diff --git a/CMakeLists.txt b/CMakeLists.txt
index 87b495a0..a6600f9e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -276,6 +276,7 @@ list (
     geom/iostream.hpp
     geom/line.hpp
     geom/line.cpp
+    geom/ops.cpp
     geom/ops.hpp
     geom/plane.cpp
     geom/plane.hpp
@@ -283,8 +284,9 @@ list (
     geom/ray.hpp
     geom/rect.cpp
     geom/rect.hpp
-    geom/sample.cpp
-    geom/sample.hpp
+    geom/sample/fwd.hpp
+    geom/sample/surface.hpp
+    geom/sample/volume.hpp
     geom/segment.cpp
     geom/segment.hpp
     geom/sphere.cpp
diff --git a/geom/aabb.hpp b/geom/aabb.hpp
index b7a6c3ad..cbd7fa72 100644
--- a/geom/aabb.hpp
+++ b/geom/aabb.hpp
@@ -138,23 +138,35 @@ namespace cruft::geom {
 
 
 ///////////////////////////////////////////////////////////////////////////////
-#include "./sample.hpp"
+#include "sample/fwd.hpp"
 
 #include <random>
 
-namespace cruft::geom {
+namespace cruft::geom::sample {
     template <size_t S, typename T>
-    struct sampler<aabb<S,T>> {
+    class volume<aabb<S,T>> {
+    public:
+        using shape_type = aabb<S,T>;
+
+        volume (shape_type const &target):
+            m_target (target)
+        { ; }
+
+
         template <typename GeneratorT>
-        static auto
-        eval (aabb<S,T> shape, GeneratorT &g)
+        auto
+        eval (GeneratorT &&g) const noexcept
         {
             point<S,T> res;
 
             for (size_t i = 0; i < S; ++i)
-                res[i] = cruft::random::uniform (shape.lo[i], shape.hi[i], g);
+                res[i] = cruft::random::uniform (m_target.lo[i], m_target.hi[i], g);
 
             return res;
         }
+
+
+    private:
+        shape_type const &m_target;
     };
 }
diff --git a/geom/ellipse.hpp b/geom/ellipse.hpp
index c98e2164..3a3bae3e 100644
--- a/geom/ellipse.hpp
+++ b/geom/ellipse.hpp
@@ -138,20 +138,29 @@ namespace cruft::geom {
 
 
 ///////////////////////////////////////////////////////////////////////////////
-#include "sample.hpp"
+#include "sample/fwd.hpp"
+
+#include "../random.hpp"
 
 #include <cmath>
 #include <random>
 
-namespace cruft::geom {
+namespace cruft::geom::sample {
     /// Specialisation for uniform sampling of ellipses
     template <typename T>
-    struct sampler<ellipse<2,T>>
-    {
+    class volume<ellipse<2,T>> {
+    public:
+        using shape_type = ellipse<2,T>;
+
+        volume (shape_type const &target):
+            m_target (target)
+        { ; }
+
+
         /// Generate a random point within the ellipse.
         template <typename GeneratorT>
-        static auto
-        eval (ellipse<2,T> shape, GeneratorT &&g)
+        auto
+        eval (GeneratorT &&g) const noexcept
         {
             // We use a two step process:
             //   * Generate a point within a unit sphere
@@ -177,9 +186,13 @@ namespace cruft::geom {
                 std::sin (phi)
             };
 
-            auto const offset = circle_pos * rho * shape.radius;
-            return shape.origin + offset.template as<cruft::vector> ();
+            auto const offset = circle_pos * rho * m_target.radius;
+            return m_target.origin + offset.template as<cruft::vector> ();
         }
+
+
+    private:
+        shape_type const &m_target;
     };
 
 
diff --git a/geom/sample.cpp b/geom/ops.cpp
similarity index 92%
rename from geom/sample.cpp
rename to geom/ops.cpp
index 19e6ea7c..333daff9 100644
--- a/geom/sample.cpp
+++ b/geom/ops.cpp
@@ -6,4 +6,4 @@
  * Copyright 2018 Danny Robson <danny@nerdcruft.net>
  */
 
-#include "sample.hpp"
+#include "ops.hpp"
diff --git a/geom/ops.hpp b/geom/ops.hpp
index d7dc2407..32b39293 100644
--- a/geom/ops.hpp
+++ b/geom/ops.hpp
@@ -6,8 +6,7 @@
  * Copyright 2015-2018 Danny Robson <danny@nerdcruft.net>
  */
 
