Re: SUMMARY AND FOLLOWUP QUESTIONS: Re: Acrobat file sizes

[Max Wyss <prodok -at- prodok -dot- ch>]

Gilda,

Yes, indeed, there was a bug in Acrobat Distiller 4, which did not take
into account the JPEG compression settings for color images. This has been
fixed with 4.05.

Any registered user of Acrobat 4 should receive the updated CD
automatically, but there have been known problems with the outsourced
registration department and the outsourced fulfillment department. So, it
can be worthwile to contact Adobe if you have registered your Acrobat 4,
and have not yet received the upgrade CD.

Now, to the follow-up questions.

There seems to be some confusion between the use of the JPEG graphic file
format in the creation/editing phase of the source document, and the use of
the JPEG compression in the creation of the PDF in Distiller. These two
topics must be kept apart.

For the source document, the JPEG graphic format is suitable for
photographic images. These are images where the color of a given pixel is
not determined by some arbitrary value, and where it does not influence the
actual appearance of the image (which is the case for photographs). For
this, the JPEG format is excellent. The JPEG graphic format uses the JPEG
compression, an algorithm which is "lossy". That means, it discards image
information which is kind of below a certain threshold. It relies on the
next neighbours. However one effect is that it creates artefacts around a
sharp contrast.

For applications where the color of a given pixel is determined by some
arbitrary value, a format is needed which does not modify that color.
Screenshots are examples for such an application. A format for these
applications must not be "lossy", but "lossless". The most widely used
lossless formats are TIFF and GIF (and all the raw formats, such as BMP on
Windoze).

TIFF can represent 8 or even 16 bit color depth per channel (and we have
three or four, depending on the color space we are using). It can use a
compression, but does not have to. The compression algorithms are part of
the run-length-encoded family.

The GIF format can represent up to 256 colors, selected out of a 8
bit-per-channel RGB range. This is, of course the reason why it leads to a
considerably smaller file size, as it uses only 8 bits color information
per pixel instead of 24 or more. The GIF format is in addition to that
compressing. The result is an even smaller file.

The raw formats do not contain any compression settings, and lead to really
big files. Some allow to set the color depth, some don't. They may have
some reason to be when it comes to fast display on a slow computer, as
there are no decompression algorithms which must run.

So, this covers the options you have when making your source documents.

For screenshots and Framemaker, I got the best results with the TIFF
format. I did process the screenshots in Adobe Photoshop: first, setting
the final dimension in which it gets placed in the Frame document, and then
a color reduction to only the needed colors, then saving as TIFF.


Now, for the compression settings in Distiller for images:

As said, this is something totally different. In a properly working
Distiller, we have several compression settings besides "none": JPEG at
several compression levels, ZIP-4-bit and ZIP-8-bit.

The JPEG compression is, as already stated "lossy". The compression levels
determine how big the losses are, and how much the quality of the resulting
image is affected. A high compression leads to a low quality, but a small
file (or image object in PDF). A low compression level leads to better
quality images, but bigger image objects. The optimum setting must be
determined for every individual document, as it is a compromise between
file size and image quality. The JPEG compression should -- particularly at
high compression levels -- only be used if the document contains only
photographic style images.

ZIP-4-bit is essentially a lossless compression. However, before it is
applied, the number of colors is reduced, so that the colors per channel
can be represented with 4 bits (which means 16 equidistant levels per
channel). Then, the ZIP compression algorithm is applied. The result is a
very small image object.

ZIP-8-bit is the only full quality lossless compression in PDF. It uses the
ZIP compression algorithm, and does not reduce the number of colors. For a
document containing screenshots, this compression setting is the only one
which ensures a sufficient quality.

So, back to your two questions:

1. Well, it would have been better to use a lossless compression file
format for your screenshots. But as you have them already, you would have
to decide if the quality is sufficient, particularly concerning the
artefacts along lines. If the compression has been set too high, and the
results are not really convincing, you might consider redoing the
screenshots. Otherwise, stay with the JPEGs, but don't make things worse in
Distiller, and use the ZIP-8-bit compression setting.

2. There are some tools which do batch converisions. Debabelizer comes to
my mind, which is said to be the industry standard for batch conversions.
If you want to do some additional processing, you might also consider the
batch processing capabilities of Adobe Photoshop.

Hope, this can help.


Max Wyss
PRODOK Engineering
Low Paper workflows, Smart documents, PDF forms
CH-8906 Bonstetten, Switzerland

Fax:  +41 1 700 20 37
e-mail:  mailto:prodok -at- prodok -dot- ch
http://www.prodok.ch



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______________________







I received several responses, but I think Dick Margulis and Graham Wyatt
had the right answer, at least for my situation (this is Dick's response,
but Graham said basically the same thing):

Be sure to upgrade to Acrobat 4.05b (or 4.05a plus the Update 1 file
that you can download from Adobe). If you are trying to use 4.0 instead
of 4.05, that is probably the source of your problem. That version is
very buggy with respect to graphics compression and file size.

This sounds like the right answer for me, because I found the following
description of the 4.05 upgrade on the Adobe web site:

   "... Proper JPEG Compression in PDF Files Created by Distiller In
   Acrobat 4.0, Distiller's JPEG compression results are the same for High,
   Medium, and Low. Acrobat 4.05 corrects this problem ..."


I just called and ordered the upgrade CD (they don't send it over the
Internet), and I assume that this will solve the problem. If it doesn't,
I'll contact the list again.  The phone number is 1 800 272 3623.  Thanks
to everyone for your help.

Here are my followup questions:

Some of the other replies I received touched on the choice of JPEG format.

We had the same problem. The answer: don't use JPGs. Frame's storage
algorithm for them makes bloat of the first magnitude. The smallest Frame
and Acrobat files you can make, in our experience are
using *.xwd (a UNIX bitmap). Second place goes to *.gif.

Personally, I would pick GIF as a more HTML friendly format  for screen
shots - JPEG is really designed for use with photographic type images.

Being a totally novice on this topic, I relied on the advice of a good
friend, who recommended JPEG. He told me that GIF is suitable more for
pictures containing large blocks of solid colour. I thought our screen
shots are complex enough to make GIF unsuitable for us.

1. Was that the wrong decision?
2. If so, is there a quick and easy way to convert hundreds of JPEGs to
GIFs?