pixel data from mex to matlab: mxArray definition..

Hello I’m trying to use TCP/IP gEth. camera using mex file. The cpp code provides by camera vendor is very useful to open library/open/close camera and trigger, also I’m able to save image bmp in a selected folder from where I can read the image (1600X1200) from matlab (imread/imshow). Everything works fine but I’m not able to transfer the pixel data from mex to matlab. Below an example of the cpp code where a simple pixel calculation is performed (*pPixel = ~*pPixel): // get the pointer to the image data

    void* pData = NULL;
    int32_t iImageOffset = 0;
    pBuffer->GetPtr (LvBuffer_UniBase, &pData);
    pBuffer->GetInt32 (LvBuffer_UniImageOffset, &iImageOffset);
    pData = (uint8_t*)pData + iImageOffset;
  for (int32_t j=0; j<(1200); j++)
        {
            uint8_t* pPixel = ((uint8_t*)pData) +  j*1600;
            for (int32_t i=0; i<(1600); i++)
            {
                for (int32_t k=0; k<iBytesPerPixel; k++)
                {
        *pPixel = ~*pPixel;

                    pPixel++;
                }
            }
      }

My problem is how to transfer the pPixel (uint_8) to matlab… I’m tring to use mxArray B; B= mxCreateNumericData(1600, 1200, mxINT8_CLASS, mxREAL); But I don’t know how to populate the matrix B. Please, any advices??

M. Thanks

Enrico

Answer 1

[Titus Edelhofer]

Hi Enrico,

as a .bmp it probably has 24 bits, i.e., you iBytesPerPixel is 3. Therefore you should allocate a 1600x1200x3 matrix:

mwSize aSize[3] = {1600, 1200, 3};
plhs[0] = mxCreateNumericArray(3, aSize, mxUINT8_CLASS, mxREAL);
uint8_t *pData = (uint8_t*) mxGetData(plhs[0]);

You might use the code you have above to loop on the indices, and put each pixel from pPixel into pData. Note though, that the indices are running differently in MATLAB then in your code, it should be something like

pData[k*1200*1600 + j*1600 + i] = *pPixel;

but I'm not 100% sure I admit ;-).

Titus

[James Tursa]

mwPointer is a generic Fortran type (integer*4 or integer*8) used to store addresses ... it is not part of the C/C++ API. I think you meant to use "uint8_t" instead. E.g.,

uint8_t *aPointer = (uint8_t *) mxGetData(plhs[0]);

[Titus Edelhofer]

Yes, of course, I mixed this up. I updated the answer. Thanks James!

[Enrico Boscolo]

very helpful !!..it's working. Many thanks Enrico