question in file:neilf_composite

[JiatengLiu]

The following two lines of code are found in the gaussian_render/neilf_composite , could you please explain the meaning of these two lines of code? And which formula it corresponds to in the paper?

pbr = rendered_pbr
rendered_pbr = pbr + (1 - rendered_opacity) * bg_color[:, None, None]

And same code in file gaussian_render/neilf

rendered_pbr = pbr + (1 - rendered_opacity) * bg_color[:, None, None]