Why is cv::Mat::data always pointing to a uchar?

Question

I try to read a NEF file using LibRaw and then put it in a cv::Mat. The NEF file stores data as 12bit, this means I need 16 bit, so I ought to use CV_16UC4 like this:

Mat img1(height, width, CV_16UC4);

Libraw stores data as ushort*[4], so I thought that this should work:

for (i = 0; i < iwidth*height; i++) {       
    img1.data[4*i+1] = Processor.imgdata.image[i][0];
    img1.data[4*i+2] = Processor.imgdata.image[i][1];
    img1.data[4*i+3] = Processor.imgdata.image[i][2];
    img1.data[4*i+4] = Processor.imgdata.image[i][3];
}

I also get a build error that data may be lost since a ushort to uchar conversion is going to take place, which makes sense, but still, how do I put data bigger than uchar in the data?

Answer 1

If you need pointer to raw data of specific type, using cv::Mat::ptr() is the best practice:

ushort* ptr = img1.ptr<ushort>();
for (i = 0; i < iwidth*height; i++) {       
    ptr[4*i+1] = Processor.imgdata.image[i][0];
    ptr[4*i+2] = Processor.imgdata.image[i][1];
    ptr[4*i+3] = Processor.imgdata.image[i][2];
    ptr[4*i+4] = Processor.imgdata.image[i][3];
}

Please see documentation.

Answer 2

cv::Mat::data uses uchar in order avoid being a template class. In order to fill it with other image data you'll need to cast the data pointer. In your case try something like this:

Mat img1(height, width, CV_16UC4);
ushort * data = reinterpret_cast< ushort* >( img1.data );

for (i = 0; i < iwidth*height; i++) {
...
}

Alternatively, instead of changing the data pointer img1.data directly in your for-loop, you could consider using

1. the templated pixel access function cv::Mat::at<T>()

   img1.at<Vec4w>(y,x) = reinterpret_cast<Vec4w>(Processor.imgdata.image[i])
   

2. use the specialized class Mat4w img(height, width) and then operator(y,x)

   img1(y,x) = reinterpret_cast<Vec4w>(Processor.imgdata.image[i])