Compiler doesn't vectorize even with simd directive

I have this function taken from [here][1]:

    bool interpolate(const Mat &im, float ofsx, float ofsy, float a11, float a12, float a21, float a22, Mat &res)
    {
       bool ret = false;
       // input size (-1 for the safe bilinear interpolation)
       const int width = im.cols-1;
       const int height = im.rows-1;
       // output size
       const int halfWidth  = res.cols >> 1;
       const int halfHeight = res.rows >> 1;
       float *out = res.ptr<float>(0);
       for (int j=-halfHeight; j<=halfHeight; ++j)
       {
          const float rx = ofsx + j * a12;
          const float ry = ofsy + j * a22;
          for(int i=-halfWidth; i<=halfWidth; ++i)
          {
             float wx = rx + i * a11;
             float wy = ry + i * a21;
             const int x = (int) floor(wx);
             const int y = (int) floor(wy);
             if (x >= 0 && y >= 0 && x < width && y < height)
             {
                // compute weights
                wx -= x; wy -= y;
                // bilinear interpolation
                *out++ =
                   (1.0f - wy) * ((1.0f - wx) * im.at<float>(y,x)   + wx * im.at<float>(y,x+1)) +
                   (       wy) * ((1.0f - wx) * im.at<float>(y+1,x) + wx * im.at<float>(y+1,x+1));
             } else {
                *out++ = 0;
                ret =  true; // touching boundary of the input
             }
          }
       }
       return ret;
    }

As suggested by [Intel Advisor][2], I added:

    #pragma omp simd
    for(int i=-halfWidth; i<=halfWidth; ++i)

However, while compiling I got:

    warning #15552: loop was not vectorized with "simd"

Googling it, I found [this][3], but it's still not clear to me how I could solve this and vectorize this loop. 

  [1]: https://github.com/perdoch/hesaff/blob/master/helpers.cpp
  [2]: https://www.google.it/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&u...
  [3]: https://software.intel.com/en-us/articles/fdiag13379