-#ifndef __UTIL_GEOM_OPS_HPP
-#define __UTIL_GEOM_OPS_HPP
+#pragma once
 
 #include "./fwd.hpp"
 #include "../point.hpp"
@@ -17,6 +16,9 @@
 
 ///////////////////////////////////////////////////////////////////////////////
 namespace cruft::geom {
+    /// Tests whether any part of two shapes overlap.
+    ///
+    /// Returns true even if the  the edges of the shapes are co-linear.
     template <
         size_t S,
         typename T,
@@ -26,6 +28,8 @@ namespace cruft::geom {
     bool
     intersects (A<S,T>, B<S,T>);
 
+
+    /// Returns a minimum squared distance between two shapes.
     template <
         size_t S,
         typename T,
@@ -50,6 +54,8 @@ namespace cruft::geom {
     T
     distance (A<S,T>, B<S,T>);
 
+
+    /// Returns an AABB for the supplied shape.
     template <
         size_t S,
         typename T,
@@ -58,6 +64,8 @@ namespace cruft::geom {
     aabb<S,T>
     bounds (K<S,T>);
 
+
+    /// Returns a maximum distance across a shape.
     template <
         size_t S,
         typename T,
@@ -66,6 +74,8 @@ namespace cruft::geom {
     T
     diameter (K<S,T>);
 
+
+    /// Returns the closest point on a shape to the supplied point.
     template <
         size_t S,
         typename T,
@@ -74,6 +84,7 @@ namespace cruft::geom {
     point<S,T>
     closest (K<S,T>, point<S,T>);
 
+
     template <
         size_t S,
         typename T,
@@ -82,6 +93,7 @@ namespace cruft::geom {
     vector<S,T>
     magnitude (K<S,T>);
 
