[Haskell] Image manipulation

[jerzy.karczmarczuk at info.unicaen.fr]

Dan Piponi adds to a short exchange: 

> jerzy.karczmarczuk:
>> [iso-8859-1] Bj�rn Wikstr�m writes: 
>>
>> > Hi! I have lots and lots of images (jpegs) that I would like to manipulate
>> > and shrink (in size). They are around 5 Mb big, so I thought this would
>> > be a good Haskell project since it's a lazy evaluating language.
>> ...
>> I must say that I don't see much use of laziness here.

> Laziness plays a big role in real world image processing. Typically,
> in applications like Apple's Shake, you build a dataflow
> representation of the image processing operations you wish to perform,
> and the final result is computed lazily so as to reduce the amount of
> computation. For example, if you blur an image, and then zoom in on
> the top left corner, then only the top left corner will be loaded up
> from the original image (assuming your image file format supports
> tiled access). You still work on tiles or scan-lines, rather than
> individual pixels, so the laziness has a 'coarse' granularity. 
> 
> But I'm not sure if this is what the original poster was talking about.

I am neither...
Still, Dan, I think that there is quite a difference between incremental
processing of signals, and images, etc., and the *lazy evaluation* of
them. Of course, a stream is consumed as economically as it can, but
not less. If you filter an image (shrinking, so some low-pass MUST be
done), a pixel must be loaded with its neighbourhood, which means *some*
scan lines.
With a JPEG this means that a 8x8 block should be loaded also with its
vicinity. But would you suggest that individual pixel processors should
be lazy? It would be useless, and probably resulting in some penalties. 

So, the laziness of Haskell for me here is less than useful.
Noooow, the lazy *generation* of streams is another story...
Generating music (low level, i.e. sound patterns) through lazy algorithms
is quite interesting. 

Jerzy Karczmarczuk