SBWO/SimpleRayTracer.jl

module SimpleRayTracer

using LinearAlgebra
using StaticArrays

using Images
using GeometryBasics
using Rotations

export OrthogonalCamera
export World, Ray
export Sphere

abstract type AbstractCamera end
abstract type AbstractObject end

struct OrthogonalCamera{T<:AbstractFloat} <: AbstractCamera
    origin::Vec3{T}
    size::Tuple{T, T}
    rotation::RotXYZ{T}
end

function get_rays(camera::OrthogonalCamera, resolution::Tuple{I, I}) where {I<:Integer}
    R = camera.rotation
    X, Y = camera.size
    Nx, Ny = resolution
    screen_pixel_position = [R*Vec3(x, y, 0.0) + camera.origin for y = LinRange(Y/2, -Y/2, Ny), x = LinRange(-X/2,  X/2, Nx)]
    [Ray(p, R*Vec3(0.0, 0.0, 1.0)) for p in screen_pixel_position]
end

struct PinHoleCamera{T<:AbstractFloat} <: AbstractCamera
    origin::Vec3{T}
    distance::T
    base::SMatrix{3, 3, T}
    size::Tuple{T, T}
end

function PinHoleCamera(origin, lookAt, up, distance, size)
    w = origin - lookAt |> normalize 
    u = up × w |> normalize
    v = w × u
    B = [u  v  w]
    PinHoleCamera(origin, distance, B, size)
end

function get_rays(camera::PinHoleCamera, resolution::Tuple{I, I}) where {I<:Integer}
    origin = camera.origin
    d = camera.distance
    X, Y = camera.size
    B = camera.base
    Nx, Ny = resolution
    [Ray(origin, B*Vec3(x, y, -d)) for y = LinRange(Y/2, -Y/2, Ny), x = LinRange(X/2,  -X/2, Nx)]
end

struct Ray{T<:AbstractFloat}
    origin::Vec3{T}
    direction::Vec3{T}
    function Ray(o::Vec3{T}, d::Vec3{T}) where {T<:AbstractFloat}
        new{T}(o, d |> normalize)
    end
end

struct HitInfo
    hit::Bool
    color::RGB{N0f8}
end

struct World
    objects::Vector
    background::RGB{N0f8}
    function World(color)
        new([], color)
    end
end

Base.push!(w::World, x::AbstractObject) = push!(w.objects, x)

struct Sphere{T<:AbstractFloat} <: AbstractObject
    origin::Vec3{T}
    radius::T
    color::RGB{N0f8}
end

function check_hit(ray::Ray{T}, sphere::Sphere{T})::Tuple{Bool, T} where {T <: AbstractFloat}
    p = ray.origin - sphere.origin
    a = ray.direction ⋅ ray.direction
    b = 2 * (p ⋅ ray.direction)
    c = p ⋅ p - sphere.radius^2
    Δ = b^2 - 4a*c
    Δ < 0.0 && return false, -1.0
    x, z = -b/(2a), √Δ/(2a)
    t = x - z
    t = t < eps() ? x + z : t
    return t < eps() ? (false, -1.0) : (true, t)
end

function trace_ray(world::World, ray::Ray)::HitInfo
    mindist = Float64 |> typemax |> prevfloat
    hit = false
    color = world.background
    for obj in world.objects
        hit, dist = check_hit(ray, obj)
        if hit && dist < mindist
            mindist = dist
            hit = true
            color = obj.color
        end
    end
    return HitInfo(hit, color)
end

function render(world, camera, resolution)
    rays = get_rays(camera, resolution)
    [trace_ray(world, ray).color for ray in rays];
end

end