带有图层的 Qt5 涂鸦 - QPaintDevice：无法破坏正在绘制的设备

Question

I'm trying to re-implement the scribble demo app for multiple layers of images. I am struggling to draw into the pixmap within the scene as the Painter complains that is being destroyed to early.

QPaintDevice: Cannot destroy paint device that is being painted

Could you please help me fix my code such that you can draw on to the roilayer pixmap with the red pen and that this layer starts transparent.

#!/usr/bin/python3

import sys
import os
from PyQt5.QtWidgets import QApplication, QGraphicsView, QGraphicsScene, QGraphicsPixmapItem
from PyQt5.QtGui import QPixmap, QImage, QPainter, QPen
from PyQt5.QtCore import Qt, QRect


class Main(QGraphicsView):

    def __init__(self):
        super().__init__()
        self.setWindowTitle("Scribble with layers")
        self.scene = QGraphicsScene()
        self.setScene(self.scene)
        self.image = QImage('sample/test.bmp')
        self.imlayer = QGraphicsPixmapItem(QPixmap.fromImage(self.image))
        self.roilayer = QGraphicsPixmapItem(QPixmap(self.image.size()))
        self.addlayer(self.imlayer)
        self.addlayer(self.roilayer)
        self.drawing = False
        self.lastPoint = None
        self.pencolour = Qt.red
        self.penwidth = 2
        self.show()

    def addlayer(self, layer):
        self.scene.addItem(layer)
        self.updateviewer()

    def updateviewer(self):
        self.fitInView(self.sceneRect(), Qt.KeepAspectRatio)

    def mousePressEvent(self, event):
        # print(event.modifiers())
        if event.button() == Qt.LeftButton:
            self.lastPoint = event.pos()
            self.drawing = True

    def mouseMoveEvent(self, event):
        if (event.buttons() & Qt.LeftButton) and self.drawing:
            self.drawlineto(event.pos())

    def mouseReleaseEvent(self, event):
        if event.button() == Qt.LeftButton and self.drawing:
            self.drawlineto(event.pos())
            self.drawing = False

    def drawlineto(self, position):
        pixmap = self.roilayer.pixmap()
        painter = QPainter(pixmap)
        painter.setPen(QPen(self.pencolour, self.penwidth, Qt.SolidLine,
                Qt.RoundCap, Qt.RoundJoin))
        painter.drawLine(self.lastPoint, position)
        self.imlayer.setPixmap(pixmap)
        self.modified = True

        rad = self.penwidth / 2 + 2
        self.update(QRect(self.lastPoint, position).normalized().adjusted(-rad, -rad, +rad, +rad))
        self.lastPoint = position


if __name__ == '__main__':
    app = QApplication([])
    main = Main()
    sys.exit(app.exec_())

Answer 1

I had the same problem. The solution is to do the following to this part of your code:

    def drawlineto(self, position):
        pixmap = self.roilayer.pixmap()
        painter = QPainter()
        painter.begin(pixmap)
        painter.setPen(QPen(self.pencolour, self.penwidth, Qt.SolidLine,
            Qt.RoundCap, Qt.RoundJoin))
        painter.drawLine(self.lastPoint, position)
        painter.end()
        self.imlayer.setPixmap(pixmap)
        self.modified = True