read part of jpeg file by pure python

[Pacino]

On 9 13 ,   7 44 , michal.zaborow... at gmail.com wrote:
> On 13 Wrz, 10:48, Pacino <xiaoyish... at gmail.com> wrote:
>
>
>
> > On 9 13 ,   4 43 , Laurent Pointal <laurent.poin... at limsi.fr> wrote:
>
> > > Pacino a écrit :
>
> > > > Hi, everyone,
>
> > > > I am wondering whether it's possible to read part (e.g. 1000*1000) of
> > > > a huge jpeg file (e.g. 30000*30000) and save it to another jpeg file
> > > > by pure python. I failed to read the whole file and split it, because
> > > > it would cost 2GB memory.
>
> > > > Can anyone help me? Any comments would be appreciated.
>
> > > Just reading parts of the *file* is easy (see tell() seek() and read()
> > > methods on files).
> > > But to extract a part of the *picture*, you must uncompress the picture
> > > in memory, grab the sub-picture, and save it back - generally with
> > > compression. I can't see how you can bypass the uncompress/compress
> > > phases and the corresponding memory use.
>
> > The most difficult part is the uncompress part. I don't want the whole
> > picture to be uncompressed in the memory, because it will consume a
> > lot of memory (2GB, as I mentioned). My goal is to uncompress part of
> > the picture into the memory.
>
> > I just read some article on this subject (http://mail.python.org/
> > pipermail/image-sig/1999-April/000713.html) , but haven't test it out
> > yet.
>
>   I have no idea what it does. Anyway - jpeg:
> 1. RGB -> HLV
> 2. divide data into 8x8 - blocks of data.
> 3. blocks are treated with discrete cosine transform.
> 4. Result is filtered to remove "fast changes".
> 5. Then result is compressed with Huffman alg.
>
> So to get part of image - you can take smaller image before step 4.
> As far as I understand code presented at:http://mail.python.org/pipermail/image-sig/1999-April/000713.html
> - full image will be loaded, and cutted.
>
> --
> Regards,
>   Micha³ Zaborowski (TeXXaS)-         -
>
> -         -

Thanks. Seems no ways to achieve the requirements.