cvQueryFrame continually returns null

[UsaidPro]

I've been trying to use Darknet to process images taken through my laptop's webcam using the demo code. The problem I'm having is that the camera opens (through cvCaptureFromCAM) but it doesn't return frames (through cvQueryFrame). I know that the camera works (can access it using other applications).

System: Windows 10, GTX 1050, Opencv 3, MS Visual Studio 2015

I've also noticed that Opencv seems to have dropped support for their C API. Any plans to move to the C++ API?

EDIT: I've confirmed that the camera works fine using the C++ API (same environment).

[AlexeyAB]

@UsaidPro Hi,

• Did you try to use your Web-cam with your own OpenCV 3.x + C++ applications?
• Do you use USB 3.0 port for Web-cam? I recommend using the port USB 3.0.

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You can try to use Darknet Yolo as DDL/SO-library and C++ application that uses this library:

1. At first change this line: https://github.com/AlexeyAB/darknet/blob/22bf10984cb7940e84db4f086ecbc25d9d5d64b5/src/yolo_console_dll.cpp#L147 to this: cv::VideoCapture cap(0); cap >> cur_frame; // where 0 - is number of web-cam

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2. Compile and run DDL/SO-library and C++ aplication that uses this library:

• Linux: set LIBSO=1 in the Makefile to build a library darknet.so and binary runable file uselib that uses this library. Then you can try to run so LD_LIBRARY_PATH=./:$LD_LIBRARY_PATH ./uselib data/voc.names cfg/yolo-voc.cfg yolo-voc.weights 0

• Windows: at first compile build\darknet\yolo_cpp_dll.sln and then compile build\darknet\yolo_console_dll.sln then you can use yolo_console_dll.exe data/voc.names cfg/yolo-voc.cfg yolo-voc.weights 0

[AlexeyAB]

Also if you want to decrease delay of your web-cam -

you can try to change these lines: https://github.com/AlexeyAB/darknet/blob/22bf10984cb7940e84db4f086ecbc25d9d5d64b5/src/yolo_console_dll.cpp#L241-L244

to these:

    std::unique_lock<std::mutex> lock(mtx);
    while (!consumed) cv_detected.wait(lock);

[AlexeyAB]

I fixed it. Just update your code from this repo. It was C API bug in OpenCV 3.x.