ashenoneart/hpvolumecloud

HPVolumeCloud is a rendering add-on for Unity's High Definition Render Pipeline (HDRP) that improves how volumetric clouds are lit. It focuses on a specific technique the author calls phi_fwd, which is a physically based way of simulating light that has bounced many times inside a thick cloud before reaching your eye, sometimes called isotropic multiple scattering. The README explains the problem it solves using before and after comparison images. In standard cloud rendering, light is usually modeled as bouncing off cloud particles once and traveling mostly in one direction, toward the sun. This makes thick clouds look wrong: their interiors get darker the deeper you look, and the underside and edges facing away from the sun can end up nearly black, since little light reaches them. Real thick clouds do not behave this way. Past a certain thickness, the inside of a cloud stops getting darker and levels off, and the underside and sides still glow softly from light that has scattered around inside the cloud many times. phi_fwd is designed as an additional lighting term added on top of normal directional sun scattering, not a replacement for it. It approximates the physics of light diffusing through a very thick, bright medium using a mathematical shortcut based on the radiative transfer equation, producing a softer, more even glow inside and around thick cloud shapes. The technique also includes checks so that thin or wispy cloud edges, where this diffusion approximation does not really apply, are not affected by it, and it tries to avoid over brightening the side of a cloud facing away from the sun. The project builds on Unity's HDRP source code and is released under the MIT license, with an additional requirement that users credit HanPi Volume Cloud and its author when distributing or crediting the work, and keep the license text intact.