Sending OpenCV images continuously from C++ to Qml

SGaist

Hi,

Out of curiosity, do you do any processing with OpenCV ?
If not, what about using QtMultimedia and the Camera type ?

sierdzio

Hm indeed, looks like the image provider API does not contain any useful API to update the view continuously. You could hack around it with the solution you linked (https://forum.qt.io/topic/38978/solved-send-qimage-from-c-to-qml-via-qquickimageprovider-or-using-a-signal/4), which requests a new image periodically using a timer. But probably a better solution is to prepare a video surface like @GrecKo suggests, or to reimplement QQuickItem. Sorry for leading you down a suboptimal path!

Yunus

@sgaist Yeah, I m doing lots of task in Opencv and I need to send opencv images to qml continuously. I used this example. But I can only send images triggering with qml buttons. I want to send the images continuously like video.

Yunus

@sierdzio Thanks for ur help. I will try the example using timer I hope it works :(

J.Hilk

@yunus

You can also take a look at one of my projects:
https://github.com/DeiVadder/Sindepile-Challenge-QML

I used that one to experiment with the QImageProvoder class and I update the ui each time a new image is available, not based on a timer.

If you don't really care about performance, you could also simply force an update each time possible, by reacting to the frameSwapped signal

Yunus

@j-hilk @sierdzio Thank you so much. I am very glad to both. I think I fixed my problem. I solved using Imageprovider and QTimer using example here. And also @J-Hilk project is working well. I ll use one of these. Thank you Qt Forum, I am very happy to be able to fix this problem

GrecKo

QQuickImageProvider is not the correct solution to display videos. Implementing a source for a VideoOutput is the sanest thing to do.
If we are advising convoluted ways to display a video, I'd like to propose writing a QAbstractTableModel containing each pixel value in its cells, and display that with a TableView and 1x1 Rectangle delegates.

Yunus

@grecko I also want the right way but I am new on Qt. Do you have an example to advise me about your method

J.Hilk

@grecko awesome idea, may I suggest an add-on .

Will need some work on the model, but I suggest only updating every other row, for a 90's era interlacing effect!
😉

GrecKo

@Yunus Here's a post where I explain it textually : https://forum.qt.io/post/474269

Here is some sample code I wrote some time ago : https://gist.github.com/oKcerG/a5b59c7583c85ff03bdec238eed8bcd8

The import part is the videoSurface setter which starts the videoSurface, and then I periodically present a frame on it.

Make sure to chose a correct QVideoSurfaceFormat, and don't do like I did for generating a QVideoFrame ( QPixmap -> QImage -> QImage in the correct format -> QVideoFrame), that's way too much conversions, but I did that only as a quick proof of concept.

There must be a more direct way to convert cv::Mat to QVideoFrame, there might even be a copyless solution but that's not my domain of expertise.

Yunus

Thank you I ll try

Muhammet Ali Asan

As far as I understand you want to read video from camera, process frames using OpenCV then display output in QML. This solution you used is not good. It uses timer with a second interval and each timeout event will send image to QML.
2 Disadvantages :

You are hardcoding your processing speed to 1 fps. Even camera provides more frames you are reading only 1 frame. To catch everything your timer ticks should be more frequent than your video reader.
In a low power system or with a camera with low fps there will be synchronization problems.

You should use QAbstractVideoFilter . There is a good example in the page. Basically it allows you to process image while it is being sent to VideoOutput by Camera in Qml. And it will be definitely more easier and efficient than your solution.