#+title: Simulated Sense of Touch

 #+author: Robert McIntyre

 #+email: rlm@mit.edu

 #+description: Simulated touch for AI research using JMonkeyEngine and clojure.

 #+keywords: simulation, tactile sense, jMonkeyEngine3, clojure

 #+SETUPFILE: ../../aurellem/org/setup.org

 #+INCLUDE: ../../aurellem/org/level-0.org

 

 

 

 * Touch

 

 Touch is critical to navigation and spatial reasoning and as such I

 need a simulated version of it to give to my AI creatures.

 

 However, touch in my virtual can not exactly correspond to human touch

 because my creatures are made out of completely rigid segments that

 don't deform like human skin.

 

 Human skin has a wide array of touch sensors, each of which speciliaze

 in detecting different vibrational modes and pressures. These sensors

 can integrate a vast expanse of skin (i.e. your entire palm), or a

 tiny patch of skin at the tip of your finger. The hairs of the skin

 help detect objects before they even come into contact with the skin

 proper. 

 

 Instead of measuring deformation or vibration, I surround each rigid

 part with a plenitude of hair-like objects which do not interact with

 the physical world. Physical objects can pass through them with no

 effect. The hairs are able to measure contact with other objects, and

 constantly report how much of their extent is covered. So, even though

 the creature's body parts do not deform, the hairs create a margin

 around those body parts which achieves a sense of touch which is a

 hybrid between a human's sense of deformation and sense from hairs.

 

 Implementing touch in jMonkeyEngine follows a different techinal route

 than vision and hearing. Those two senses piggybacked off

 jMonkeyEngine's 3D audio and video rendering subsystems. To simulate

 Touch, I use jMonkeyEngine's physics system to execute many small

 collision detections, one for each "hair". The placement of the

 "hairs" is determined by a UV-mapped image which shows where each hair

 should be on the 3D surface of the body.

 

 

 * Defining Touch Meta-Data in Blender

 

 Each geometry can have a single UV map which describes the position

 and length of the "hairs" which will constitute its sense of

 touch. This image path is stored under the "touch" key.  The image

 itself is grayscale, with black meaning a hair length of 0 (no hair is

 present) and white meaning a hair length of =scale=, which is a float

 stored under the key "scale". If the pixel is gray then the resultant

 hair length is linearly interpolated between 0 and =scale=.

 

 #+begin_src clojure

 (defn tactile-sensor-profile

   "Return the touch-sensor distribution image in BufferedImage format,

    or nil if it does not exist."

   [#^Geometry obj]

   (if-let [image-path (meta-data obj "touch")]

     (load-image image-path)))

 #+end_src

 

 

 ** TODO add image showing example touch-uv map

 ** TODO add metadata display for worm

 

 * Triangle Manipulation Functions

 

 The rigid bodies which make up a creature have an underlying

 =Geometry=, which is a =Mesh= plus a =Material= and other important

 data involved with displaying the body. 

 

 A =Mesh= is composed of =Triangles=, and each =Triangle= has three

 verticies which have coordinates in XYZ space and UV space.

  

 Here, =(triangles)= gets all the triangles which compose a mesh, and

 =(triangle-UV-coord)= returns the the UV coordinates of the verticies

 of a triangle.

 

 #+begin_src clojure

 (defn triangles

   "Return a sequence of all the Triangles which compose a given

    Geometry." 

   [#^Geometry geom]

   (let

       [mesh (.getMesh geom)

        triangles (transient [])]

     (dorun

      (for [n (range (.getTriangleCount mesh))]

        (let [tri (Triangle.)]

          (.getTriangle mesh n tri)

         ;; (.calculateNormal tri)

         ;; (.calculateCenter tri)

          (conj! triangles tri))))

     (persistent! triangles)))

    

 (defn mesh-triangle

   "Get the triangle specified by triangle-index from the mesh within

   bounds."

   [#^Mesh mesh triangle-index]

   (let [scratch (Triangle.)]

