Thread / OpenCV Segmentation Fault

[LS-KS]

Hello folks,
I'm trying to visualize some image processing using cv2.
In general the following code works as intended and I would have started to implement my processing Code. But unfortunately sometimes I get segmentation faults if I repeatedly stop/start the VideoThread.

I use qml to Image Item with QuickImageProvider to visualize the video feed.

Interestingly I get console output from receiving signals in my qml file when the VideoThreads quit() method is already called.

Can anyone help? Google couldn't

import sys
import cv2
from PySide6.QtGui import QImage
from PySide6.QtCore import Signal, Slot, Qt, QThread
from PySide6.QtQuick import QQuickImageProvider
from PySide6.QtQml import QQmlImageProviderBase


class VideoThread(QThread):
    frameChanged = Signal(QImage)

    def __init__(self, parent=None):
        QThread.__init__(self, parent)
        self.capture = cv2.VideoCapture(0)
        self.capture.set(cv2.CAP_PROP_FPS, 30)
        self.running = True
        self.detecting = False
        # initialize haar cascade face detection
        self.faceCascade = cv2.CascadeClassifier(cv2.data.haarcascades + 'haarcascade_frontalface_default.xml')
        print("VideoThread initialization finished")

    def run(self):

        while self.running:
            print("getframe...")
            ret, frame = self.capture.read()
            if ret:
                if self.detecting:
                    # Convert the frame to grayscale
                    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
                    # Detect faces in the frame
                    faces = self.faceCascade.detectMultiScale(gray, scaleFactor=1.1, minNeighbors=5)
                    # Draw rectangles around the faces
                    for (x, y, w, h) in faces:
                        cv2.rectangle(frame, (x, y), (x + w, y + h), (255, 0, 0), 2)
                rgbImage = cv2.cvtColor(frame, cv2.COLOR_BGR2RGB)
                h, w, _ = rgbImage.shape
                qImage = QImage(rgbImage.data, w, h, QImage.Format_RGB888)
                self.frameChanged.emit(qImage)
                print("...emitted frame")

    def quit(self):
        print("try closing")
        self.running = False
        self.capture.release()
        super().quit()
        super().wait()
        self.deleteLater()
        print("closed")

    def start(self):
        print("VideoThread: start")
        self.running = True
        super().start()
        print("VideoThread: started")

    def detect(self):
        self.detecting = not self.detecting


class VideoPlayer(QQuickImageProvider):
    imageChanged = Signal(QImage)

    def __init__(self):
        super().__init__(QQmlImageProviderBase.Image, QQmlImageProviderBase.ForceAsynchronousImageLoading)
        self.videoThread = None
        self.image = None

    def requestImage(self, id, size, requestedSize):
        if self.image:
            img = self.image
        else:
            img = QImage(1280, 720, QImage.Format_RGBA8888)
            img.fill(Qt.black)
        return img

    @Slot(QImage)
    def updateImage(self, frame):
        print("new image in updateImage")
        self.image = frame
        self.imageChanged.emit(frame)

    @Slot()
    def start(self):
        print("Starting Video feed...")
        if not self.videoThread:
            self.videoThread = VideoThread()
            self.videoThread.frameChanged.connect(self.updateImage)
        self.videoThread.running = True
        self.videoThread.start()

    @Slot()
    def stop(self):
        print("Finishing Video feed.")
        if self.videoThread:
            self.videoThread.quit()
            # self.videoThread.deleteLater()
            self.videoThread = None
            print("Finished Video feed.")

    @Slot()
    def toggleDetection(self):
        self.videoThread.detect()

[Christian Ehrlicher]

@LS-KS said in Thread / OpenCV Segmentation Fault:

qImage = QImage(rgbImage.data, w, h, QImage.Format_RGB888)

Default problem when using this QImage ctor and not reading the docs I would guess: "The buffer must remain valid throughout the life of the QImage and all copies that have not been modified or otherwise detached from the original buffer"

Make a deep copy of the image.

[SGaist]

Hi,

Your thread stopping looks wrong. You change running to false and immediately nuke your capture while in fact, the run function might still be doing something. The deallocation should rather happen as cleanup at the end of the run method.

[LS-KS]

@Christian-Ehrlicher ,
Thank you for your replys.
I moved the super().wait() just one line up. And it now never crushes.
But somehow this way it feels wrong?!?

I think i will have time to read the docs today.

@SGaist : what do you mean by 'clean up at the end of the run function'?

[JonB]

@LS-KS said in Thread / OpenCV Segmentation Fault:

I moved the super().wait() just one line up. And it now never crushes.

@SGaist : what do you mean by 'clean up at the end of the run function'?

I have not analyzed all your code, but I think from your def quit(self) you should move at least the lines

self.capture.release()
self.deleteLater()

out and put them as the last lines in def run(self) after the while self.running loop.

[LS-KS]

@JonB

Thank you for your reply.

self.deleteLater()

terminates the whole thread and must stay in the quit() method.

But moving

self.capture.release()

into run() method was the solution!