+
     template <
         size_t S,
         typename T,
@@ -90,5 +102,3 @@ namespace cruft::geom {
     K<S,T>
     scale (K<S,T>, T);
 }
-
-#endif
diff --git a/geom/sample.hpp b/geom/sample.hpp
deleted file mode 100644
index f73ba559..00000000
--- a/geom/sample.hpp
+++ /dev/null
@@ -1,175 +0,0 @@
-/*
- * This Source Code Form is subject to the terms of the Mozilla Public
- * License, v. 2.0. If a copy of the MPL was not distributed with this
- * file, You can obtain one at http://mozilla.org/MPL/2.0/.
- *
- * Copyright 2015-2018 Danny Robson <danny@nerdcruft.net>
- */
-
-#pragma once
-
-#include "../coord/fwd.hpp"
-#include "../extent.hpp"
-#include "../random.hpp"
-
-#include "ops.hpp"
-
-#include <cstddef>
-#include <random>
-
-
-///////////////////////////////////////////////////////////////////////////////
-namespace cruft::geom {
-    /// A function object that selects a uniformly random point inside a shape
-    /// using a provided random generator. The point will lie within the shape,
-    /// inclusive of boundaries.
-    ///
-    /// May be specialised for arbitrary shapes but uses rejection sampling
-    /// as a safe default. This implies that execution may not take a constant
-    /// time.
-    template <typename ShapeT>
-    struct sampler {
-        /// Returns a point which lies within the supplied shape, inclusive
-        /// of borders.
-        template <typename GeneratorT>
-        static auto
-        eval (ShapeT const &shape, GeneratorT &&g)
-        {
-            auto b = bounds (shape);
-
-            while (true) {
-                auto p = sample (b, g);
-                if (intersects (shape, p))
-                    return p;
-            }
-        }
-    };
-
-
-    ///////////////////////////////////////////////////////////////////////////
-    /// A convenience function that calls sample::fn to select a random point
-    /// in a provided shape.
-    template <
-        typename ShapeT,
-        typename GeneratorT  // random generator
-    >
-    auto
-    sample (ShapeT const &shape, GeneratorT &&gen)
-    {
-        return sampler<ShapeT>::eval (shape, std::forward<GeneratorT> (gen));
-    }
-
-
-    ///////////////////////////////////////////////////////////////////////////
-    std::vector<cruft::point2f>
-    poisson_sample (cruft::extent2i, float distance, int samples);
-
-
-    ///////////////////////////////////////////////////////////////////////////
-    namespace surface {
-        /// A generator of samples that lie on the surface of a shape
-        template <typename ShapeT>
-        class sampler;
-
-
-        template <typename ShapeT>
-        sampler (ShapeT const&) -> sampler<std::decay_t<ShapeT>>;
-
-
-        //---------------------------------------------------------------------
-        template <size_t S, typename T>
-        class sampler<cruft::extent<S,T>> {
-        public:
-            sampler (cruft::extent<S,T> _target):
-                target (_target)
-            { ; }
-
-            template <typename GeneratorT>
-            cruft::point<S,T>
-            operator() (GeneratorT &&gen) const noexcept
-            {
-                cruft::point<S,T> p;
-
-                for (size_t i = 0; i < S; ++i)
-                    p[i] = random::uniform (T{0}, target[i], gen);
-
-                return p;
-            }
-
-        private:
-            cruft::extent<S,T> target;
-        };
-
-
-        /// Approximate a poisson disc sampling through the "Mitchell's Best
-        /// Candidate" algorithm.
-        ///
-        /// Try to keep adding a new point to a set. Each new point is the
-        /// best of a set of candidates. The 'best' is the point that is
-        /// furthest from all selected points.
-        ///
-        /// \return A vector of the computed points
-        template <typename SamplerT, typename DistanceT, typename GeneratorT>
-        auto
-        poisson (SamplerT const &target,
-                 GeneratorT &&gen,
-                 DistanceT &&minimum_distance,
-                 size_t candidates_count)
-
-        {
-            using point_type = decltype (target (gen));
-            using value_type = typename point_type::value_type;
-
-            std::vector<point_type> selected;
-            std::vector<point_type> candidates;
-
-            // prime the found elements list with an initial point we can
-            // perform distance calculations on
-            selected.push_back (target (gen));
-
-            // keep trying to add one more new point
-            while (1) {
-                // generate a group of candidate points
-                candidates.clear ();
-                std::generate_n (
-                    std::back_inserter (candidates),
-                    candidates_count,
-                    [&] (void) {
-                        return target (gen);
-                    }
-                );
-
-                // find the point whose minimum distance to the existing
-                // points is the greatest (while also being greater than the
-                // required minimum distance);
-                auto best_distance2 = std::numeric_limits<value_type>::lowest ();
-                size_t best_index = 0;
-
-                for (size_t i = 0; i < candidates.size (); ++i) {
-                    auto const p = candidates[i];
-                    auto d2 = std::numeric_limits<value_type>::max ();
-
-                    // find the minimum distance from this candidate to the
-                    // selected points
-                    for (auto q: selected)
-                        d2 = cruft::min (d2, cruft::distance2 (p, q));
-
-                    // record if it's the furthest away
-                    if (d2 > best_distance2 && d2 > cruft::pow (minimum_distance (p), 2)) {
-                        best_distance2 = d2;
-                        best_index = i;
-                    }
-                }
-
-                // if we didn't find a suitable point then we give up and
-                // return the points we found, otherwise record the best point
-                if (best_distance2 <= 0)
-                    break;
-
-                selected.push_back (candidates[best_index]);
-            }
-
-            return selected;
-        }
-    }
-}
diff --git a/geom/sample/fwd.hpp b/geom/sample/fwd.hpp
new file mode 100644
index 00000000..e5a05373
--- /dev/null
+++ b/geom/sample/fwd.hpp
@@ -0,0 +1,33 @@
+/*
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ *
+ * Copyright 2015-2018 Danny Robson <danny@nerdcruft.net>
+ */
+
+#pragma once
+
+namespace cruft::geom::sample {
+    /// Provides an interface to randomly sample points within the volume of
+    /// a supplied shape.
+    ///
+    /// This class _should_ be specialised for performance. Though a basic
+    /// rejection sampling implementation is defined as a fallback.
+    template <typename ShapeT>
+    class volume;
+
+    template <typename ShapeT>
+    volume (ShapeT const&) -> volume<std::decay_t<ShapeT>>;
+
+
