Nebula 1994 June/NEBULA_SE.ISO/Documents/FAQ/PostScript-faq/PSlevel2.txt

Text File | 1993-01-27 | 29.1 KB | 650 lines

PostScript(R) LEVEL 2 -- QUESTIONS & ANSWERS
============================================

PostScript Level 2, the first major new release of PostScript
software since its introduction, is a unification and enhancement
of the PostScript language based on the needs voiced by users of
PostScript printers and Display PostScript(R) workstations,
Independent Software Vendors (ISVs), and Original Equipment
Manufacturers (OEMs). PostScript Level 2 contains a number of
performance enhancements, is easier for software developers to use,
and contains important new functionality such as device-independent
color, forms handling and patterns support.

*** What is PostScript Level 2?

First, let's look at the current state of the PostScript language.
The baseline of the language is defined by the PostScript Language
Reference Manual, also known as the "red book." The red book defines
the basic PostScript language imaging model functionality for line
art, sampled images, text, and the RGB color model. Since its
introduction in 1985, the PostScript language has been considerably
extended for greater programming power, efficiency, and
flexibility.

Typically, these language extensions have been designed to adapt the
PostScript language to new imaging technologies or system
environments. While these extensions have introduced new
functionality and flexibility to the language, the basic imaging
model remains unchanged. The principal language extensions are:

+  Color:  The color extensions provide a cyan-magenta-yellow-black
(CMYK) color model for specifying colors and a colorimage operator
for painting sampled images. They also include additional rendering
controls for color output devices.

+  Composite fonts:  The composite font extensions enhance the basic
font facility to support character sets that are very large or have
complex requirements for encoding or character positioning.

+  Display PostScript:  The Display PostScript system enables
workstation applications to use the PostScript language and imaging
model for managing the appearance of the display. Some of the
extensions are specialized to interactive display applications,
such as concurrent execution and support for windowing systems.
Other extensions are more general and are intended to improve
performance or programming convenience.

When Adobe decided to add additional functionality to the PostScript
language, we did not want to add the functionality in a piecemeal
fashion and have it exist in some devices but not others.  This makes
life difficult for independent software vendors (ISVs) who write
PostScript language programs. PostScript Level 2 integrates the
original PostScript language, all previous language extensions, and
new language features into the core PostScript language imaging
model. PostScript Level 2 ensures application developers consistent
functionality across all Level 2 devices. When an application images
to a Level 2 device, it can be assured that a wide range of features
will exist on that device and that these features can be exploited
to their fullest for increased performance and functionality.

*** What are the features of PostScript Level 2?

PostScript Level 2 consolidates all of the current language
extensions into one unified language and adds many new features. It
is also upward compatible with the current generation of PostScript
devices. Here is a brief list of what comprises PostScript Level 2:

    + Existing PostScript language
    + Color extensions
    + Composite font extensions
    + Display PostScript extensions
    + Improved memory management
    + CIE-based device-independent color
    + Improved printer hardware features support
    + Data and image compression and decompression
    + Optimized graphics and text operators from the
      Display PostScript system
    + New halftoning algorithms
    + Forms support
    + Patterns support
    + Binary language encodings
    + ATM font rendering technology

*** What are the color extensions to the PostScript language?

The color extensions were added to the language in 1988 to provide
more complete color functionality. With the original PostScript
language, color could be specified using the red-green-blue (RGB)
and hue-saturation-brightness (HSB) color models.

The color extensions include cyan-magenta-yellow-black (CMYK) color
model, black generation and undercolor removal functions, screen and
transfer functions for four separate color components, and a
colorimage operator for rendering color sampled images.

The color extensions are currently found in PostScript color
printers from Canon, QMS, Oce, and NEC as well as all
implementations of the Display PostScript system.

*** Why would you want the CMYK color extensions in a black and white
    printer?

In a nut-shell, compatibility between black-and-white and color
Level 2 devices.

Today, ISVs must handle PostScript color printers differently.  For
example, current monochrome laser printers does not contain the CMYK
color extensions, and as a result PostScript language programs must
emulate this functionality, which results in slower performance. All
Level 2 implementations will include the CMYK color extensions as
standard.

*** What are the composite font extensions to the PostScript
    language?

The composite font technology is a general solution that extends the
basic PostScript language font mechanism to enable the encoding of
very large character sets and handle non-horizontal writing modes.

A Type 1 PostScript font has room for encoding only 256 distinct
characters. A typical Japanese font has over 7,000 Kanji, katakana
and hiragana characters. The composite font technology allows you
to create one "composite" font that is made up from any number of
"base" fonts. In addition, the composite font technology allows you
to include two sets of metrics (character spacing details) in the
font:  one for a horizontal-writing mode, and one for a vertical-
writing mode.

*** Why would you want the composite font extensions in a roman
    printer?

This technology is currently implemented only in Japanese language
PostScript devices, but the composite font technology is a general
solution that applies to any language. It allows for the creation
of one composite font that combines two or more fonts. For example,
you may wish to combine a text font (such as Times-Roman) with a
special font (such as Zapf-Dingbats) and have all characters at your
disposal within a single font.

