Why fixedSize error occurs depends on default parameter? - c++

Question

I was trying to find a way how to make default parameter of Mat type, but it was complicated to make. But, today I found out this code OutputArray _hist = Mat() and I thought this could be simply a default parameter of Mat type. Therefore, I could make this code and it worked well, but I couldn't still understand one thing.
int myGetHistogram(InputArray _src, OutputArray _hist = Mat())
{
Mat src = _src.getMat();
_hist.create(512,512,CV_8U);
Mat histImage = _hist.getMat();
...
rectangle(histImage, max_pt1, max_pt2, Scalar(0), -1);
return max_pt1.x/(histImage.cols/256);
}
In this code, this error message shows up.
OpenCV Error: Assertion failed (!fixedSize() || ((Mat)obj)->size.operator()() == _sz)*
If I set the default parameter as OutputArray _hist = Mat(512,512,CV_8U) instead of OutputArray _hist = Mat(), then the error is removed.
Why does this problem happen?

Answer 1

In either case you're creating an OutputArray from a temporary Mat (i.e. this constructor), so you won't be able to change the size or datatype.
Take inspiration from the OpenCV code. Use cv::noArray() to make the output parameter optional, and then cv::OutputArray::needed to determine how to initialize your cv::Mat histImage.
#include <opencv2/opencv.hpp>
int myGetHistogram(cv::InputArray _src, cv::OutputArray _hist = cv::noArray())
{
cv::Mat src = _src.getMat();
cv::Size const HISTOGRAM_SIZE(512, 512);
cv::Mat histImage;
if (_hist.needed()) {
_hist.create(HISTOGRAM_SIZE, CV_8U);
histImage = _hist.getMat();
} else {
histImage = cv::Mat(HISTOGRAM_SIZE, CV_8UC1);
}
// ... whatever
return 1;
}
int main()
{
cv::Mat a(4, 4, CV_8UC1);
cv::Mat b;
myGetHistogram(a);
myGetHistogram(a, b);
return 0;
}