Why do we need to invert the y and z axes?

[Echo-jyt]

        ## projection
        landmarks2d = util.batch_orth_proj(landmarks2d, codedict['cam'])[:,:,:2]; landmarks2d[:,:,1:] = -landmarks2d[:,:,1:]#; landmarks2d = landmarks2d*self.image_size/2 + self.image_size/2
        landmarks3d = util.batch_orth_proj(landmarks3d, codedict['cam']); landmarks3d[:,:,1:] = -landmarks3d[:,:,1:] #; landmarks3d = landmarks3d*self.image_size/2 + self.image_size/2
        trans_verts = util.batch_orth_proj(verts, codedict['cam']); trans_verts[:,:,1:] = -trans_verts[:,:,1:]

Why do we need to invert the coordinate axis here?