The VideoLAN Forums

Posted: **26 Apr 2009 10:48**

I am trying to access the VLC's decoded video output using vmem module in my C# application. I am using a wrapper posted in this forum viewtopic.php?f=32&t=47385. The VLC version that I am using is 0.9.9.
VLC is ran with these arguments:

Code: Select all

            string my_dolock_str = Marshal.GetFunctionPointerForDelegate(my_dolock).ToInt32().ToString();
            string my_unlock_str = Marshal.GetFunctionPointerForDelegate(my_unlock).ToInt32().ToString();
            string[] argv = new string[] { //"-I", "dummy"
                                          "--ignore-config"
                                         //, "--no-overlay"
                                         //, "--noaudio"
                                         , "--vout", "vmem"
                                         , "--vmem-width", VideoWidth.ToString()
                                         , "--vmem-height", VideoHeight.ToString()
                                         , "--vmem-pitch", (VideoWidth * 4).ToString()
                                         , "--vmem-chroma", "RV32"
                                         , "--vmem-lock", my_dolock_str
                                         , "--vmem-unlock", my_unlock_str
                                         , "--vmem-data", ctx_ptr.ToInt32().ToString()
            };

I have implemented the lock functions in two ways, both giving me run-time errors.

1- Using GDI+:

Code: Select all

        public static IntPtr dolock(IntPtr ctx)
        {
            Rectangle rect = new Rectangle(0, 0, VideoWidth, VideoHeight);
            bmpd = bmp.LockBits(rect,ImageLockMode.ReadOnly,PixelFormat.Format32bppArgb);
            IntPtr tmpRes = bmpd.Scan0;
            return tmpRes;
        }

        public static void unlock(IntPtr ctx)
        {
            bmp.UnlockBits(bmpd);
        }

where in my Form class:

Code: Select all

        static Bitmap bmp;
        static BitmapData bmpd;

2- Allocating the picture data in application's unmanaged space:

Code: Select all

        public static IntPtr dolock(IntPtr ctx)
        {
            return ctx;
        }

        public static void unlock(IntPtr ctx)
        {
        }

where in my Form_Load have:

Code: Select all

        ctx_ptr = Marshal.AllocHGlobal(VideoWidth * VideoHeight * 4);

The result of debugging shows that the vmem module invokes the lock callback functions correctly. But, as soon as the dolock functions returns with the pointer to the surface, the application is halted with the following error:

The instruction at ... referenced memory at "0x000000". The memory could not be "written" .

Any suggestions?

Posted: **29 Apr 2009 17:48**

First of all return address of the allocated memory (ctx_ptr in your code) instead of ctx argument.
Secondly try using [UnmanagedFunctionPointer(CallingConvention.Cdecl)] attribute before your lock/unlock methods delegates.

Posted: **30 Apr 2009 09:02**

First of all return address of the allocated memory (ctx_ptr in your code) instead of ctx argument.
Secondly try using [UnmanagedFunctionPointer(CallingConvention.Cdecl)] attribute before your lock/unlock methods delegates.

Thank you versor. It seems that the CallingConvention attribute was the problem.

The VideoLAN Forums

vmem in C# (using Marx's wrapper)

vmem in C# (using Marx's wrapper)

Re: vmem in C# (using Marx's wrapper)

Re: vmem in C# (using Marx's wrapper)