Leonidas_1501:

the depth of the melting pool can be calculated with those informations


It is not for the depth of the melting pool  it is the distance from the camera plane of the bounding box corners.
With a fully calibrated camera you need to know the Z of these points to backproject in the word coordinates (if these points are co-planar the depth is the same for all the corners).
Take a look at:
  
      

      Medium – 12 Aug 20
  

  
    

Inverse Projection Transformation 4

  Depth and Inverse Projection

  
    Reading time: 7 min read
  

  

  
    
    
  

  


E.g if you know that all these points are co-planar on the floor you can backproject from the pixel coords to the floor with a fully calibrated camera.