     (.getTriangle mesh triangle-index scratch)

     scratch))

 

 (defn triangle-vertex-indices

   "Get the triangle vertex indices of a given triangle from a given

    mesh."

   [#^Mesh mesh triangle-index]

   (let [indices (int-array 3)]

     (.getTriangle mesh triangle-index indices)

     (vec indices)))

 

 (defn vertex-UV-coord

   "Get the UV-coordinates of the vertex named by vertex-index"

   [#^Mesh mesh vertex-index]

   (let [UV-buffer

         (.getData

          (.getBuffer

           mesh

           VertexBuffer$Type/TexCoord))]

     [(.get UV-buffer (* vertex-index 2))

      (.get UV-buffer (+ 1 (* vertex-index 2)))]))

 

 (defn triangle-UV-coord

   "Get the UV-cooridnates of the triangle's verticies."

   [#^Mesh mesh width height triangle-index]

   (map (fn [[u v]] (vector (* width u) (* height v)))

        (map (partial vertex-UV-coord mesh)

             (triangle-vertex-indices mesh triangle-index))))

 #+end_src

 

 * Schrapnel Conversion Functions

 

 It is convienent to treat a =Triangle= as a sequence of verticies, and

 a =Vector2f= and =Vector3f= as a sequence of floats. These conversion

 functions make this easy. If these classes implemented =Iterable= then

 this code would not be necessary. Hopefully they will in the future.

 

 #+begin_src clojure

 (defn triangle-seq [#^Triangle tri]

   [(.get1 tri) (.get2 tri) (.get3 tri)])

 

 (defn vector3f-seq [#^Vector3f v]

   [(.getX v) (.getY v) (.getZ v)])

 

 (defn point->vector2f [[u v]]

   (Vector2f. u v))

 

 (defn vector2f->vector3f [v]

   (Vector3f. (.getX v) (.getY v) 0))

 

 (defn map-triangle [f #^Triangle tri]

   (Triangle.

    (f 0 (.get1 tri))

    (f 1 (.get2 tri))

    (f 2 (.get3 tri))))

 

 (defn points->triangle

   "Convert a list of points into a triangle."

   [points]

   (apply #(Triangle. %1 %2 %3)

          (map (fn [point]

                 (let [point (vec point)]

                   (Vector3f. (get point 0 0)

                              (get point 1 0)

                              (get point 2 0))))

               (take 3 points))))

 #+end_src

 

 * Triangle Affine Transforms

 

 The position of each hair is stored in a 2D image in UV

 coordinates. To place the hair in 3D space we must convert from UV

 coordinates to XYZ coordinates. Each =Triangle= has coordinates in

 both UV-space and XYZ-space, which defines a unique [[http://mathworld.wolfram.com/AffineTransformation.html ][Affine Transform]]

 for translating any coordinate within the UV triangle to the

 cooresponding coordinate in the XYZ triangle.

 

 #+begin_src clojure

 (defn triangle->matrix4f

   "Converts the triangle into a 4x4 matrix: The first three columns

    contain the vertices of the triangle; the last contains the unit

    normal of the triangle. The bottom row is filled with 1s."

   [#^Triangle t]

   (let [mat (Matrix4f.)

         [vert-1 vert-2 vert-3]

         ((comp vec map) #(.get t %) (range 3))

         unit-normal (do (.calculateNormal t)(.getNormal t))

         vertices [vert-1 vert-2 vert-3 unit-normal]]

     (dorun 

      (for [row (range 4) col (range 3)]

        (do

          (.set mat col row (.get (vertices row)col))

          (.set mat 3 row 1))))

     mat))

 

 (defn triangle-transformation

   "Returns the affine transformation that converts each vertex in the

    first triangle into the corresponding vertex in the second

    triangle."

