How to understand "To sample the cone, we cannot follow MipNeRF [3] to use the multivariate Gaussian, since the multivariate Gaussian is anisotropic but the pre-filtering in our Tri-Mip encoding is isotropic."

[Terry10086]

Thanks for your impressive work. I am confused about the sentence mentioned in your paper "To sample the cone, we cannot follow MipNeRF [3] to use the multivariate Gaussian, since the multivariate Gaussian is anisotropic but the pre-filtering in our Tri-Mip encoding is isotropic." I mean why does Tri-Mip need an isotropic characteristic? What is isotropic? Does it mean the same color is observed from different views?

Could you please give more explanation?