PyQt4 video player crashes when moving window

Question

I've written a simple PyQt4 GUI that plays an OpenCV VideoCapture. This requires converting frames from numpy arrays to QImages. I'm using OpenCV so that I can detect circles using my findCircles method.

However, when I pass my frames to findCircles, the program crashes when the window is moved. This problem does not occur when I don't search for circles. I don't understand why this is happening, as I'm under the impression that the work is being done on a different thread than the GUI since I call findCircles from the run method of a QThread.

Note that I don't receive a normal error message in the console; Python crashes like such:

Here is the video file I've been using to test my player. I'm running Python 2.7.6 on Windows 8.1.

import sys
import cv2.cv as cv, cv2
from PyQt4.Qt import *
import time

def numpyArrayToQImage(array):
    if array != None:
        height, width, bytesPerComponent = array.shape
        bytesPerLine = bytesPerComponent * width;
        cv2.cvtColor(array, cv.CV_BGR2RGB, array)
        return QImage(array.data, width, height, bytesPerLine, QImage.Format_RGB888)
    return None

def findCircles(frame):
    grayFrame = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
    blurredFrame = cv2.medianBlur(grayFrame, 3)
    circles = cv2.HoughCircles(blurredFrame, cv.CV_HOUGH_GRADIENT, 1, 30, param1=50, param2=30, minRadius=30, maxRadius=35)

    if circles is not None: 
        for i in circles[0]:
            cv2.circle(frame, (i[0], i[1]), i[2], (255, 0, 0), 1) # Perimeter
            cv2.circle(frame, (i[0], i[1]), 3, (0, 255, 0), -1) # Center

class VideoThread(QThread):
    frameProcessed = pyqtSignal(QImage)

    def __init__(self, video, videoLabel):
        QThread.__init__(self)
        self.video = video
        self.fps = self.video.get(cv.CV_CAP_PROP_FPS)
        self.frameCount = self.video.get(cv.CV_CAP_PROP_FRAME_COUNT)
        self.startingSecond = 0
        self.videoLabel = videoLabel

    def run(self):
        clockAtStart = time.clock()

        while True:
            runtime = self.startingSecond + (time.clock() - clockAtStart)
            currentFrame = int(runtime * self.fps)

            if currentFrame < self.frameCount - 1:
                self.video.set(cv.CV_CAP_PROP_POS_FRAMES, currentFrame)
                frame = self.video.read()[1]
                findCircles(frame) # Removing this line removes the issue
                self.frameProcessed.emit(numpyArrayToQImage(frame))
                time.sleep(.02)
            else:
                break

class MainWindow(QMainWindow):
    def __init__(self):
        super(MainWindow, self).__init__()
        self.initUI()

    @pyqtSlot(QImage)
    def updateVideoLabel (self, image):
        self.videoLabel.setPixmap(QPixmap.fromImage(image))
        self.videoLabel.update()

    def initUI(self):
        self.setGeometry(300, 300, 500, 375)
        self.setMinimumHeight(250)
        self.createWidgets()
        self.addWidgets()

    def startNewVideo(self):
        self.video = cv2.VideoCapture(unicode(QFileDialog.getOpenFileName(self, "Open video").toUtf8(), encoding="UTF-8"))
        self.videoThread = VideoThread(self.video, self.videoLabel)
        self.videoThread.frameProcessed.connect(self.updateVideoLabel)
        self.playVideoFrom(0)

    def playVideoFrom(self, frame):
        self.videoThread.startingSecond = frame / self.videoThread.fps
        self.videoThread.start()

    def createWidgets(self):
        self.populateMenuBar()
        self.videoLabel = QLabel()
        self.videoLabel.setStyleSheet('background-color : black;');

    def populateMenuBar(self):
        self.menuBar = self.menuBar()
        fileMenu = QMenu('File', self)
        openAction = QAction('Open video...', self)
        openAction.triggered.connect(self.startNewVideo)
        fileMenu.addAction(openAction)
        self.menuBar.addMenu(fileMenu)

    def addWidgets(self):
        mainLayout = QVBoxLayout()
        mainLayout.addWidget(self.videoLabel, 1)
        centralWidget = QWidget()
        self.setCentralWidget(centralWidget)
        centralWidget.setLayout(mainLayout)

if __name__ == '__main__':
    app = QApplication(sys.argv)
    player = MainWindow()
    player.show()
    sys.exit(app.exec_())

Answer 1

I've tested your program, and it crashes when it finds no circles as indicated in the error message:

Traceback (most recent call last):
  File "test_opencv_tkinter.py", line 53, in run
    findCircles(frame) # Removing this line removes the issue
  File "test_opencv_tkinter.py", line 26, in findCircles
    if len(circles) > 0:
TypeError: object of type 'NoneType' has no len()

I've made some changes in the findCircles(frame) function, as follows, and it runs without error, even when I move the window around on the screen.

def findCircles(frame):
    grayFrame = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
    blurredFrame = cv2.medianBlur(grayFrame, 3)
    circles = cv2.HoughCircles(grayFrame,cv.CV_HOUGH_GRADIENT,1,20,
                            param1=50,param2=30,minRadius=0,maxRadius=0)
    if circles == None: 
        print "no circles found"
        return
    if len(circles) > 0:
        print "found circles ", len(circles[0])
        for i in circles[0]:
            cv2.circle(frame, (i[0], i[1]), i[2], (255, 0, 0), 1) # Perimeter
            cv2.circle(frame, (i[0], i[1]), 3, (0, 255, 0), -1) # Center