How to use cv::Mat::convertTo

Question

I'm writing a function which takes a cv::Mat of arbitrary type, converts it to a float image, processes it and converts it back to it's original type. The problem is that non of the simple ways I've come up with works. Here's what I tried so far:

cv::Mat process(const cv::Mat& input)// input might be float
{
    cv::Mat_<float> result(input.size());

   // Generate result based on input.
    result = ...;

    // Now convert result back to the type of input:
#if 1
    // Version 1: Converting in place crashes with:
    // OpenCV Error: Assertion failed (!fixedType() || ((Mat*)obj)->type() == mtype) in cv::_OutputArray::create,
    // file ...\OpenCV\modules\core\src\matrix.cpp, line 1365
    if (result.type() != input.type())
        result.convertTo(result, input.type());

#else
    // Version 2: Not what you'd expect
    if (result.type() != input.type())
    {
        cv::Mat tmp;
        result.convertTo(tmp, input.type());
        result = tmp;// This line doesn't replace result, but converts tmp back to float.
    }
#endif
    return result;
}

The calling function:

int main(int argc, char* argv[])
{
    cv::Mat_<float> a =  cv::Mat_<float>::zeros(256, 256);
    cv::Mat a1 = process(a);

    cv::Mat_<uint16_t> b =  cv::Mat_<uint16_t>::zeros(256, 256);
    cv::Mat b1 = process(b);
    assert(b1.type()==CV_16UC1);
    return 0;
}

So, what would be the standard way of doing this? I'm using OpenCV 2.4.10 on Windows.

Answer 1

The problem is the templated cv::Mat_<float> . Apparently cv::Mat::convertTo() cannot take a Mat_<> as an output. Additionally cv::Mat_<float>::operator=() works differently than cv::Mat:operator=() . It implicitly converts the image to the appropriate format.

It makes sense once you think of it.