where y 0 i represents the initial position of the i-th VP. Given the target a i , the loss function in this stage consists of four components: the negative log-likelihood (NLL) loss for VP types, a truncated L2 loss for VP 3D coordinates, an L1 distance loss for VP-VP pairs and VP-space pairs, and a regularization loss to constrain movement between consecutive iterations. Therefore, at iteration r, the training loss function for this stage is expressed as: