I've written a mini ray tracer for the computer language shootout. The
original version was a 66-line OCaml program which I ported to C++:

  http://www.ffconsultancy.com/free/ray_tracer/comparison.html

I have since ported the program to C (icc and gcc), SML (sml-nj and mlton)
and Java (j2se and gcj) and Simon Geard kindly ported it to Fortran 90.

I'd like to see it ported to other languages as well, particularly Haskell
and I'm keen to hear any constructive criticisms of my existing
implementations.

It is interesting to note that the SML is 50% longer than the OCaml. When
compiled with Mlton it is also significantly faster. Here are my timings
(1.2GHz Athlon t-bird) for 128x128 resolution, 4x4 oversampling and 6
levels of spheres:

Mlton    1.316s  mlton ray.sml
IFC      1.361s  ifort -O3 -u -static-libcxa -o raytracer raytracer.f90
C++      1.605s  g++-3.4 -O3 -funroll-all-loops -ffast-math ray.cpp -o ray
ocamlopt 1.932s  ocamlopt -inline 100 ray.ml -o ray
SML-NJ   2.245s  sml ray.sml
G95      3.351s  g95 -O3 -ffast-math ray.f90 -o ray
C        5.971s  gcc-3.4 -lm -std=c99 -O3 -ffast-math ray.c -o ray
Java     6.492s  javac ray.java
GCJ     20.316s  gcj-3.4 --main=ray -Wall -O3 -ffast-math ray.java -o ray
ocamlc  41.047s  ocamlc ray.ml -o ray

Note: the OCaml and C++ implementations timed here are not those on the site
- they are more optimised and longer (keeping within 100 LOC).

The compile times are also interesting:

ocamlc   0.099s
IFC      0.333s
SML-NJ   0.625s
C        0.655s
ocamlopt 0.714s
GCJ      0.723s
G95      0.941s
Java     1.362s
Mlton    8.267s
C++      8.676s

Here is the SML (for Mlton):

(* The Great Computer Language Shootout
   http://shootout.alioth.debian.org/
   contributed by Jon Harrop, 2005
   Compile: mlton ray.sml *)

fun real n = Real.fromInt n
(* Mlton does not provide a "for loop" construct. *)
fun for (s, e, f) = if s=e then () else (f (real s); for (s+1, e, f))

val delta = 0.00000001 (* FIXME: Where is mach eps in the std libs? *)
val infinity = 1.0 / 0.0 (* FIXME: Where is infinity? *)
val pi = 4.0 * Real.Math.atan 1.0 (* FIXME: Where is pi? *)

(* 3D vector
   SML typically uses tuples rather than records (as in the OCaml
   implementation). *)
type vec = real * real * real
(* SML allows operator precedences to be declared when defining infix
   operators. *)
infix 2 *| fun s *| (x, y, z) = (s*x+0.0, s*y, s*z)
infix 1 +| fun (x1, y1, z1) +| (x2, y2, z2) =
               (x1+x2+0.0, y1+y2+0.0, z1+z2+0.0)
infix 1 -| fun (x1, y1, z1) -| (x2, y2, z2) =
               (x1-x2+0.0, y1-y2+0.0, z1-z2+0.0)
fun dot (x1, y1, z1) (x2, y2, z2) = x1*x2 + y1*y2 + z1*z2+0.0
fun unitise r = (1.0 / Real.Math.sqrt (dot r r)) *| r

(* Node in the scene tree *)
datatype scene =
         Sphere of vec * real
       | Group of vec * real * scene list

(* Find the first intersection of the given ray with this sphere *)
fun ray_sphere (orig, dir) center radius =
    let
        val v = center -| orig
        val b = dot v dir
        val disc = b * b - dot v v + radius * radius
    in
        if disc < 0.0 then infinity else
        let val disc = Real.Math.sqrt disc in
            let val t2 = b + disc in
            if t2 < 0.0 then infinity else
            (fn t1 => if t1 > 0.0 then t1 else t2) (b - disc)
            end end end

(* Accumulate the first intersection of the given ray with this sphere *)
(* This function is used both for primary and shadow rays. *)
fun intersect (orig, dir) scene =
    let fun of_scene (scene, (l, n)) =
            case scene of
                Sphere (center, radius) =>
                let val l' = ray_sphere (orig, dir) center radius in
                    if l' >= l then (l, n) else
                    (l', unitise (orig +| l' *| dir -| center))
                end
              | Group (center, radius, scenes) =>
                let val l' = ray_sphere (orig, dir) center radius in
                    if l' >= l then (l, n) else
                    foldl of_scene (l, n) scenes
                end in
        of_scene (scene, (infinity, (0.0, 0.0, 0.0)))
    end

