag:
            pen.setColor(Qt.red)
            painter.setPen(pen)
            #painter.drawLine(5, 20, 100, 20) 
            w, h = self.size().width(), self.size().height()
            painter.drawLine(self.cameraViewfinder.pos(), 
                             self.cameraViewfinder.pos()+QPoint(w, h)-self.cameraViewfinder.pos()*2)  
               
            self.flag = False                     # попробуйте изменить размер окна            
        
        
if __name__ == "__main__":
    import sys
    app = QApplication(sys.argv)
    wform = form ()
    wform.resize(400, 300)
    wform.show()
    sys.exit(app.exec_())

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Как работает QPainter?

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