PDF graphics bewilderment

["Thomas Michanek" <thomas.michanek@xxxxxx>]

Dear Framers and Adobers,

I cannot get any sense out of my PDF experiments from FrameMaker.
Why does some color graphics end up "blocky" and "blurry", while
others end up crisp and clear? Please enlighten me on how Acrobat
Distiller is supposed to handle color graphics. And excuse me for
a long message; I want to provide as much details as necessary.

Background: I use FM 5.5.6 on Windows 2000 SP1 with Acrobat 4.05a.
The FM files contain color screendumps taken with PaintShop Pro,
saved as 24-bit TIFFs and also as 8-bit and 4-bit color-reduced
TIFFs, and imported at 96/150/200 dpi (depending on window size).
I create PDFs by printing a PS file using the Acrobat Distiller
printer, created using AdobePS 5.1.2 (UI 1.0.2) and Adist4.ppd.
I view the PDFs with Smoothing turned on, and when I talk about
"blurry" graphics, it's not the effect of smoothing in Acrobat.

I want to create PDFs for on-line viewing purposes. I use a
pre-made Distiller joboptions file with the following settings:
* Resolution 600 dpi, Binary format, non-optimized
* Downsampling to 72 dpi, Automatic compression, Medium quality
(I think that's all the relevant ones in this case)

I thought that downsampling to 72 dpi would reduce the image
data, so that zooming in on a high-dpi image would reveal no
further details, but only make the image "blocky". However, this
is not the case: the 150/200-dpi 8-bit and 4-bit TIFFs can be
zoomed in to reveal all pixels (details) of the original image.
Have I misunderstood what downsampling to 72 dpi means?

Further, the high-dpi 24-bit TIFFs look very bad: they are
pixelated (low-res), blurry and contain color artifacts.
Text and icons cannot be distinguished. None of this affects
the 8/4-bit TIFFs or the 96-dpi 24-bit TIFFs.
Why are some 24-bit TIFFs handled differently???

To overcome the problems with 24-bit TIFFs, I have tried using
downsampling to 144 dpi, as well as no downsampling at all.
Both results in larger PDF files (expected), and the 24-bit TIFFs
now have the same quality and "behavior" as the 8/4-bit TIFFs.
Great, but why doesn't this work for 72 dpi downsampling?

I have also experimented with downsampling to 10 dpi, just to
check the downsampling. The result baffles me: only the 24-bit
TIFFs are affected in the way I would expect, but the 8/4-bit
TIFFs are unaffected and appear in full glory!

This makes me wonder: are there bugs or undocumented limitations
on how the Distiller handles 24-bit color graphics (as opposed
to 8/4-bit color graphics), and how downsampling works?
Are these problems in any way related to Windows 2000?
Or have I completely misunderstood these concepts?

I can provide sample files, if someone needs to see the
effects I'm trying to describe, or to trouble-shoot for me.

Very many thanks in advance,

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Thomas Michanek, Technical Writer
IAR Systems AB, Sweden: http://www.iar.com
mailto:Thomas.Michanek@iar.se
Tel: +46 18 167800, Fax: +46 18 167838
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