Scanning docs

[KA4HJH]

>For schematics JPEG is just missapplying the technology... JPEG is 
>intended for
>pictures may them be colorless or not. Things like drawings requires you to
>reduce the compression ration so that more components are still there for the
>lossy DCT based transform will give good enough results. There was a separate
>standard developed for black/white images such as drawings, this standard is
>called JBIG but I have rarely seen it implemented.

I've NEVER seen that used.

>GIF is really an old
>technology, the versions are GIF-87 and GIF-89a as some may recall, the later
>is nowdays only used. These are byte oriented and uses a palette of 256
>colours and then uses an Lempel-Ziv-Welch (LZW) compression technique. The LZW
>is still causing patent trouble if I recall things rigth.

Rather like that MPEG Layer III encoder. LZ isn't patented, LZW is.

>Anyway, in order to
>get around those troubles AND make use of the fact that 24-bit colours had hit
>the streets the PNG format was developed.

Belive it or not, the latest version of Netscape for Macintosh MIGHT 
have PNG support. It didn't a year ago. I had to use a different 
browser to look at some--or download them first and open them with 
something else (yeah, right).

>PNG contains prefilters just as the
>full JPEG standard has (nobody implements the full standard in reality, there
>are many options not in use and people is really runing the JFIF format, a
>reduced JPEG format).

I wondered what the deal was with that. I see JFIF all the time.

>  PNG also uses the Lempel-Ziv compression and in fact the
>GZIP compression engine is what was intended. GZIP compression normally
>outruns LZW on larger datafiles and todays pictures really fall into this
>category. With propper pre-filtering etc you should do even better. GIF is
>really just a legacy format.

In PhotoShop it's referred to as "Compuserve GIF" format.

>TIFF is a hairy format with a number of options
>and few really implement and accept all of them.

Tell me about that one! (I"ve seen some funky JPEG's, too).

>For instance is the LZW
>section of TIFF not commonly used.

Saves space but it really slows down the opens and saves a lot on 
large images. Useless for pre-press work unless you're really short 
on disk space.

>Only the raw-format TIFF seems to be
>accepted widely, but that is not with any compression.

I'd never send someone a compressed TIFF. I can hear the reply now...


>The PDF framework will just encapsule JPEGs most of the times, so the issues
>relating to JPEG and compression will ripple over to PDFs aswell. I don't
>recall if PDF supplied other compression schemes aswell, but it should.

In Acrobat Distiller 4 there are separate settings for color and 
grayscale bitmaps. You have a choice of JPEG or ZIP (or let the 
Distiller decide automatically) as well as image quality and optional 
downsampling (with choice of method and finished resolution).

Monochrome bitmaps are slightly different. You have a choice of CCIT 
Group 3 or 4, ZIP, or "Run Length" (CCIT Group 4 appears to be the 
default), as well as downsampling.

Finally there's a single checkbox for "compress text and line art", 
which I assume is LZW.

Note that you can create different combinations of these settings and 
save them. No swapping things around.


So what the heck is CCIT?


>My preference is like this:
>When you take a photo of a synth - use JPEG.
>When you scan a schematic - use PNG.
>When you make a document - deliver PDF.

I'll have to PNG a try. It hasn't been used much here in the Mac 
world (so far), but then I mostly do pre-press.


A final note: not all encoders are alike. I get quite a lot of 
variation in finished file size using different programs, even with 
the same settings (supposedly). Experimentation is a good idea.


-- 
Terry Bowman, KA4HJH
"The Mac Doctor"