Pyqt crashes when trying to show opencv videostream

Question

I tried code from this answer and it crashes with error Process finished with exit code -1073740771 (0xC000041D) after some time (2-10 sec.) and sometimes with 0xC0000005. It crashes immediately if I try to drag the window. However when I put time.sleep(0.1) in run it works fine. If I use sleeps shorter than 0.1 it crashes again.

from PyQt5.QtWidgets import QApplication, QMainWindow, QWidget, QLabel,QMessageBox
from PyQt5.QtGui import QImage, QPixmap
from PyQt5.QtCore import QThread, pyqtSignal, pyqtSlot, Qt
import cv2
import sys
import time

class CamThread(QThread):
    changemap = pyqtSignal('QImage')

    def run(self):
        cap = cv2.VideoCapture(0)
        cap.set(cv2.CAP_PROP_FRAME_WIDTH, 640)
        cap.set(cv2.CAP_PROP_FRAME_HEIGHT, 480)

        while True:
            ret, img_rgb = cap.read()
            if ret:
                self.rgb = cv2.cvtColor(img_rgb, cv2.COLOR_BGR2RGB)
                self.convert = QImage(self.rgb.data, self.rgb.shape[1], self.rgb.shape[0], QImage.Format_RGB888)
                self.p = self.convert.scaled(640, 480, Qt.KeepAspectRatio)
                self.changemap.emit(self.p)
                #time.sleep(0.1)


class App(QWidget):
    def __init__(self):
        super().__init__()
        self.title = 'webcam'

        self.initUI()

    @pyqtSlot('QImage')
    def setImage(self, image):
        self.label.setPixmap(QPixmap.fromImage(image))

    def initUI(self):
        self.setWindowTitle(self.title)
        self.setGeometry(100, 100, 640, 480)
        self.resize(640, 480)
        self.label = QLabel(self)
        self.label.resize(640, 480)
        thr = CamThread(self)
        thr.changemap.connect(self.setImage)
        thr.start()

app = QApplication(sys.argv)
win = App()
#win.setAttribute(Qt.WA_DeleteOnClose, True)
win.show()
app.exit(app.exec_())

I thought that the problem is somewhere in signals/slots but haven't been able to find anything relevant.

Windows 10
Python - 3.7
Pyqt - 5.12
OpenCV - 3.4.5.20

change `self.rgb` to `rgb`, `self.convert` to `convert` and `self.p` to `p` — eyllanesc, Mar 22 '19 at 19:44
execute it in the CMD, there you should surely give you more information — eyllanesc, Mar 22 '19 at 19:49
Still crushes. Shows nothing in CMD not even previous error. — Mur4al, Mar 22 '19 at 19:58
mmm, why do you want an image beyond 100ms ?, a camera provides a 30FPS image that is 30 ms, what is the FPS of your camera? — eyllanesc, Mar 22 '19 at 19:59
I don't actually. Just want to know what the problem is in case i meet it later. It takes ~ 0.01 sec. for a frame if I only use OpenCV even with blur and some other processing. And 10 fps camera is not the best what one could want from CV application, I suppose. — Mur4al, Mar 22 '19 at 20:06

Mur4al · Accepted Answer · 2019-03-23T19:21:17.083

Fixed it using QMutex and QWaitCondition to prevent update call while main thread is already updating. Apparently, issue was in that. eyllanesc, I'm new here as you see, should I make an answer in original thread?

from PyQt5.QtWidgets import QApplication, QMainWindow, QWidget, QLabel, QMessageBox
from PyQt5.QtGui import QImage, QPixmap
from PyQt5.QtCore import QThread, pyqtSignal, pyqtSlot, Qt, QMutex, QWaitCondition
import cv2
import sys
import time


class CamThread(QThread):
    changemap = pyqtSignal('QImage')

    def __init__(self, mutex, condition):
        super().__init__()
        self.mutex = mutex
        self.condition = condition

    def run(self):
        cap = cv2.VideoCapture(0)
        cap.set(cv2.CAP_PROP_FRAME_WIDTH, 640)
        cap.set(cv2.CAP_PROP_FRAME_HEIGHT, 480)
        while True:
            try:
                ret, img_rgb = cap.read()
                if ret:
                    rgb = cv2.cvtColor(img_rgb, cv2.COLOR_BGR2RGB)

                    #any other image processing here

                    convert = QImage(rgb.data, rgb.shape[1], rgb.shape[0], QImage.Format_RGB888)
                    p = convert.scaled(640, 480, Qt.KeepAspectRatio)
                    self.changemap.emit(p)
                    self.condition.wait(self.mutex)

            except:
                print('error')


class App(QWidget):
    time = 0

    def __init__(self):
        super().__init__()
        self.title = 'webcam'
        self.mutex = QMutex()
        self.condition = QWaitCondition()
        self.initUI()

    @pyqtSlot('QImage')
    def setImage(self, image):
        self.mutex.lock()
        try:
            self.label.setPixmap(QPixmap.fromImage(image))
        finally:
            self.mutex.unlock()
            self.condition.wakeAll()

    def initUI(self):
        self.mutex.lock()
        self.setWindowTitle(self.title)
        self.setGeometry(100, 100, 640, 480)
        self.resize(640, 480)
        self.label = QLabel(self)
        self.label.resize(640, 480)
        self.thr = CamThread(mutex = self.mutex,condition=self.condition)
        self.thr.changemap.connect(self.setImage)
        self.thr.start()


app = QApplication(sys.argv)
win = App()
win.show()
app.exit(app.exec_())

N.B. You still need to properly stop thread and close camera connection in this example.

Pyqt crashes when trying to show opencv videostream

1 Answers1