bevy/crates/bevy_pbr/src/atmosphere/bruneton_functions.wgsl

// Copyright (c) 2017 Eric Bruneton
// All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions
// are met:
// 1. Redistributions of source code must retain the above copyright
//    notice, this list of conditions and the following disclaimer.
// 2. Redistributions in binary form must reproduce the above copyright
//    notice, this list of conditions and the following disclaimer in the
//    documentation and/or other materials provided with the distribution.
// 3. Neither the name of the copyright holders nor the names of its
//    contributors may be used to endorse or promote products derived from
//    this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
// ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
// LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
// CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
// SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
// CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
// ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
// THE POSSIBILITY OF SUCH DAMAGE.
//
// Precomputed Atmospheric Scattering
// Copyright (c) 2008 INRIA
// All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions
// are met:
// 1. Redistributions of source code must retain the above copyright
//    notice, this list of conditions and the following disclaimer.
// 2. Redistributions in binary form must reproduce the above copyright
//    notice, this list of conditions and the following disclaimer in the
//    documentation and/or other materials provided with the distribution.
// 3. Neither the name of the copyright holders nor the names of its
//    contributors may be used to endorse or promote products derived from
//    this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
// ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
// LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
// CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
// SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
// CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
// ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
// THE POSSIBILITY OF SUCH DAMAGE.

#define_import_path bevy_pbr::atmosphere::bruneton_functions

#import bevy_pbr::atmosphere::{
    types::Atmosphere,
    bindings::atmosphere,
}

// Mapping from view height (r) and zenith cos angle (mu) to UV coordinates in the transmittance LUT
// Assuming r between ground and top atmosphere boundary, and mu= cos(zenith_angle)
// Chosen to increase precision near the ground and to work around a discontinuity at the horizon
// See Bruneton and Neyret 2008, "Precomputed Atmospheric Scattering" section 4
fn transmittance_lut_r_mu_to_uv(r: f32, mu: f32) -> vec2<f32> {
  // Distance along a horizontal ray from the ground to the top atmosphere boundary
    let H = sqrt(atmosphere.top_radius * atmosphere.top_radius - atmosphere.bottom_radius * atmosphere.bottom_radius);

  // Distance from a point at height r to the horizon
  // ignore the case where r <= atmosphere.bottom_radius
    let rho = sqrt(max(r * r - atmosphere.bottom_radius * atmosphere.bottom_radius, 0.0));

  // Distance from a point at height r to the top atmosphere boundary at zenith angle mu
    let d = distance_to_top_atmosphere_boundary(r, mu);

  // Minimum and maximum distance to the top atmosphere boundary from a point at height r
    let d_min = atmosphere.top_radius - r; // length of the ray straight up to the top atmosphere boundary
    let d_max = rho + H; // length of the ray to the top atmosphere boundary and grazing the horizon

    let u = (d - d_min) / (d_max - d_min);
    let v = rho / H;
    return vec2<f32>(u, v);
}

// Inverse of the mapping above, mapping from UV coordinates in the transmittance LUT to view height (r) and zenith cos angle (mu)
fn transmittance_lut_uv_to_r_mu(uv: vec2<f32>) -> vec2<f32> {
  // Distance to top atmosphere boundary for a horizontal ray at ground level
    let H = sqrt(atmosphere.top_radius * atmosphere.top_radius - atmosphere.bottom_radius * atmosphere.bottom_radius);

  // Distance to the horizon, from which we can compute r:
    let rho = H * uv.y;
    let r = sqrt(rho * rho + atmosphere.bottom_radius * atmosphere.bottom_radius);

  // Distance to the top atmosphere boundary for the ray (r,mu), and its minimum
  // and maximum values over all mu- obtained for (r,1) and (r,mu_horizon) -
  // from which we can recover mu:
    let d_min = atmosphere.top_radius - r;
    let d_max = rho + H;
    let d = d_min + uv.x * (d_max - d_min);

    var mu: f32;
    if d == 0.0 {
        mu = 1.0;
    } else {
        mu = (H * H - rho * rho - d * d) / (2.0 * r * d);
    }

    mu = clamp(mu, -1.0, 1.0);

    return vec2<f32>(r, mu);
}

/// Simplified ray-sphere intersection
/// where:
/// Ray origin, o = [0,0,r] with r <= atmosphere.top_radius
/// mu is the cosine of spherical coordinate theta (-1.0 <= mu <= 1.0)
/// so ray direction in spherical coordinates is [1,acos(mu),0] which needs to be converted to cartesian
/// Direction of ray, u = [0,sqrt(1-mu*mu),mu]
/// Center of sphere, c = [0,0,0]
/// Radius of sphere, r = atmosphere.top_radius
/// This function solves the quadratic equation for line-sphere intersection simplified under these assumptions
fn distance_to_top_atmosphere_boundary(r: f32, mu: f32) -> f32 {
  // ignore the case where r > atmosphere.top_radius
    let positive_discriminant = max(r * r * (mu * mu - 1.0) + atmosphere.top_radius * atmosphere.top_radius, 0.0);
    return max(-r * mu + sqrt(positive_discriminant), 0.0);
}

/// Simplified ray-sphere intersection
/// as above for intersections with the ground
fn distance_to_bottom_atmosphere_boundary(r: f32, mu: f32) -> f32 {
    let positive_discriminant = max(r * r * (mu * mu - 1.0) + atmosphere.bottom_radius * atmosphere.bottom_radius, 0.0);
    return max(-r * mu - sqrt(positive_discriminant), 0.0);
}

fn ray_intersects_ground(r: f32, mu: f32) -> bool {
    return mu < 0.0 && r * r * (mu * mu - 1.0) + atmosphere.bottom_radius * atmosphere.bottom_radius >= 0.0;
}