Confusion about the usage of "opacity" in the code

[lnSong]

Thank you for your great work! I'm a bit confused about the usage of "opacity" in the code.

During forward inference, the formula used is:

outputs["rgb_gaussians"] + outputs["rgb_sky"] * (1.0 - outputs["opacity"])

However, when rendering the background separately, the formula is:

Background_rgb = results["Background_rgb"] * results["Background_opacity"] + green_background * (1 - results["Background_opacity"])

The key difference between the two is whether outputs["opacity"] is multiplied by the foreground. Which one is correct?