Keypoint Validation in "InitTranslation"

[Daaohame]

Hi,

I have some questions about the class InitTranslation used for SMPL refinement, specifically regarding the check for valid keypoints –
https://github.com/zju3dv/EasyMocap/blob/206c95065f0da64581804528d9e182fd35de5e88/myeasymocap/operations/init.py#L44C1-L45C1

for i in range(kpts1.shape[0]):
  k2d = keypoints[i, :nJoints]
  if k2d[:, -1].sum() < nJoints / 2:
      mywarn('[{}] No valid keypoints in frame {}'.format(self.__class__.__name__, i))
      params['Th'][i] = params['Th'][i-1]
      continue

1. Why do you use k2d[:, -1].sum()? How do you handle cases where negative and positive values offset each other in the sum?
2. What is the rationale behind the threshold nJoints / 2?

Thank you for your assistance.