(* Trace a single ray into the scene *)
fun ray_trace light (orig, dir) scene =
    let val (lambda, n) = intersect (orig, dir) scene in
        if lambda >= infinity then 0.0 else
        let val g = 0.0 - dot n light in
            (* If we are on the shadowed side of a sphere then don't
               bother casting a shadow ray as we know it will intersect
               the same sphere. *)
            if g <= 0.0 then 0.0 else
            let
                val orig = orig +| lambda *| dir +| delta *| n
                val dir = (0.0, 0.0, 0.0) -| light
            in
                let val (l, _) = intersect (orig, dir) scene in
                    if l >= infinity then g else 0.0
                end end end end

(* Recursively build the scene tree *)
fun create level r (x, y, z) =
    let val obj = Sphere ((x, y, z), r) in
        if level = 1 then obj else
        let val r' = 3.0 * r / Real.Math.sqrt 12.0 in
            let fun aux x' z' =
                    create (level-1) (0.5 * r) (x-x', y+r', z+z') in
                let val objs = [aux (~r') (~r'), aux r' (~r'),
                                aux (~r') r', aux r' r', obj] in
                    Group ((x, y, z), 3.0 * r, objs)
                    end end end end

(* Build a scene and trace many rays into it, outputting a PGM image *)
val () =
    let
        val level = 6 (* Number of levels of spheres *)
        val ss = 4 (* Oversampling *)
        (* Resolution *)
        val n = case CommandLine.arguments () of
                    [s] => (case Int.fromString s of
                                SOME n => n | _ => 256)
                  | _ => 256
        val scene = create level 1.0 (0.0, ~1.0, 0.0) (* Scene tree *)
    in
        (fn s => print ("P5\n"^s^" "^s^"\n255\n")) (Int.toString n);
        for (0, n, fn y => for (0, n, fn x =>
            let val g = ref 0.0 in
                for (0, ss, fn dx => for (0, ss, fn dy =>
                    let val n = real n
                        val x = x + dx / real ss - n / 2.0
                        val y = n - 1.0 - y + dy / real ss - n / 2.0 in
                        let val eye = unitise (~1.0, ~3.0, 2.0)
                            val ray = ((0.0, 0.0, ~4.0),
                                       unitise (x, y, n)) in
                            g := !g + ray_trace eye ray scene
                        end
                    end));
                let val g = 0.5 + 255.0 * !g / real (ss*ss) in
                    print (String.str(Char.chr(Real.trunc g)))
                end
            end))
    end

Also, the C++ code includes some optimisations not in the C code because it
already exceeded the 100 LOC limit of the shootout. Specifically,
specialised calls for shadow rays. This shaves off another 30% or so.

(* The Great Computer Language Shootout
   http://shootout.alioth.debian.org/
   Contributed by Jon Harrop, 2005
   Compile: ocamlopt -inline 100 ray.ml -o ray *)

(* This implementation differs from the original in several ways:

   Uses an implicit scene, generated as a ray is traced rather than being
   precalculated and stored explicitly in a tree.

   Specialized shadow-ray intersection functions. *)

let delta = sqrt epsilon_float and pi = 4. *. atan 1.

(* 3D vector and associated functions *)
type vec = {x:float; y:float; z:float}
let ( *| ) s r = {x = s *. r.x; y = s *. r.y; z = s *. r.z}
let ( +| ) a b = {x = a.x +. b.x; y = a.y +. b.y; z = a.z +. b.z}
let ( -| ) a b = {x = a.x -. b.x; y = a.y -. b.y; z = a.z -. b.z}
let dot a b = a.x *. b.x +. a.y *. b.y +. a.z *. b.z
let unitise r = (1. /. sqrt (dot r r)) *| r

(* A semi-infinite ray starting at "orig" and with direction "dir". *)
type ray = { orig: vec; dir: vec }

(* Calculate the parametric intersection of the given ray with the given
sphere. *)
let ray_sphere orig dir center radius =
  let v = center -| orig in
  let b = dot v dir in
  let disc = b *. b -. dot v v +. radius *. radius in
  if disc < 0. then infinity else
    let disc = sqrt disc in
    (fun t2 -> if t2 < 0. then infinity else
       ((fun t1 -> if t1 > 0. then t1 else t2) (b -. disc))) (b +. disc)

(* Calculate whether or not the given ray intersects the given sphere. *)
let ray_sphere' orig dir center radius =
  let v = center -| orig in
  let b = dot v dir in
  let disc = b *. b -. dot v v +. radius *. radius in
  if disc < 0. then false else b +. sqrt disc >= 0.

(* Ratio of the radii of one level of spheres to the next. *)
let s = 6. /. sqrt 12.

