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Re: PDF: multiple slides per page to one slide per page?

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In article <a782d5b7-1aa6-473f-9940-a9bdac96b6d0
@g3g2000prf.googlegroups.com>, dmathog@gmail.com says...

> Does this suggest a need for a "pass through" setting for images?
> 
> It would certainly be simpler than the decompress, (carefully) store
> at full decompressed resolution for all intermediate steps, recompress
> method.  It might be something like
> 
>   dColorImageFilter=/asis
> 
> Naively it seems like this should be possible, since the PDF is
> essentially just a bag of objects, and I can't think of a reason why
> an image object couldn't be copied from one bag to the other, albeit
> possibly wrapped in another object if the image scale needs to be
> changed.

It can't be done because, basically, Ghostscript and pdfwrite aren't 
copying things from one 'bag' to another at all.

What actually happens is that Ghostscript fully interprets the PDF file, 
converting all the instructions into graphics primitives (things which 
make marks, like text, images, linework etc). 

These are passed into the rendering pipeline, where pdfwrite intercepts 
them before they are rendered, and re-emits them as PDF obejcts, 
suitably wrapped up in a PDF file with all the furniture required.

So the image you are talking about is converted from a DCT-compressed 
PDF object into an uncompressed sequence of bytes representing the 
samples of an image. These are then recompressed back into a PDF object.

There is no way in the existing scheme to have a 'pass-through' mode, 
because the graphics library needs to see the image in that decompressed 
fashion, not as a DCT compressed stream.

In a former life I did do some work on this and it looks to me like it 
is possible to reverse the decoding of a DCT image. If you know the 
exact way the image was originally comrpessed (which is stored in the 
JPEG information, as it is required for decompression) then it should be 
possible to rerun the DCT compression and get the same JPEG data out, no 
additional artefacts.

I never got finished with it though, there always seems to be something 
more important......



				Ken

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Re: PDF: multiple slides per page to one slide per page? pipitas <pipitas@googlemail.com> - 2011-04-25 04:35 -0700
  Re: PDF: multiple slides per page to one slide per page? pipitas <pipitas@googlemail.com> - 2011-04-25 05:55 -0700
  Re: PDF: multiple slides per page to one slide per page? David Mathog <dmathog@gmail.com> - 2011-06-21 15:03 -0700
    Re: PDF: multiple slides per page to one slide per page? David Mathog <dmathog@gmail.com> - 2011-06-22 15:35 -0700
      Re: PDF: multiple slides per page to one slide per page? ken <ken@spamcop.net> - 2011-06-23 07:56 +0100
        Re: PDF: multiple slides per page to one slide per page? David Mathog <dmathog@gmail.com> - 2011-06-23 13:56 -0700
          Re: PDF: multiple slides per page to one slide per page? ken <ken@spamcop.net> - 2011-06-24 07:57 +0100
            Re: PDF: multiple slides per page to one slide per page? David Mathog <dmathog@gmail.com> - 2011-06-24 09:33 -0700
              Re: PDF: multiple slides per page to one slide per page? ken <ken@spamcop.net> - 2011-06-24 20:05 +0100
              Re: PDF: multiple slides per page to one slide per page? ken <ken@spamcop.net> - 2011-06-25 11:51 +0100
                Re: PDF: multiple slides per page to one slide per page? David Mathog <dmathog@gmail.com> - 2011-06-27 10:21 -0700
                Re: PDF: multiple slides per page to one slide per page? ken <ken@spamcop.net> - 2011-06-28 08:08 +0100
                Re: PDF: multiple slides per page to one slide per page? David Mathog <dmathog@gmail.com> - 2011-06-27 10:33 -0700
                Re: PDF: multiple slides per page to one slide per page? David Mathog <dmathog@gmail.com> - 2011-06-28 09:22 -0700
                Re: PDF: multiple slides per page to one slide per page? mathog <dmathog@gmail.com> - 2011-06-30 10:34 -0700
                Re: PDF: multiple slides per page to one slide per page? tlvp <tPlOvUpBErLeLsEs@hotmail.com> - 2011-06-30 23:25 -0400

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