+    /// Provides an interface to randomly sample points across the surface of
+    /// a supplied shape.
+    ///
+    /// The class _must_ be specialised for each shape.
+    template <typename ShapeT>
+    class surface;
+
+    template <typename ShapeT>
+    surface (ShapeT const&) -> surface<std::decay_t<ShapeT>>;
+}
diff --git a/geom/sample/surface.hpp b/geom/sample/surface.hpp
new file mode 100644
index 00000000..3653ac0b
--- /dev/null
+++ b/geom/sample/surface.hpp
@@ -0,0 +1,105 @@
+/*
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ *
+ * Copyright 2015-2018 Danny Robson <danny@nerdcruft.net>
+ */
+
+#pragma once
+
+#include "volume.hpp"
+
+#include "../ops.hpp"
+
+#include "../../coord/fwd.hpp"
+#include "../../extent.hpp"
+#include "../../random.hpp"
+
+#include <cstddef>
+#include <random>
+
+
+///////////////////////////////////////////////////////////////////////////////
+namespace cruft::geom::sample {
+    /// Approximate a poisson disc sampling through the "Mitchell's Best
+    /// Candidate" algorithm.
+    ///
+    /// Try to keep adding a new point to a set. Each new point is the
+    /// best of a set of candidates. The 'best' is the point that is
+    /// furthest from all selected points.
+    ///
+    /// \return A vector of the computed points
+    template <typename SamplerT, typename DistanceT, typename GeneratorT>
+    auto
+    poisson (SamplerT const &target,
+             GeneratorT &&gen,
+             DistanceT &&minimum_distance,
+             size_t candidates_count)
+
+    {
+        using point_type = decltype (target.eval (gen));
+        using value_type = typename point_type::value_type;
+
+        std::vector<point_type> selected;
+        std::vector<point_type> candidates;
+
+        // prime the found elements list with an initial point we can
+        // perform distance calculations on
+        selected.push_back (target.eval (gen));
+
+        // keep trying to add one more new point
+        while (1) {
+            // generate a group of candidate points
+            candidates.clear ();
+            std::generate_n (
+                std::back_inserter (candidates),
+                candidates_count,
+                [&] (void) {
+                    return target.eval (gen);
+                }
+            );
+
+            // find the point whose minimum distance to the existing
+            // points is the greatest (while also being greater than the
+            // required minimum distance);
+            auto best_distance2 = std::numeric_limits<value_type>::lowest ();
+            size_t best_index = 0;
+
+            for (size_t i = 0; i < candidates.size (); ++i) {
+                auto const p = candidates[i];
+                auto d2 = std::numeric_limits<value_type>::max ();
+
+                // find the minimum distance from this candidate to the
+                // selected points
+                for (auto q: selected)
+                    d2 = cruft::min (d2, cruft::distance2 (p, q));
+
+                // record if it's the furthest away
+                if (d2 > best_distance2 && d2 > cruft::pow (minimum_distance (p), 2)) {
+                    best_distance2 = d2;
+                    best_index = i;
+                }
+            }
+
+            // if we didn't find a suitable point then we give up and
+            // return the points we found, otherwise record the best point
+            if (best_distance2 <= 0)
+                break;
+
+            selected.push_back (candidates[best_index]);
+        }
+
+        return selected;
+    }
+
+
+    /// A surface sampler specialisation for 2d extents.
+    ///
+    /// The actual work is handed off to the volume sampler, as it's
+    /// equivalent in 2 dimensions.
+    template <typename T>
+    class surface<extent<2,T>> :
+        public volume<extent<2,T>>
+    { using volume<extent<2,T>>::volume; };
+}
diff --git a/geom/sample/volume.hpp b/geom/sample/volume.hpp
new file mode 100644
index 00000000..95dec1ec
--- /dev/null
+++ b/geom/sample/volume.hpp
@@ -0,0 +1,105 @@
+/*
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ *
+ * Copyright 2015-2018 Danny Robson <danny@nerdcruft.net>
+ */
+
+#pragma once
+
+#include "../../coord/fwd.hpp"
+#include "../../extent.hpp"
+#include "../../random.hpp"
+
+#include "../ops.hpp"
+
+#include <cstddef>
+#include <random>
+
+
+///////////////////////////////////////////////////////////////////////////////
+namespace cruft::geom::sample {
+    ///////////////////////////////////////////////////////////////////////
+    /// A convenience function that calls sample::fn to select a random
+    /// point in a provided shape.
+    ///
+    /// This function is useful because we can perform use partial
+    /// specialisation if we pass the functionality off to a well known
+    /// class.
+    template <
+        typename ShapeT,
+        typename GeneratorT  // random generator
+    >
+    auto
+    eval (ShapeT const &shape, GeneratorT &&gen)
+    {
+        volume query (shape);
+        return query.eval (std::forward<GeneratorT> (gen));
+    }
+
+
+    /// A function object that selects a uniformly random point inside a
+    /// shape using a provided random generator. The point will lie
+    /// within the shape, inclusive of boundaries.
+    ///
+    /// This should be specialised for anything complex, but will work
+    /// (with a varying performance hit) for anything that provides
+    /// bounds` and `intersects`.
+    template <typename ShapeT>
+    class volume {
+    public:
+        explicit volume (ShapeT const &target):
+            m_target (target)
+        { ; }
+
+
+        /// Returns a point which lies within the supplied shape, inclusive
+        /// of borders.
+        ///
+        template <typename GeneratorT>
+        auto
+        eval (GeneratorT &&g) const
+        {
+            auto b = bounds (m_target);
+
+            while (true) {
+                auto p = ::cruft::geom::sample::eval (b, g);
+                if (intersects (m_target, p))
+                    return p;
+            }
+        }
+
+    private:
+        ShapeT const &m_target;
+    };
+
+
+    ///////////////////////////////////////////////////////////////////////
+    /// A specialisation of the volume sampler for extents.
+    template <size_t S, typename T>
+    class volume<cruft::extent<S,T>> {
+    public:
+        using shape_type = extent<S,T>;
+
+        explicit volume (shape_type const &target):
+            m_target (target)
+        { ; }
+
+
+        template <typename GeneratorT>
+        auto
+        eval (GeneratorT &&gen) const noexcept
+        {
+            cruft::point<S,T> p;
+
+            for (size_t i = 0; i < S; ++i)
+                p[i] = random::uniform (T{0}, m_target[i], gen);
+
+            return p;
+        }
+
+    private:
+        shape_type const &m_target;
+    };
+}
\ No newline at end of file
diff --git a/geom/tri.hpp b/geom/tri.hpp
index acd69f8b..19a1d9a7 100644
--- a/geom/tri.hpp
+++ b/geom/tri.hpp
@@ -10,7 +10,7 @@
 #define CRUFT_GEOM_TRI_HPP
 
