QtMultimedia app freezes when switching between cameras on a captureSession

[raulgd]

Hi,

As I was doing exercises from the QML Book - Image capture chapter 11, I wanted to capture a video stream from a camera selected from a combobox, and then switch to another camera in the combobox, then change the camera in the captureSession so it now renders the selected camera video stream output.

Problem is, as I was following the example, I found a few inconsistencies like getting a list of cameras into the combobox (which I worked around), but the problem is that as I switch cameras in the captureSession, the application completely freezes, this happens mostly when more than one camera is attached at my computer and of course more than one item in the combobox, so I fear something might be missing.

Here is a proof of concept of the issue, please anyone feel free to check it out and tell me if I'm missing something or doing something wrong when switching the captureSession camera so my app doesn't freeze. I'm using Qt 6.5 LTS from the commercial distribution on Windows 11.

PoC Link: https://github.com/raul-guerrero/WebcamSwitchPoC

Here is my Main.qml just so you can review here as well:

import QtQuick
import QtQuick.Window
import QtQuick.Controls
import QtMultimedia

Window {
    width: 800
    height: 600
    visible: true
    title: qsTr("Hello World")

    property Camera previousCamera: null

    MediaDevices {
        id: mediaDevices
    }

    CaptureSession {
        id: captureSession
        videoOutput: videoOutput
        camera: Camera {
            cameraDevice: mediaDevices.defaultVideoInput
            active: true
        }
        onCameraChanged: {
            console.log("triggered onCameraChanged event")
            captureSession.camera.start()
        }
    }

    Item {
        x: 20
        y: 20
        width: 640
        height: 480

        VideoOutput {
            id: videoOutput
            anchors.fill: parent
        }
    }

    ComboBox {
        id: combobox_select_camera
        x: 670
        y: 20
        width: 110
        height: 32
        model: ListModel {
            Component.onCompleted: function() {
                previousCamera = captureSession.camera
                for (var i = 0; i < mediaDevices.videoInputs.length; i++) {
                    this.append({ description: mediaDevices.videoInputs[i].description })
                }
            }
        }
        textRole: "description"
        displayText: captureSession.camera.cameraDevice.description
        onActivated: function (index) {
            if (previousCamera != null
                    && previousCamera.cameraDevice.description !== mediaDevices.videoInputs[index].description) {
                if (previousCamera.active) {
                    previousCamera.stop()
                }
                captureSession.camera.cameraDevice = mediaDevices.videoInputs[index]
                previousCamera = captureSession.camera
            }
        }
    }
}

Thanks

[JoeCFD]

@raulgd did you try to stop the current camera first and then switch to another camera?

[raulgd]

@JoeCFD thanks for the reply, yes you can see in the onActivated code that I stop the camera

[raulgd]

Ok so I finally got it working.
The problem seems to be the integration between camera, video output and capture session, every time you want to change camera you have to recreate all objects in the UI for it to work again.
I would have expected that this would be handled internally by the aforementioned objects but it seems it's not the case.
I updated the repo with a working version where I dynamically destroy and recreate the dependent objects and it works now.

https://github.com/raul-guerrero/WebcamSwitchPoC