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Date:         Fri, 22 Jan 1993 14:45:21 CDT
Reply-To:     Jeff Bone <JBONE@DOPIG.UAB.EDU>
From:         Jeff Bone <JBONE@DOPIG.UAB.EDU>
Subject:      Scanning FAQ Update, Part 4 of 4

-------------         PAGEMAKER/QUARK LISTSERV               -----------------
-------------   S C A N N I N G    F A Q    UPDATE 1/22/93   -----------------
Part 4 of 4

                   Questions and Answers Section 2


-- The question I have is where do they come up with setting the
-- LPI to 53, 83, and 106. Are these magic numbers or what?

   Almost!  The screen rulings (LPI) are based on a mathematical formula
   that takes into consideration, among other things, the occilation frequency
   of the video clock generator in the laser control circuit.  What?  Suffice
   it to say that the rulings are practically magical.  As a matter of fact
   most screen frequencies and angles used in four color separating in more
   expensive imagesetters are arrived at with a great deal of secrecy because
   these little differences mean the world to those little color rosettes.



-- What exactly is the difference between a PICT and a TIFF file format?

   For the most part they contain the same type data when it comes to bitmap
   information, that is each pixel in the image.  But the PICT file
   specification (created by Apple and based on Quickdraw) can also contain
   vector data.  Vector data is resolution independant.  For example, instead
   of a series of pixels making up a line across an image in a bitmap, a
   vector line would be an x,y coordinate where the line starts and stops
   and then a value for its thickness and color.

   As you can see the PICT file can be much more complex than a TIFF file and
   this is why Aldus along with Microsoft created the TIFF file specification.
   They needed a simple yet "rich" file type that could handle greyscale data
   from a scanned file and be exchanged between various machines and operating
   systems.  TIFF is the most widely supported bitmap file format to date.  It
   is now capable of handling not only greyscale data but color data as well
   and even has the ability to store data in a compressed form (LZW and
   Packbits in the 5.0 spec, CCITT IV and V, and JPEG/M-JPEG in the 6.0
   spec).  PICT currently has only one method for compressing a file,
   through the use of the Quicktime/Quickpress extension from Apple, this
   allows a PICT file to be stored as a JPEG compressed file and read by
   any application without conversion first.



-- Which format is best for a specific image (i.e. a simple B&W, a scaled
-- image, a color image, etc.)

   It depends entirely on your situation.  Remember, that more pleasing it is
   -- the more memory and the slower your system will be in manipulating them.

   In my opinion TIFF is the best file format for all of these examples
   mentioned.  The basic TIFF file (5.0) can handle 1-bit, 4-bit, and 8-bit
   greyscale data, and can handle 8-bit, 16-bit, and 24-bit color data.  The
   most common forms however are 1-bit and 8-bit greyscale, and 8-bit and 24-
   bit color.  With the two basic forms of compression recognized in the
   TIFF spec you can successfully compress all of these types of images so
   that they take up less space in storage without worrying about using
   some secondary compression utility.




-- Why are there so many different DPI scales?  What common laser printers
-- use what DPI?  What DPI do image-setters use?  What DPI can PageMaker
-- handle?  MicroSoft Word?  Quark?  What about an Image Writer or a
-- Stylewriter?

   DPI or Dots Per Inch is a clever marketing tool that the early laserprinter
   manufacturers came up with that really doesn't mean a whole lot.  A few
   years ago, imagesetter manufacturers and scanner makers were talking in
   the more descriptive SPSI (spots per square inch) and LPI (lines per
   inch) measurements.  It was very common for imagesetters to produce 1.6
   million SPSI at 133 LPI, or 1270dpix1270dpi with 256 grey scales.
   Marketing types couldn't compete because the laserprinter seemed paltry
   in comparison, 90,000 SPSI at 53 LPI, or 300dpix300dpi with 14 grey
   scales.

   Anyway DPI has become dominant because laserprinter manufacturers had more
   advertising money.  Most laserprinters are now showing their DPI uniformly
   anyway, so you can simply square the DPI and get an accurate SPSI.

   So DPIxDPI = SPSI which is much more useful in determining how much better
   one laserprinter is over another one.  For instance, the original Apple
   Laserwriter is 90,000 spsi (300x300), the NeXT Laserprinter is 160,000 spsi
   (400x400) which is 1.7 times better not 1.3 as the 400/300 ratio would
   suggest.  Where this really gets interesting is with the new crop of 600dpi
   lasers.  For instance, the new QMS 960 is 360,000 spsi, or 4 times better
   not 2 times as the 600/300 ratio would suggest.

   Below is a list of some common printers and their SPSI:

    Printer                         SPSI           Type
    ------------------------------------------------------
    Epson LX-80                       14,400    9-pin Dot Matrix
    Apple Imagewriter                 20,760    18-pin Dot Matrix
    Apple Laserwriter                 90,000    Canon CX Laserprinter
    HP LaserJet III                   90,000    Canon SX Laserprinter
    Panasonic KXI1124                129,600    24-pin Dot Matrix
    Canon BJ10e                      129,600    Canon 36-valve Bubble Jet
    NeXT Laserprinter                160,000    Fujitsu Laserprinter
    Dataproducts LZ960               160,000    Fujitsu Laserprinter
    Compaq PageMarq 20               320,000    Xerox Laserprinter
    HP LaserJet IV                   360,000    Canon BX Laserprinter
    Lasermaster L1000                600,000    Fujitsu Laserprinter
    Linotronic 170                 1,612,900    Infrared Laser imagesetter
    Linotronic 330                11,444,689    Helium Neon Laser imagesetter
    AGFA SelectSet 5000           21,622,500    Helium Argon Laser imagesetter

-- What are the best ways (if any) to compress a graphics file once it's
-- been scanned?

   The TIFF format provides a Compressed TIFF version -- that is usually
   recognized anytime that Uncompressed TIFF is.  Other formats can be
   manually compressed -- but will need to be manually uncompressed anytime
   you need them.  Manual compression is used more for archiving the file
   for storage and can save a significant amount of space.

   Without loosing any of the data, most forms of compression are based on the
   LZH (Lempel-Zimpel-Huffman) algorithm, they are:

    For the Mac:         For the PC:        For UNIX Machines;
        Stuffit              PKZip             Unix Compress
        CompactPro           ARJ               Zoo
        Disk Doubler         Zoo               PKZip
                                               ARJ

   They are all good and most will compress a file within 5-10% of the other.

-- My Hewlett-Packard ScanJet came with a rudimentary scanning package.
-- Is there better scanning software than DeskScan?  What are the main
-- features to look for in scanning software?

   Concensus favorite for grey scale scanning is Ofoto.  Look especially
   for software that gives you the ability to scan and save in all the
   common file formats that you will want to use, and your scanning software
   should definitely allow you to crop and rotate before or during saving.
   If you don't already own an image processing package, check out Photo-
   Styler for the PC and PhotoShop for the Mac.  These packages won't
   require separate scanning software but rather allow for the addition of
   a "plug-in" module that will allow scanning from inside these applications.


 Finally
 -------

    If you have any questions or additions to this FAQ, please feel free to
 send them directly to me:

    Jeff Bone, Electronic Imaging Coordinator
    University of Alabama at Birmingham
    School of Medicine
    933 19th Street South, CHSB-19 Room 320
    Birmingham, Alabama  35294-2041

    voice (205) 934-4396
      fax (205) 934-3749
    email jbone@dopig.uab.edu
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