[Gambas-user] Not a Gambas, but a C++ question: GTKMM, Video and... Threads!

[Benoît Minisini]

Le 22/11/2018 à 02:24, First Last a écrit :
> I want to apologize because this is for a C++ project, not a Gambas one.
> I'm really clueless at the moment, and I know there are people in this 
> list that knows an awful lot, so I'm trying to pick someone's brains.
> 
> Once upon a time, I got myself a coupla pan-tilt IPCameras. Nothing 
> fancy really.
> Zero Linux support, they offer Mac and Win drivers, apps, and maybe even 
> chocolate. But not for Linux.
> 
> Contacted the manufacturer, they sent me a nice NDA -which I signed- and 
> afterwards an SDK with Linux, Mac and Win dev libs, but just Mac and Win 
> examples.
> The SDK has an absolute lack of Linux examples, as expected. Also, as I 
> am a mere mortal and not a multibillion corporation, the maker will give 
> me exactly zero support.
> 
> Because of the NDA I cannot go to details, but let's say I successfully 
> connected to one of the cams (the other, an older model, makes the SDK 
> segfault).
> I can connect and authenticate to the cam, and I can spawn a video 
> retrieval thread from the main app, which seems to work. All of this, 
> again, in C++ and GTKMM.
> The worker thread retrieves video frame by frame in a buffer, along with 
> camera events. So far so good, it works.
> 
> Problem 1: Cannot figure how to dump these frames to a GTKMM window, 
> control, or surface (the Win example uses Direct3D, not applicable 
> here). Can anyone point me in the right direction?
> Problem 2: I will have to cross the thread boundary to do this. Again, 
> not a clue. Help?

You can create a pipe in the main thread with the pipe() system call. 
Then the worker thread will write the frame to the pipe, and the main 
thread will read the frames from it.

That way, your main thread will just have to call select() on the pipe 
to get the data when it is available.

> 
> The SDK is 32bit only. My main machine is 64bit. I am using VSCode in a 
> 32bit VM to develop, but I would really like to have a Gambas version 
> eventually, like in a lib so anyone else can use it. Also, the Win 
> example in a 32bit Win VM does not show video, but it does work in the 
> real hardware. So, I want to make sure that when I don't see anything is 
> not because of the VM, but because something went wrong.
> 
> Problem 3: Is there any way to make Gambas work as a 32bit, both in the 
> IDE and the interpreter, in a 64bit machine?
> Problem 4: If it is at all possible to have a solution for Problem 3 
> above: How do I solve the threading issue in Gambas?
> 
> Thanks, you may now continue talking BASIC.
> zxMarce.
> 

Then I suggest you make a 32-bits executable that just implements the 
worker thread, and that writes the frame data to a named pipe.

Then another 64-bits process will run this 32 bits executable, tell him 
which named pipe to use, and will be able to get the data from this pipe.

That way, the main process can be written in any language, and can do 
what it wants with the frames.

The only problem you will have is converting the camera frame data 
format into an image that can be displayed by your graphical toolkit. 
But maybe your SDK already handles that?

Note: afaik pipes are the fastest and easiest way to exchange data 
between processes or threads on Linux. Shared memory may be faster (for 
big amount of data), but requires additional synchronization primitives, 
so it is harder to use correctly.

Regards,

-- 
Benoît Minisini