   [#^Triangle tri-1 #^Triangle tri-2]

   (.mult 

    (triangle->matrix4f tri-2)

    (.invert (triangle->matrix4f tri-1))))

 #+end_src

 

 * Triangle Boundaries

   

 For efficiency's sake I will divide the UV-image into small squares

 which inscribe each UV-triangle, then extract the points which lie

 inside the triangle and map them to 3D-space using

 =(triangle-transform)= above. To do this I need a function,

 =(inside-triangle?)=, which determines whether a point is inside a

 triangle in 2D UV-space.

 

 #+begin_src clojure

 (defn convex-bounds

   "Returns the smallest square containing the given vertices, as a

    vector of integers [left top width height]."

   [uv-verts]

   (let [xs (map first uv-verts)

         ys (map second uv-verts)

         x0 (Math/floor (apply min xs))

         y0 (Math/floor (apply min ys))

         x1 (Math/ceil (apply max xs))

         y1 (Math/ceil (apply max ys))]

     [x0 y0 (- x1 x0) (- y1 y0)]))

 

 (defn same-side?

   "Given the points p1 and p2 and the reference point ref, is point p

   on the same side of the line that goes through p1 and p2 as ref is?" 

   [p1 p2 ref p]

   (<=

    0

    (.dot 

     (.cross (.subtract p2 p1) (.subtract p p1))

     (.cross (.subtract p2 p1) (.subtract ref p1)))))

 

 (defn inside-triangle?

   "Is the point inside the triangle?"

   {:author "Dylan Holmes"}

   [#^Triangle tri #^Vector3f p]

   (let [[vert-1 vert-2 vert-3] (triangle-seq tri)]

     (and

      (same-side? vert-1 vert-2 vert-3 p)

      (same-side? vert-2 vert-3 vert-1 p)

      (same-side? vert-3 vert-1 vert-2 p))))

 #+end_src

 

 

 

 * Sensor Related Functions

 

 These functions analyze the touch-sensor-profile image convert the

 location of each touch sensor from pixel coordinates to UV-coordinates

 and XYZ-coordinates.

 

 #+begin_src clojure

 (defn sensors-in-triangle

   "Locate the touch sensors in the triangle, returning a map of their

    UV and geometry-relative coordinates."

   [image mesh tri-index]

   (let [width (.getWidth image)

         height (.getHeight image)

         UV-vertex-coords (triangle-UV-coord mesh width height tri-index)

         bounds (convex-bounds UV-vertex-coords)

         

         cutout-triangle (points->triangle UV-vertex-coords)

         UV-sensor-coords

         (filter (comp (partial inside-triangle? cutout-triangle)

                       (fn [[u v]] (Vector3f. u v 0)))

                 (white-coordinates image bounds))

         UV->geometry (triangle-transformation

                       cutout-triangle

                       (mesh-triangle mesh tri-index))

         geometry-sensor-coords

         (map (fn [[u v]] (.mult UV->geometry (Vector3f. u v 0)))

              UV-sensor-coords)]

   {:UV UV-sensor-coords :geometry geometry-sensor-coords}))

 

 (defn-memo locate-feelers

   "Search the geometry's tactile UV profile for touch sensors,

    returning their positions in geometry-relative coordinates."

   [#^Geometry geo]

   (let [mesh (.getMesh geo)

         num-triangles (.getTriangleCount mesh)]

     (if-let [image (tactile-sensor-profile geo)]

       (map

        (partial sensors-in-triangle image mesh)

        (range num-triangles))

       (repeat (.getTriangleCount mesh) {:UV nil :geometry nil}))))

 

 (defn-memo touch-topology

   "Return a sequence of vectors of the form [x y] describing the

    \"topology\" of the tactile sensors. Points that are close together

    in the touch-topology are generally close together in the simulation."

   [#^Gemoetry geo]

   (vec (collapse (reduce concat (map :UV (locate-feelers geo))))))

 

 (defn-memo feeler-coordinates

   "The location of the touch sensors in world-space coordinates."

   [#^Geometry geo]

   (vec (map :geometry (locate-feelers geo))))

 #+end_src

 

 * Physics Collision Objects

 #+begin_src clojure

 (defn get-ray-origin

   "Return the origin which a Ray would have to have to be in the exact

   center of a particular Triangle in the Geometry in World

   Coordinates."

