single-column matrix. Similarly to Mat::row and Mat::col, this is an O(1) operation.
@param d index of the diagonal, with the following values:
- `d=0` is the main diagonal.
- - `d>0` is a diagonal from the lower half. For example, d=1 means the diagonal is set
+ - `d<0` is a diagonal from the lower half. For example, d=-1 means the diagonal is set
immediately below the main one.
- - `d<0` is a diagonal from the upper half. For example, d=-1 means the diagonal is set
+ - `d>0` is a diagonal from the upper half. For example, d=1 means the diagonal is set
immediately above the main one.
*/
Mat diag(int d=0) const;