Capture OpenCV video and present it on QVideoWidget

borisruna

Hello everyone ! I am trying to capture the output of a webcam and present it to a QVideoWidget using the following :

import sys
import cv2
from PyQt5.QtMultimedia import QVideoFrame, QVideoSurfaceFormat
from PyQt5.QtMultimediaWidgets import QVideoWidget
from PyQt5.QtWidgets import QWidget, QApplication, QVBoxLayout
from PyQt5.QtCore import pyqtSignal, pyqtSlot, Qt, QThread, QSize
from PyQt5.QtGui import QImage


class Thread(QThread):
    changePixmap = pyqtSignal(QImage)

    def run(self):
        cap = cv2.VideoCapture(0)
        while True:
            ret, frame = cap.read()
            if ret:
                # https://stackoverflow.com/a/55468544/6622587
                rgbImage = cv2.cvtColor(frame, cv2.COLOR_BGR2RGB)
                h, w, ch = rgbImage.shape
                bytesPerLine = ch * w
                convertToQtFormat = QImage(rgbImage.data, w, h, bytesPerLine, QImage.Format_RGB888)
                p = convertToQtFormat.scaled(600, 600, Qt.KeepAspectRatio)
                self.changePixmap.emit(p)


class VideoPlayer(QWidget):

    def __init__(self, parent=None):
        super(VideoPlayer, self).__init__(parent)
        self.videoWidget = QVideoWidget()
        self.video_surface = self.videoWidget.videoSurface()
        video_surface_format = QVideoSurfaceFormat(QSize(600, 600),
                                                   QVideoFrame.pixelFormatFromImageFormat(QImage.Format_RGB888))
        self.video_surface.start(video_surface_format)
        layout = QVBoxLayout()
        layout.addWidget(self.videoWidget)
        self.setLayout(layout)

        th = Thread(self)
        th.changePixmap.connect(self.update_image)
        th.start()

    @pyqtSlot(QImage)
    def update_image(self, image):
        self.video_surface.present(QVideoFrame(image))


if __name__ == '__main__':
    app = QApplication(sys.argv)
    player = VideoPlayer()
    player.setWindowTitle("Player")
    player.resize(600, 600)
    player.show()
    sys.exit(app.exec_())

However I get a black frame and immediately a grey (like there is no output). I get no warnings or errors on the console.
An insight on this issue would be highly appreciated !

Thank you

SGaist

Hi,

th is local to your __init__. It will be destroyed when out of scope.

CristianMaureira

In PySide we have a similar example (without the capture part) https://doc.qt.io/qtforpython/examples/example_external__opencv.html and there we do use self.th = Thread(self) so I second @SGaist comment.

borisruna

Thank you very much !