wxOpenCVCameras is a crude untested example of how to retrieve and display images from multiple cameras, using OpenCV to grab images from a camera and wxWidgets to display the images.
Every camera has its own worker thread CameraThread, which grabs a frame
from a camera with cv::VideoCapture, converts the frame from cv::Mat
to wxBitmap, and creates a resized thumbnail also converted to wxBitmap.
Additionally, benchmarking data (times for grabbing image, converting
it to wxBitmap, creating a thumbnail...) are collected.
The full resolution bitmap, the thumbnail bitmap, and the benchmarking data
are stored in CameraFrameData class. As wxBitmap is implemented as
copy-on-write in a probably thread-unsafe manner, the wxBitmaps are stored
as raw pointers and CameraFrameDatas in std::unique_ptr.
CameraFrameData is then added by a worker thread to a container
shared between the GUI thread and camera threads. The container is std::vector
(see CameraGridFrame::m_newCameraFrameData and CameraThread::CameraSetupData::frames) protected
by wxCriticalSection (see CameraGridFrame::m_newCameraFrameDataCS
and CameraThread::CameraSetupData::framesCS). The GUI thread (a wxFrame-derived CameraGridFrame)
then uses a fixed-frequency wxTimer to update the camera display with images stored
in the container.
The GUI has a crude control of the camera (thread) by using wxMessageQueue to pass
the commands (such as setting the thread sleep time or getting/setting one of
cv::VideoCaptureProperties) from the GUI to the camera thread.
A camera can be added either as an integer (e.g., 0 for a default webcam) or as an URL.
There is a couple of preset camera URLs offered via a menu, but these are not guaranteed
to stay online and are mostly time-limited. When a camera is added, settings from menu
"Defaults for New Cameras" are used.
Output from all cameras is displayed in a single frame as thumbnails (CameraPanels
in a wxWrapSizer). Left doubleclicking a thumbnail opens a new frame (OneCameraFrame)
showing the camera output in the full resolution. Right clicking a thumbnail shows
a popup menu allowing crude communication with the camera (thread).
In the debug build, various diagnostic messages are output with wxLogTrace(TRACE_WXOPENCVCAMERAS, ...).
wxOpenCVCameras uses internet streams as camera sources. If an application connects to multiple
hardware cameras with the same parameters, an entirely different approach should probably be used.
For example, a single worker thread for capturing from all the cameras, using cv::VideoCapture::waitAny()
with cv::VideoCapture::retrieve() and one or more worker threads for processing the captured
images. Such approach would be not only less thread-hungry, the frames from multiple cameras
should be better synchronized as well.
To convert cv::Mat to wxBitmap, the code uses ConvertMatBitmapTowxBitmap() from the
wxOpenCVTest project, so all the information
provided there applies here is as well.
Removing a camera (i.e., stopping a thread) may sometimes take a while so that the program
appears to be stuck. However, this happens when the worker thread is stuck in an OpenCV call
(e.g., opening/closing cv::VideoCapture or grabbing the image) that may sometimes take a while,
where the thread cannot test whether to stop.
Requires OpenCV v4.2+, wxWidgets v3.1+, and a compiler supporting C++11 standard.