(* Find the first intersection point of the given ray with the scene. *)
let intersect level orig dir =
  let rec of_scene center radius lambda normal level =
    if level = 1 then
      let lambda' = ray_sphere orig dir center radius in
      if lambda' >= lambda then lambda, normal else
        lambda', unitise (orig +| lambda' *| dir -| center)
    else
      if ray_sphere orig dir center (3. *. radius) >= lambda
      then lambda, normal else
        let accu = of_scene center radius lambda normal 1 in
        let r = 0.5 *. radius and l = level - 1 in let r' = s *. r in
        let aux dx dz (lambda, normal) =
          of_scene (center +| {x=dx; y=r'; z=dz}) r lambda normal l in
        let mr' = -.r' in
        aux r' mr' (aux r' r' (aux mr' r' (aux mr' mr' accu))) in
  of_scene {x=0.; y= -1.; z=0.} 1. infinity {x=0.; y=0.; z=0.} level

(* Find if the given ray intersects the scene. *)
let intersect' level orig dir =
  let rec of_scene center radius level =
    if level = 1 then ray_sphere' orig dir center radius else
      (* Exploit short-circuit evaluation of boolean comparisons to
terminate
         this function early. *)
      ray_sphere' orig dir center (3. *. radius) &&
        (of_scene center radius 1 ||
           let r = 0.5 *. radius and l = level - 1 in let r' = s *. r in
           of_scene (center +| {x= -.r'; y=r'; z= -.r'}) r l ||
             of_scene (center +| {x= r'; y=r'; z= -.r'}) r l ||
             of_scene (center +| {x= -.r'; y=r'; z= r'}) r l ||
             of_scene (center +| {x= r'; y=r'; z= r'}) r l) in
  of_scene {x=0.; y= -1.; z=0.} 1. level

(* Trace a single ray by casting it into the scene and, if it intersects
   anything, casting a second ray toward the light to determine occlusion.
*)
let rec ray_trace l light orig dir =
  let lambda, n = intersect l orig dir in
  if lambda = infinity then 0. else
    let g = -. dot n light in
    (* If we are on the shadowed side of a sphere then don't bother casting
a
       shadow ray as we know it will intersect the same sphere. *)
    if g <= 0. then 0. else
      let p = orig +| lambda *| dir +| delta *| n in
      if intersect' l p ({x=0.; y=0.; z=0.} -| light) then 0. else g

(* Ray trace the scene at the given resolution. *)
let () =
  (* Resolution *)
  let n = match Sys.argv with [| _; l |] -> int_of_string l | _ -> 256 in
  (* Light direction *)
  let light = unitise {x= -1.; y= -3.; z=2.} in
  (* Number of levels of spheres, and oversampling. *)
  let level = 6 and ss = 4 in

  Printf.printf "P5\n%d %d\n255\n" n n;
  for y = n - 1 downto 0 do
    for x = 0 to n - 1 do
      (* Average each pixel over ss*ss separate rays. *)
      let g = ref 0. in
      for dx = 0 to ss - 1 do
        for dy = 0 to ss - 1 do
          (* Calculate the origin and direction of this ray. *)
          let orig = {x=0.; y=0.; z= -4.} in
          let dir = unitise {x = float (x - n / 2) +. float dx /. float ss;
                             y = float (y - n / 2) +. float dy /. float ss;
                             z = float n} in
          g := !g +. ray_trace level light orig dir
        done
      done;
      let g = int_of_float (0.5 +. 255. *. !g /. float (ss*ss)) in
      Printf.printf "%c" (char_of_int g)
    done
  done

Here's the C++:

// The Great Computer Language Shootout
// http://shootout.alioth.debian.org/
// Contributed by Jon Harrop, 2005
// Compile: g++ -Wall -O3 -ffast-math ray.cpp -o ray

#include <vector>
#include <iostream>
#include <limits>
#include <cmath>

using namespace std;

numeric_limits<double> real;
double delta = sqrt(real.epsilon()), infinity = real.infinity(), pi = M_PI;

// 3D vector
struct Vec {
  double x, y, z;
  Vec(double x2, double y2, double z2) : x(x2), y(y2), z(z2) {}

// Semi-infinite ray
struct Ray { Vec orig, dir; Ray(Vec o, Vec d) : orig(o), dir(d) {} };

// Scene tree
// In this implementation, a node in the scene tree is represented by a
single
// struct which is either a group of scene trees with a spherical bound or,
// implicitly, a single sphere if the group has no children.
// This is not equivalent to the variant type used to represent a node in
the
// original OCaml implementation because this representation cannot
associate
// data with only leaf nodes (such as color, reflectivity etc.) but requires
// significantly less C++ code and gives room to implement a specialized
// shadow-ray intersection algorithm.
struct Scene {
  vector<Scene> child; // Child nodes in the scene tree
  Vec center; // Center of the sphere or spherical bound
  double radius; // Radius of the sphere or spherical bound

  Scene(Vec c, double r) : child(), center(c), radius(r) {}

fun s *| (x, y, z) : vec = (s*x, s*y, s*z)

This won't have an effect on performance, but the above style of
let-nesting (used throughout the code), which is probably a result of
a conservative translating from OCaml, is not necessary in SML. In
SML, you can flatten the nested let-expressions. In other words, you
can flatten an expression of the form

  let val ... in
  let val ... in
    ...
  let val in ... end ... end end

into

  let val ...
      val ...
      ...
  in ... end