How to pass managed Bitmap from C# to wrapped unmanaged OpenCV's Mat

Question

I have a project with 3 parts:

1. Managed C# project with a callback which gives me a Bitmap (it should have PixelFormat = Format24bppRgb). I've tried several ways to convert the Bitmap into something I could pass to part 2, that's the last thing I've tried:

   public int BufferCB(IntPtr pBuffer, int BufferLen)
   {
       byte[] aux = new byte[BufferLen];
       Marshal.Copy(pBuffer, aux, 0, BufferLen);
       String s_aux = System.Text.Encoding.Default.GetString(aux);
       wrappedClassInstance.GetBitmap(s_aux);
   }
   

2. Managed C++/CLI to wrap item 3:

   int WrappedClass::GetBitmap(array<System::Byte>^ s_in) {
       pin_ptr<unsigned char> pin = &s_in[0];
       unsigned char* p = pin;
       return privateImplementation->GetBitmap(p);
   }
   

3. Unmanaged C++ application that uses OpenCV. I want to load this data into a Mat with:

   Mat myMat;
   int NativeClass::GetBitmap(unsigned char *s_in) {
   
       // If there's no input:
       if (!s_in) {
           return -1;
       }
       /* h and w are defined elsewhere */
       myMat = Mat(h, w, CV_8UC3, (void *)s_in, Mat::AUTO_STEP));
       imwrite("test.bmp", myMat);
       return 0;
   }
   

When the the imwrite function is reached an exception is thrown: "System.Runtime.InteropServices.SEHException (0x80004005)". It doesn't say much, but I'm guessing the data I passed into the Mat got corrupted when I've marshalled it.

Before, I was trying to pass the data without a wrapper:

[DllImport("mydll.dll", ...)]
static extern void GetBitmap(IntPtr pBuffer, int h, int w);

void TestMethod(IntPtr pBuffer, int BufferLen)
{
    // h and w defined elsewhere
    // GetBitmap is essentially the same as in item 3.
    GetBitmap(pBuffer, BufferLen, h, w);
}

and that worked (it saved the Bitmap into the file), but because the DLL stays attached until I kill the process that solution is not good enough for me. I also don't want to "mirror" the Mat class into my project, as I know Mat should accept data from some char*.

Please help, how can I do this? Am I doing wrong type conversions?

Thank you.

Answer 1

I've changed part 2 to memcpy unmanaged memory into native memory, just to be sure. But the real problem with my code was that everytime I wanted to get a new Mat, I'd call Mat constructor:

Mat myMat;
int MyClass::GetBitmap(unsigned char* s_in) {
    myMat = Mat(...)
}

Instead, I did:

Mat myMat;
int MyClass::GetBitmap(unsigned char* s_in) {
    Mat aux;
    aux = Mat(...)
    aux.copyTo(myMat);
    aux.release();
}

... and now my code is working perfectly.

EDIT: Removed some parts which used new Mat(...), it was a typo and it wouldn't compile because I'm using Mat, not Mat*.