unpack 12 bit mono images

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I'm trying to process images from a Hamamatsu Orca flash 4 camera using Opencv and having some trouble with 12 bit mono images. They are bit packed with 2 pixels in 3 bytes as described:

MONO12 Packed Format

This is output data format of DCAM_PIXELTYPE_MONO12. P is the pixel transmitted by the camera. B is the byte in buffer received data.

Buffer / Pixel Data: B[0] P[0] bits 11 ... 4, B[1] P[1] bits 3 ... 0 | P[0] bits 3 ... 0, B[2] P[1] bits 11 ... 4, ... ... B[m-2] P[n-1] bits 11 ... 4, B[m-1] P[n] bits 3 ... 0 | [n-1] bits 3 ... 0, B[m] P[n] bits 11 ... 4

The following code is supposed to unpack the image but am having trouble following and hope someone might help me understand a few things: first, why would you mix BYTE and WORD with char and ushort, second, why use a LUT to unpack the bits instead of transferring the buffer directly to 2 arrays containing char and ushort, and third, why is this not working as written. ie, isn't pImage a pointer to the unpacked 12 bit image? Maybe that pImage array of chars needs to be transferred to yet another array of ushort 2 bytes at a time first? Is there a more efficient way to do this?

void unpack_mono12_image( void* pSrcTop, int32 srcRowbytes, void* pDstTop, int32 dstRowbytes, int32 width, int32 height )
{
    WORD lut1[65536];
    WORD lut2[65536];
    // make lut to unpack MONO12
    int i, j;
    for( i=0; i<65536; i++ )
    {
        WORD w = (WORD)i;
        BYTE* p = (BYTE*)&w;

        lut1[i] = (p[0] << 4) + (p[1] & 0x0F);
        lut2[i] = (p[1] << 4) + ((p[0] & 0xF0) >> 4);
    }

    // unpack MONO12 and copy
    char* src = (char*)pSrcTop;
    char* dst = (char*)pDstTop;

    for( i=0; i<height; i++ )
    {
        WORD* pDst = (WORD*)(dst + dstRowbytes * i);
        BYTE* pSrc = (BYTE*)(src + srcRowbytes * i);
        for( j=0; j<width/2; j++ )
        {
            *pDst++ = lut1[*(WORD*)pSrc++];
            *pDst++ = lut2[*(WORD*)pSrc++];
            pSrc++;
        }
    }
}

...

int32 rowbytes      = bufframe.width * 2;
    int32 framebytes    = rowbytes * bufframe.height;
    char* pImage = new char[ framebytes ];
    memset( pImage, 0, framebytes );

    unpack_mono12_image( bufframe.buf, bufframe.rowbytes, pImage, rowbytes, bufframe.width, bufframe.height );
                     cv::Mat img(bufframe.width, bufframe.height, CV_16U, pImage);
                     ushort* pointer_to_data_start = img.ptr<ushort>();
                     memcpy(pointer_to_data_start, pImage, bufframe.height * rowbytes);
                     cv::imshow("image", img);
                     cv::waitKey(30);
    delete pImage;