The VideoLAN Forums

Posted: **23 Aug 2010 13:26**

I'm attempting to write a small C# class that will return a WriteableBitmap that's hooked to the output of a VLC instance via vmem. VLC starts up, calls the dolock() function once, then immediately crashes reporting an error in libswscale_plugin.dll.

Here's the relevant code:

Code: Select all

    static class render
    {
        static Dispatcher dispatcher = Dispatcher.CurrentDispatcher;

        [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
        unsafe delegate IntPtr CLock(IntPtr ctx);
        
        [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
        unsafe delegate void CUnLock(IntPtr ctx);

        static CLock _lock;
        static CUnLock _unlock;

        static IntPtr pix_ptr;

        static WriteableBitmap wbmp;
        static int _w, _h;

        static VideoLanClient Vlc;
        static VlcMediaPlayer VlcPlayer;

        public static BitmapSource start(string mrl, int w, int h)
        {
            _w = w;
            _h = h;

            wbmp = new WriteableBitmap(_w, _h, 96, 96, PixelFormats.Bgr32, null);
            pix_ptr = Marshal.AllocHGlobal(_w * _h * 4);

            _lock += do_lock;
            _unlock += do_unlock;

            string path = System.IO.Path.GetDirectoryName(System.Reflection.Assembly.GetExecutingAssembly().Location);

            string[] args = new string[] { @"--vout=vmem", 
                                           @"--vmem-width=" + _w.ToString(),
                                           @"--vmem-height=" + _h.ToString(),
                                           @"--vmem-pitch=" + (_w * 4).ToString(),
                                           @"--vmem-chroma=RV32",
                                           @"--vmem-lock=" + Marshal.GetFunctionPointerForDelegate(_lock).ToInt32().ToString(),
                                           @"--vmem-unlock=" + Marshal.GetFunctionPointerForDelegate(_unlock).ToInt32().ToString(),
                                           @"--vmem-data=" + pix_ptr.ToInt32().ToString()
                                         };
            
            Vlc = new VideoLanClient(path, args);
            VlcPlayer = Vlc.NewMediaPlayer(IntPtr.Zero);

            VlcMedia desc = Vlc.NewMedia(mrl);
            VlcPlayer.Load(desc);
            desc.Dispose();

            VlcPlayer.Play();
            return wbmp;
        }

        static IntPtr do_lock(IntPtr ctx)
        {
            return ctx;
        }

        static void do_unlock(IntPtr ctx)
        {
            dispatcher.Invoke(DispatcherPriority.Render, new Action(delegate()
            {
                wbmp.WritePixels(new System.Windows.Int32Rect(0, 0, _w, _h), ctx, wbmp.BackBufferStride, 0);
            }));
        }

    }

If I remove the vmem command-line argument, the file plays just fine in a desktop window.

What am I doing wrong?? Is there someone here who has successfully done this that can point me in the right direction?

Thanks!

Posted: **23 Aug 2010 19:30**

It's crashing because you're not correct functions pointers or video memory pointers.

Posted: **24 Aug 2010 02:40**

It's crashing because you're not correct functions pointers or video memory pointers.

Forgive my ignorance, but I'm not fluent in C++, so most of the sample code is useless to me. It seems my understanding of how this works is incorrect, conceptually.

Can you explain specifically what I'm doing incorrectly?

1. Allocate a video buffer where size = width * height * bytes per pixel;
2. Pass a pointer to the buffer to VLC using the vmem-data argument;
3. Pass pointers to my lock and unlock functions to VLC using the vmem-lock and vmem-unlock arguments;
4. When a video frame is about to be decoded, VLC calls my lock function with a pointer to the buffer (from vmem-data);
5. I return the same pointer back to VLC (because my data structure is only video memory, no other fields);
6. VLC renders the video frame into the buffer;
7. VLC calls my unlock function which displays the image.

From debugging I know that:

1. VLC is calling my lock function as expected;
2. The pointer that VLC is passing to the lock function is the same as the pointer passed via vmem-data.

I'm reasonably confident that at least this part is correct. So what is it exactly that VLC expects that I'm not giving it?

Thanks for your help.

Posted: **24 Aug 2010 08:40**

It's crashing because you're not correct functions pointers or video memory pointers.

Actually it turns out my code was fine. It works fine with 0.9.9, but produces the libswscale_plugin crash with 1.0.x.

Does anyone have a clue why this might be?

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