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it is the same as: (just for clarifying)

 Mat aux = cur_img(Rect(0,0,100,100));
 Mat res_img = aux.reshape(1,1);

When you do this operation:

 Mat aux =  image(Rect(0,0,100,100));

Your are calling this operator

  C++: Mat Mat::operator()(const Rect& roi) const

This operation makes a new header for the specified sub-array of *this, thus it is a 0(1) operation, that is, no matrix data is copied. So, matrix elements are no longer stored continuosly without gaps at the end of each row.

  Mat res_img = aux.reshape(1,1);

No data is copied. That is, this is an O(1) operation. Consequently, if you change the number of rows the matrix must be continuous

 bool myCheckMatContinuity(const Mat& m)
 {
      //return (m.flags & Mat::CONTINUOUS_FLAG) != 0;
      return m.rows == 1 || m.step == m.cols*m.elemSize();
 }

 Mat aux = image(Rect(0,0,100,100));
 bool cont= myCheckMatContinuity(aux); //false!
 Mat res_img = aux.reshape(1,1); //error

it is the same as: (just for clarifying)

 Mat cur_img = imread(fileList.fileNames[i],0);
 Mat aux = cur_img(Rect(0,0,100,100));
 Mat res_img = aux.reshape(1,1);

When you do this operation:

 Mat aux =  image(Rect(0,0,100,100));

Your are calling this operator

  C++: Mat Mat::operator()(const Rect& roi) const

This operation makes a new header for the specified sub-array of *this, thus it is a 0(1) operation, that is, no matrix data is copied. So, matrix elements are no longer stored continuosly without gaps at the end of each row.

  Mat res_img = aux.reshape(1,1);

No data is copied. That is, this is an O(1) operation. Consequently, if you change the number of rows the matrix must be continuous

 bool myCheckMatContinuity(const Mat& m)
 {
      //return (m.flags & Mat::CONTINUOUS_FLAG) != 0;
      return m.rows == 1 || m.step == m.cols*m.elemSize();
 }

 Mat aux = image(Rect(0,0,100,100));
 bool cont= myCheckMatContinuity(aux); //false!
 Mat res_img = aux.reshape(1,1); //error