Confusion about the inverse-depth-smooth-loss

[SJoJoK]

Hello! I'm currently doing some research on NeRF, and I found your work SinNeRF. It's an awesome work, and I download the code (in this repository). While reading the code, I found that you use kornia.losses.inverse_depth_smoothness_loss to calculate the "self-supervised inverse depth smoothness loss" which you describe in the paper (equation 4) . The conflict is that, while kornia.losses.inverse_depth_smoothness_loss using the first-order gradient of the RGB Image according to the official document and its source code, the equation in paper use the second-order gradient of RGB Image. Thanks for any advice and help.