Re: [libreoffice-users] A word of warning about PDF text

Le Fri, 31 Jan 2014 13:22:41 +1000,
Peter West <lists@pbw.id.au> a écrit :

> A word of warning about text retrieved from PDF documents.
>
> Recovering text blocks from PDFs is inherently risky.  PDF is a page 
> definition format, and so it has no notion of the semantics of the
> text it contains. It places bits of text at certain positions on the
> page. You can create a whole page of text by taking the individual
> characters and their attributes and position on the page, shuffling
> them, and writing them to the file.  That will produce a readable
> file, but try extracting the text from that file. Unless you have a
> very, very smart text extractor that reverse-engineers the process of
> creating the page, then calculates the _visual_ order of the text
> elements, you will end up with gibberish.
>
> _Most_ pdf text, _most_ of the time, is laid on the page in visual 
> order, but in even the best-behaved files, you are likely to be
> surprised.
>
> If you don't _know_ that your PDF text extractor program is
> completely visually accurate by design, don't tell your boss that
> you can easily extract that PDF text, without allowing time for
> proof-reading every page. You will get burned.

It is why I open the pdf file into a separated program and use the
mouse to select the text, and copy/past or Ctrl-C/Ctrl-V. That way, I
have full control on how the text will appear when I select it.

And I use other programs like pdfimages, pdftppm and convert to
extract the images directly from the pdf. They can be turned or
mirrored, it is why convert is useful too. When they are split in small
pieces, pdftoppm give me an exact copy of each page of the pdf, each
page into a ppm file, which is converted in jpeg. In that case, gimp is
useful to extract only the images from these files and cut the text.

The script I use for the images is joined. To use it, place it
somewhere in your path, control it is executable, go into the
directory where your pdf file is, and run 'pdf2jpg'. It will only issue
a help message. Be aware it will extract all the pdf files in that
directory on the fly. Be also aware that, if the final output is jpeg
files, ppm files are automatically used as middle men when needed,
the conversion will be much slower and they can use a lot of space on
the disk.

So, if you want to extract pictures from a 100MB pdf file, count at
least 2GB of temporary disk usage to be safe in all cases. (estimation
from memory, so make you own tests if you don't have a lot of free disk
space)

Also, with some distributions, you may have to adjust the name of the
pdfimages and pdftoppm commands in the script. They are part of poppler
on gentoo (poppler-utils or something like that on Debian), in the past,
they was part of xpdf.

Dominique

> I don't know how LO extracts PDF text; perhaps it is very
> sophisticated. I have my doubts.

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