### Human height estimation using a calibrated camera

Hello OpenCVers!

I am working on an algorithm to estimate ~~heights ~~the height of detected people in ~~the ~~a video, and I'm stuck.

The part that I have working is the detection of people using the HoG algorithm, so I have a bounding box for every person in the frame. And I have calibrated the camera, so I have my intrinsic and extrinsic camera parameters.

The problem is that now I have a formula for the perspective projection with 2 unknowns: **height** of the object and the **distance** from the object to the camera.
I am using **one** mono web camera to detect people so I have no information about the distance from the object to the camera. And the height is what I'm trying to estimate, so I don't have that as well.

I know this problem is solvable if I use a kinect or a stereo camera in order to get the distance, but I'm limited to only one mono web camera.

Does anyone have an idea on how to approach this problem?
I have read about using reference objects but I can't figure out how to use them to help my problem.