 #include "../point.hpp"
-#include "sample.hpp"
+#include "sample/fwd.hpp"
 
 #include <cstddef>
 #include <random>
@@ -33,11 +33,13 @@ namespace cruft::geom {
     using tri3f = tri<3,float>;
 
 
-    namespace surface {
+    namespace sample {
         template <size_t S, typename T>
-        class sampler<tri<S,T>> {
+        class surface<tri<S,T>> {
         public:
-            sampler (tri<S,T> _target):
+            using shape_type = tri<S,T>;
+
+            explicit surface (tri<S,T> _target):
                 base (_target.a),
                 v0 (_target.b - _target.a),
                 v1 (_target.c - _target.a)
@@ -53,7 +55,8 @@ namespace cruft::geom {
 
         private:
             cruft::point<S,T> base;
-            cruft::vector<S,T> v0, v1;
+            cruft::vector<S,T> v0;
+            cruft::vector<S,T> v1;
         };
     };
 
diff --git a/tools/poisson.cpp b/tools/poisson.cpp
index 07aa3539..fb363f50 100644
--- a/tools/poisson.cpp
+++ b/tools/poisson.cpp
@@ -1,7 +1,7 @@
 #include "cmdopt.hpp"
 #include "functor.hpp"
 #include "geom/aabb.hpp"
-#include "geom/sample.hpp"
+#include "geom/sample/surface.hpp"
 
 #include <cstdlib>
 #include <iostream>
@@ -25,10 +25,11 @@ main (int argc, char **argv)
     opts.scan (argc, argv);
 
     std::cout << "<svg height='" << area.h << "' width='" << area.h << "'>";
-    for (auto p: cruft::geom::surface::poisson (cruft::geom::surface::sampler (area),
-                                               std::default_random_engine (),
-                                               cruft::functor::constant (distance),
-                                               samples))
+    namespace sample = cruft::geom::sample;
+    for (auto const &p: sample::poisson (sample::surface {area},
+                                         std::default_random_engine (),
+                                         cruft::functor::constant (distance),
+                                         samples))
     {
         std::cout << "<circle cx='" << p.x << "' cy='" << p.y << "' r='1' />";
     }