   [geom tri]

   (let [new (Vector3f.)]

     (.calculateCenter tri)

     (.localToWorld geom (.getCenter tri) new) new))

 

 (defn get-ray-direction

   "Return the direction which a Ray would have to have to be to point

   normal to the Triangle, in coordinates relative to the center of the

   Triangle."

   [geom tri]

   (let [n+c (Vector3f.)]

     (.calculateNormal tri)

     (.calculateCenter tri)

     (.localToWorld

      geom

      (.add (.getCenter tri) (.getNormal tri)) n+c)

     (.subtract n+c (get-ray-origin geom tri))))

 #+end_src

 

 

 * Skin Creation

 

 #+begin_src clojure

 (defn touch-fn

   "Returns a function which returns tactile sensory data when called

    inside a running simulation."

   [#^Geometry geo]

   (let [feeler-coords (feeler-coordinates geo)

         tris (triangles geo)

         limit 0.1

         ;;results (CollisionResults.)

         ]

     (if (empty? (touch-topology geo))

       nil

       (fn [node]

         (let [sensor-origins 

               (map

                #(map (partial local-to-world geo) %)

                feeler-coords)

               triangle-normals 

               (map (partial get-ray-direction geo)

                    tris)

               rays

               (flatten

                (map (fn [origins norm]

                       (map #(doto (Ray. % norm)

                               (.setLimit limit)) origins))

                     sensor-origins triangle-normals))]

           (vector

            (touch-topology geo)

            (vec

             (for [ray rays]

               (do

                 (let [results (CollisionResults.)]

                   (.collideWith node ray results)

                   (let [touch-objects

                         (filter #(not (= geo (.getGeometry %)))

                                 results)]

                     (- 255

                        (if (empty? touch-objects) 255

                            (rem 

                             (int

                              (* 255 (/ (.getDistance

                                         (first touch-objects)) limit)))

                             256))))))))))))))

   

 (defn touch! 

   "Endow the creature with the sense of touch. Returns a sequence of

    functions, one for each body part with a tactile-sensor-proile,

    each of which when called returns sensory data for that body part."

   [#^Node creature]

   (filter

    (comp not nil?)

    (map touch-fn

         (filter #(isa? (class %) Geometry)

                 (node-seq creature)))))

 #+end_src

 

 * Visualizing Touch

 

 #+begin_src clojure

 (defn view-touch 

   "Creates a function which accepts a list of  touch sensor-data and

    displays each element to the screen."

   []

   (view-sense

    (fn 

      [[coords sensor-data]]

      (let [image (points->image coords)]

        (dorun

         (for [i (range (count coords))]

           (.setRGB image ((coords i) 0) ((coords i) 1)

                    (gray (sensor-data i)))))

        image))))

 #+end_src

 

 * Headers 

 #+begin_src clojure

 (ns cortex.touch

   "Simulate the sense of touch in jMonkeyEngine3. Enables any Geometry

   to be outfitted with touch sensors with density determined by a UV

   image. In this way a Geometry can know what parts of itself are

   touching nearby objects. Reads specially prepared blender files to

   construct this sense automatically."

   {:author "Robert McIntyre"}

   (:use (cortex world util sense))

   (:use clojure.contrib.def)

   (:import (com.jme3.scene Geometry Node Mesh))

   (:import com.jme3.collision.CollisionResults)

   (:import com.jme3.scene.VertexBuffer$Type)

   (:import (com.jme3.math Triangle Vector3f Vector2f Ray Matrix4f)))

 #+end_src   

 

 

 * Source Listing

 * Next

 

 

 * COMMENT Code Generation

 #+begin_src clojure :tangle ../src/cortex/touch.clj

 <<skin-main>>

 #+end_src

 

 #+begin_src clojure :tangle ../src/cortex/test/touch.clj 

 #+end_src