In VB6, how do I pass the array returned by ImagetoArray() to a C++ function by reference not by value?

I have been puzzled by the problem for days, and your help is sincerely appreciated.

I use VB6 and a NI1430 board to grab images from a UNIQ1830-CL camera.  After the image is obtained, I use ImageToArray to get the data, and pass the array to a C++ function to do centroiding for a subregion of the image.  The C++ function resides in a custom-made dll named  'mathLib'.

The signature of the C++ function is:

STDMETHODIMP Cmath::centroid(VARIANT *image, int left, int width, int top, int height, VARIANT *theCentroid) ;

To call the C function from VB6, I do:

dim centroid as variant

dim imageData as variant

...

let imageData = CWIMAQImage.ImageToArray 

Let centroid = mathLib.centroid(imageData, regionLeft, regionWidth, regionTop, regionHeight)  

...

I tried the NI1430 and a board from another company.  The 'imageData' look the same in VB6 - both are 2D Integer arrays of size 1024x1024.  However when the array is passed to the C++ routine, there is a 5 times difference in processing speed.  I examined the array on the C++ side.  It turns out that the 'vt' field has different values.  For the array coming from NI1430, vt = 0x2002 (i.e. VT_ARRAY & VT_I2), which incates it's passed in by value.  While for the array coming from the other board, vt = 0x6002 (i.e. VT_BYREF & VT_ARRAY & VT_I2), which says it's passed by reference.  Since I call the C++ function for many sub regions in the image, and each call results in a new copy of the NI image array, the slowness in speed for the NI board is profound.

So my question is: In VB6, how do I get a reference to the array returned by ImagetoArray() ?  I've tried to obtain the address of the array, pass the first element of the array, etc.  none of them worked.  Please help.