[pdftex] pdftex compression -- proposed addition to manual

[White, George]

I agree that the pdftex FAQ should provide a brief discussion of compression
with 
pointers to more detailed information.  There used to be a good overview of
image
formats on a WWW server at Los Alamos National Lab, but I haven't seen any
good
single source recently, so I'll keep looking.  In any case, the key points
for the FAQ 
should include:

1.  pdftex does not eliminate the need for a proper image manipulation tool:
for best
     results it will be safest to convert individual images to PDF.  
2.  consider altering image color model and resolution as well as the type
of compression
     (or even converting between line art and image form -- each has
advantages in certain
     situations)
3.  digital image storage and processing is a rapidly evolving field, so
expect new and 
     better algorithms to appear and make their way into PDF in the future.

> -----Original Message-----
> From:	Ben Crowell [SMTP:crowell01 at lightandmatter.com]
> Sent:	Sunday, August 26, 2001 12:06 AM
> To:	pdftex at tug.org
> Subject:	[pdftex] pdftex compression -- proposed addition to manual
> 
> Reinhard Kotucha wrote about JPEGs being further compressible,
> and Alan Shutko suggested this was happening because GS
> was downsampling. My own experience is based on scanning photos
> into a lossless format, then converting to JPEG or PNG with Graphic
> Converter, and then using pdftex, which does Flate compression.
> When I did this, the PDF output was very close to being the same
> size as the original image files, i.e. Flate was not achieving
> much further compression. This could mean either that Reinhard
> Kotucha's images originated from digital cameras that did less
> than optimal JPEG compression, or, as Alan suggests, that the
> images got downsampled. The latter interpretation, however,
> would not explain the wide variation in compression ratios
> observed by Reinhard.
> 
	[White, George]  
	Don't forget that jpeg supports variable compression levels.  You
can't really
	talk about an "optimal" level without knowing more about the uses to
which a
	document will be put and the characteristics of the image.

> Reinhard wrote:
> >If I understand your mail correctly, you want to discourage people to
> >apply further compression to jpeg files.  Theoretically, if a
> >compression algorithm is ideal, then there shouldn't be any other
> >algorithm that is able to further compress that file.  This is
> >obviously not true for jpeg.
> My intention was simply to explain to people how to get better
> compression. There is nothing wrong with doing Flate compression,
> since it is lossless and almost never /increases/ the size of the
> file.
> 
> If it's true that some digital cameras do a suboptimal job of
> JPEG compression, then I think image-processing software is
> probably the best tool for improving the compression. I wouldn't
> just send it through Flate and hope it fixes the problem.
> Also, people may want to be able to adjust the level of
> JPEG compression. Typically for a PDF destined for internet
> distribution, the level of compression you want is /much/ more
> harsh than what you'd get from a digital camera.
> 
> On a different topic relating to compression, Otfried Cheong
> e-mailed me off-list, saying that when pdftex reads a PNG
> or TIFF image as an input, it unpacks it into an uncompressed
> pixel array, and then applies no further compression except for
> Flate. This would seem to conflict with the information
> George N. White III gave in a previous post, or maybe their
> statements are really not contradictory and there's something I
> don't understand. (Otfried, I apologize if I'm misrepresenting
> what you said, but when I tried to reply to your e-mail off
> list and ask you to post it to the list, my e-mail bounced back.)
> 
> George also wrote on the list:
> >Unfortunately, this subject goes far beyond what can reasonably be
> >included in a pdftex FAQ.
> It seems to me that the page or so of text
> we've been working on in this thread is about the right amount
> to go in the pdftex manual.
> I think there ought to be at least /some/ information on compression
> in the pdftex manual, even if it's just a pointer to where to
> get more details.
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