Multithreaded Image Processing in Clojure

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Multithreaded Processing

Examples

Purpose

An example Clojure script illustrating how to run concurrent threads that perform independent tasks, and how to combine their results afterwards.

The same result could be obtained using the built-in clojure pmap function. This script illustrates how to instantiate java Thread directly from Clojure, and how to launch and join threads for concurrent execution.

Code

; Albert Cardona 20081011
;
; This file illustrates how to create a generic, reusable macro that uses as
; many threads as cores your CPU has.
; The macro, named "multithreader" and declared below, takes as arguments:
;  - the starting index
;  - the ending index (non-inclusive)
;  - any function and
;  - .. its required arguments
;
;  Then an example is shown on how to process an image line by line, where any
;  number of threads are editing one line each at a time, without collision.
;  In particular, we declare the "line-randomizer" function, which simply sets
;  each pixel of a 32-bit image to a random value from 0 to 1.
;  Then the function "line-randomizer" is invoked by giving it and a new image
;  to the "multithreader" macro, and then the resulting image is shown.
;
;
; As a step-by-step introduction, this file starts by declaring first a set of functions that illustrate:
; - how to loop from a starting to an ending index:
;     * "do-it"
;     * "do-it-iterating"
;     * "do-it-looping"
; - how to make a function that uses multiple threads:
;     * "do-it-multithreading"
; - ... and finally, how the "do-it-multithreading" function is broken apart into two: the "printer" function and the "multithreader" macro.
; - how to extract a macro from the "do-it-multithreading" function
; - how to declare a function inside a closure (the "line-randomizer"
;   function), so that the function has access to an otherwise invisible
;   variable (in this case, the Random number generator)
;
; Finally, the "multithreader" macro and the "line-randomizer" function are
; put to use, and the resulting image is show.
; 
; Note that the "multithreader" macro may be reused with ANY function you want.
; All you need to do is to subdivide an image in a way that makes sense to you,
; (such as by lines) and apply to each subdivision your function, in a
; multithreaded way, with the "multithreader" macro.
;
; See http://clojure.org for general documentation on Clojure
; See [[Clojure_Scripting]] for help on
; using Clojure with ImageJ
; See [[Scripting_Help]] for how to use fiji's built-in scripting languages and how to create scripts for ImageJ.
;
; As a final note: Clojure runs native. There is no such thing as a clojure
; interpreter: all clojure code compiles to java byte code, which runs native
; on the JVM.
; Sometimes though Clojure will use reflection, which may slow down processing.
; To avoid it, just add type declarations with #^ (see below for examples)
; where they really make a difference.
; To tell the compiler to warn you when reflection is used, uncomment this line:
; (set! *warn-on-reflection true)
;
; The ';' indicates a commented line, as you may have already guessed.
;
; Have fun -- Albert Cardona


(defn do-it [start end]
  "Print all numbers from start to end (exclusive)"
  (map println
       (range start end)))

; Invoke like
; (do-it 0 10)

(defn do-it-iterating [start end]
  "Print all numbers from start to end (inclusive)"
  (doseq [i (range start (+ 1 end))]
    (println i)))

(defn do-times [start end]
  "Print all numbers from start to end (exclusive)"
  (dotimes [i (range start end)]
    (println i)))

; Invoke like
; (do-it-iterating 0 10)

; Crude looping
(defn do-it-looping [start end]
  "Print all numbers from start to end (non-inclusive)"
  (loop [i start]
    (if (<= i end)
      (do
        (println i)
        (recur (inc i))))))

; Invoke like
; (do-it-looping 0 10)


(import '(java.util.concurrent.atomic AtomicInteger))

(defn do-it-multithreaded [start end]
  "Print all numbers from start to end (inclusive), multithreaded"
  (let [ai (AtomicInteger. start)]
    (defn looper []
      (loop [i (.getAndIncrement ai)]
        (if (< i (+ 1 end))
          (do
            (println i)
            (try
              (Thread/sleep 100)
              (catch Exception e (.printStackTrace e)))
            (recur (.getAndIncrement ai))))))
    (dotimes [i (.availableProcessors (Runtime/getRuntime))]
      (.start (Thread. looper)))))

; Invoke like
; (do-it-multithreaded 0 10)

; Now separated:

(defn printer [i]
  "Print the given number i and then sleep 100 ms"
  (println i)
  (try
    (Thread/sleep 100)
    (catch Exception e (.printStackTrace e))))

(defmacro multithreader [start end fun & args]
  "Call function fun with numeric arguments from start to end (non-inclusive), multithreaded"
  ; Below, the # is shorthand for (gensym <a-name>) to create unique names
  `(let [ai# (AtomicInteger. ~start)]
    ; Define a new function to represent one Thread
    ; (All functions implement Runnable, not only Callable)
    (defn looper []
      (loop [i# (.getAndIncrement ai#)]
        (if (< i# ~end)
          (do
            ; Execute the function given as argument
            ; with the current i and whatever other args it needs
            (~fun i# ~@args)
            (recur (.getAndIncrement ai#))))))
    ; Create as many looper threads as cores the CPU has
    (let [threads# (map (fn [x#] (Thread. looper (str "Looper-" x#)))
                       (range (.availableProcessors (Runtime/getRuntime))))]
      ; Start all threads
      (doseq [t# threads#]
        (.start t#))
      ; Wait until all threads are done
      (doseq [t# threads#]
        (.join t#)))))

; Invoke like:
;(multithreader 0 10 printer)

; So now we can define a processing function that will edit, for example, a line
; in an image, and then apply the multithreader function to process the image
; with as many threads as desired. In this case, as many as cores the CPU has:

(import '(java.util Random))

; Define line-randomizer function inside a closure
; so the Random seed and distribution is shared.
; Could use built-in rand function as well, but then can't control seed.

(let [r (Random. (System/currentTimeMillis))]
  (defn line-randomizer [row #^floats pixels width]
    "Randomize the value of all pixels in the given row of the image contained in the unidimensional array of pixels"
    (let [offset (int (* width row))]
      (dotimes [i width]
        (aset pixels (+ i offset) (.nextFloat r))))))

; Execute like

(import '(ij IJ ImagePlus)
        '(ij.process FloatProcessor))

; Use the float[] from an ImageJ image. An array of floats
; would be created like:
; #^floats pixels (make-array Float/TYPE (* width height))
; Note the #^floats, which is shorthand for (with-meta (make-array ...) {:floats}) and adds an obvious type declaration to avoid reflection.

(let [width (int 512)   ; without the cast, 512 would be a Number, not a primitive. Number (like Integer, Float, etc.) need unboxing, which is costly.
      height (int 512)
      #^ImagePlus imp (IJ/createImage "Random image" "32-bit" width height 1)
      #^floats pixels (.getPixels (.getProcessor imp))]
  (multithreader 0 height
                 line-randomizer pixels width)
  (.setMinAndMax (.getProcessor imp) 0 1)
  (.show imp))



See also

Clojure Scripting