Lucifer

Lucifer is a physically based renderer, written as a tutorial on how to implement raytracing and pathtracing in Rust.

Lucifer tries to showcase a very clean and easy-to-understand implementation of the basics of 3D rendering using raytracing.

The first few commits are structured as a tutorial, showing the implementation of the core structures and algorithms of a path tracing renderer.

Building

Lucifer is written in Rust and may, depending on when you're trying to build it, require on the nightly release of the Rust compiler.

# Install Rust via Rustup
$ curl https://sh.rustup.rs -sSf | sh

# Install nightly release, if necessary
$ rustup install nightly
$ rustup default nightly

# Build lucifer
$ cargo build --release

# Run lucifer to generate the example output
$ cargo run --release example.png

Core Concepts

Space

Space is represented using the Vector and Point types, taken from the cgmath crate. Point represents positions in space, while Vector represents directions and distances between points.

A Ray is a straight line through space. It has an origin, a direction, and a possibly infinite length. Rays represent the path photons travel unhindered after being emitted, until interacting (possibly being absorbed or reflected) with the surface of an object.

An Intersection describes a location where a photon hits, and interacts with, an object. It consists of the point in space where the intersection occurred, the object's surface's normal vector, the distance from the photon's origin, and a boolean indicating whether the surface was hit from the object's inside or outside.

A Geometry is the abstraction of a physical shape. It provides methods for intersections tests between itself and Rays.

Energy

Light is represented using the Radiance structure, which models the radiant intensity of light coming from a given direction. Radiant intensity defines the color and brightness of light.

The color of a surface is represented by Albedo. An Albedo is a multiplicative factor for Radiance, with the assumption that any value is between zero and one. I.e. an Albedo cannot increase Radiance and cannot turn Radiance negative.

The appearance of a surface is modeled as a set of Effects, each representing a specific type of interaction between light and the surface. The set of Effects is called Bsdf.

Each effect is composed of an effect type, a Distribution, and some combination of Radiance, Albedo, and Ior. There are five different effects:

1. Emission is light emitted by a surface, independently of incoming light. This is the primary Effect for light sources.

   Emission is defined by a Radiance value and a distribution function.

2. Diffuse Reflection is light scattered by the surface. This is the primary Effect for rough surfaces.

   Diffuse reflection is defined by an Albedo value and a distribution function. The Distribution is centered on the surface normal

3. Specular Reflection is light reflected by the surface. This is the primary Effect for shiny surfaces and mirrors.

   Specular reflection is defined by an Albedo value and a distribution function. The Distribution is centered on the reflected incidence direction.

4. Diffuse Refraction is light transmitted, but scattered by the surface. This is the primary Effect for light-transmitting, but non-transparent objects.

   Diffuse refraction is defined by an Albedo value and a distribution function. The Distribution is centered on the mirrored surface normal.

5. Specular Refraction is light transmitted and refracted by the surface. This is the primary Effect for clear, transparent objects.

   Specular refraction can turn into reflection, if the angle of incidence is low.

   Specular refraction is defined by an Albedo and Ior value and a distribution function. The Distribution is centered on the refracted incidence direction.

The Material trait is responsible to compute the surface's appearance at the point of intersection. The generated set of Effects and their attributes can vary depending on the attributes of the Intersection.