performance issues with multi-dimensional array

[Amnon]

I hope it's not the billion time you get the question.
I wanted to use clojure for image processing. I have a 3 dimensional
array I'm passing to clojure from java.
I then loop on the array to manipulate it.
Even the simplest task takes about half a minutes (I expected it to be
over in less than a second)

For example, I am turning an array to gray scale.
I tried to erase all my false improvements and squeeze everything to a
single function:

grey should get a 3Dim array (WxHx3 colors) and return it in a gray
scale.
Any help would be highly appreciated.
Amnon

[Konrad Hinsen]

On 24 Nov 2009, at 17:30, Amnon wrote:

> I hope it's not the billion time you get the question.
> I wanted to use clojure for image processing. I have a 3 dimensional
> array I'm passing to clojure from java.
> I then loop on the array to manipulate it.
> Even the simplest task takes about half a minutes (I expected it to be
> over in less than a second)

Could you give some more details? For example, what is the Java class
you use for the 3D arrays? How are you accessing it from Clojure? How
do you do the loops? How do you create the destination array, and how
do you fill it with the grey-scale values? I suspect that many of
these steps could be optimized, but it's hard to say without knowing
how your current code works.

Konrad.

[Christophe Grand]

It would be easier with some of your code. Meanwhile, check this post http://clj-me.cgrand.net/2009/10/15/multidim-arrays/ (it doesn't apply as is to clojure 1.0, you have to replace #^objects by #^"[Ljava.lang.Object;").

Christophe

[Amnon]

Hi Konrad,
In the original post, I put in the code, it was removed by the post
editor.
The way I wanted it implement, I have the main in java (gui and
stuff). I create a 3D array in java, it's an int array (i.e. int arr[]
[][] ) and call with that array to clojure.

It can be done in java at least 3 orders of magnitude faster so I'm
sure there is something I'm doing wrong.
I'll appreciate any suggestion.
Thanks,
Amnon

I've squeezed my example into two functions:

(defn light [arr x y]
(+ (aget arr x y 0) (aget arr x y 1) (aget arr x y 2))
)

(defn grey [arr]
(dotimes [x (length arr)]
(dotimes [y (alength (aget arr 0))]
(let [lg (int (/ (light arr x y) 3))]
(aset arr x y 0 lg)
(aset arr x y 1 lg)
(aset arr x y 2 lg)
))))

[David Nolen]

Read Christophe's post about multi-dim arrays. Reflection is getting in the way here.

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[Amnon]

I did, and it works like a charm. Couldn't figure out how to hint an
array of objects (what should I put instead of the old #^objects?
I still get it to work in 300 ms (which is not great but something I
can live with).
Thanks for the replies,

> > clojure+u...@googlegroups.com<clojure%2Bunsu...@googlegroups.com >

[David Nolen]

Check out this thread: http://groups.google.com/group/clojure/browse_frm/thread/4c52d4c3c19201b6/dd80d5e5bde4dc61

[Christophe Grand]

On Thu, Nov 26, 2009 at 11:49 AM, Amnon <amnon....@gmail.com> wrote:

I did, and it works like a charm. Couldn't figure out how to hint an
array of objects (what should I put instead of the old #^objects?
I still get it to work in 300 ms (which is not great but something I
can live with).

You have to replace #^objects with #^"[Ljava.lang.Object;" (don't use a more specialized type: it wouldn't help with aget/aset).

 

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