*** What are the Display PostScript Extensions to the PostScript
    language?

The Display PostScript extensions address the needs of using the
PostScript language imaging model in a display environment. It
includes extensions to deal specifically with displays and windowing
systems as well as many optimized operators to increase performance
which is critical in an interactive display environment.

*** Why would you want the Display PostScript extensions in a
    printer?

Most of the functionality in PostScript Level 2 that comes from the
Display PostScript extensions result in improved performance. This
includes clipping, rectangle operators, and binary language
encoding to name a few. Each of the new Level 2 features that come
from the Display PostScript extensions are detailed later in this
document.

Another obvious reason is for compatibility between Display
PostScript applications and PostScript Level 2 printers.

*** Can you tell me more about the rest of the PostScript Level 2
    features?

Sure. Here a brief overview of the important features and benefits
of PostScript Level 2:

Filters
-------

+ A filter transforms data as it is being read from or written to a
file. The language supports filters for ASCII encoding of binary
data, compression and decompression, and embedded subfiles.
Properly used, these filters reduce the storage and transmission
cost of page descriptions, especially ones containing sampled
images. => Reduced storage requirements, greater performance.

+ ASCII encoding of binary data:  ASCII/85 (represent binary data
in ASCII format with only a 125% expansion of data), and ASCII/HEX
(current method of representing binary data in ASCII format but with
a 200% expansion of data). => Compact representation of binary data
in a portable ASCII representation.

+ Compression and decompression filters:  CCITT Group 3 & 4
(monochrome images), run-length encoding (monochrome and grayscale
images), LZW (~2:1 compression of text files),  DCT (20-200:1
compression of color images using the proposed JPEG standard).
=> Improved performance due to reduced transmission times. PostScript
files on disk can also be made much smaller, saving disk space.

Binary Encoding
---------------

+ In addition to the standard ASCII encoding, the language syntax
includes two binary-encoded representations. These binary encodings
improve efficiency of generation, representation, and
interpretation. However, they are less portable than the ASCII
encoding and are suitable for use only in controlled environments.
=> Performance, compactness.

Improved underlying implementation
----------------------------------

+ Improved font disk cache. We have improved the backup of the font
cache on printers with a hard disk. Font access methods for reading
the font back into RAM are more efficient. Also, the management of
the disk is improved, so it does not become fragmented. => Performance,
enhanced functionality.

+ ATM font rendering technology. => Improved performance (4-5 times
faster in raw character building speed) and improved quality (most
evident at small point sizes and low resolutions).

Improved memory management system
---------------------------------

+ One pool of memory available for all resource needs (page image,
font cache, path storage, downloadable fonts, etc.). Memory
allocated dynamically to meet needs. In general, memory is more
efficiently shared among different uses and arbitrary memory
restrictions have been eliminated. => Eliminates arbitrary memory
restrictions for imaging of more complex graphics.

+ Opportunistic memory management scheme. In the current system, the
PostScript language program must manage memory on a per page basis.
New memory management operators allow more flexibility for programs
to explicitly release unused memory resources by removing individual
entries from dictionaries and removing font definitions in an order
unrelated to the order in which they were created. => More efficient
use of available memory.

+ Automatic memory reclamation. VM is reclaimed automatically for
composite objects that are no longer accessible, such as strings
used by the show operator. A "garbage collector" will automatically
reclaim other unused memory. => More efficient use of available memory.

Optimized graphics operators
----------------------------

+ Rectangle operators. New operators for filling, clipping and
stroking rectangles; all highly optimized. For example, rectfill is
3 times faster than an equivalent moveto, lineto, lineto, lineto,
closepath, fill. => Performance, convenience.

+ Graphics state objects provide a fast way to switch between
graphics states, which define the current line weight, color, font,
etc. In existing printers, graphics states are stored on a stack,
so accessing an arbitrary graphics state is somewhat cumbersome.
With graphics state objects, the graphics state can be associated
with a name, and retrieved  by simply requesting the name.
=> Performance, convenience.

+ Halftone specification. New halftone dictionaries provide a more
precise way of specifying the halftone dots, and makes switching
between halftone screens faster. (The spot function is not
reinterpreted.) => Performance, convenience, enhanced functionality.

+ User paths are self-contained procedures that consists entirely
of path construction operators and their coordinate operands. User
path operators perform path construction and painting as a single
operation; this is both convenient and efficient. There is a user
path cache to optimize interpretation of user paths that are invoked
repeatedly. => Performance, convenience.

+ Stroke adjustment. For very thin lines, there is a trade-off
between perfect positioning and  consistent line width.  Depending
on the placement of such a line, it could end up being rendered as
either 1 or 2 pixels wide, which is a noticeable difference. To
account for this, PostScript language programs often include logic
to slightly alter the coordinates of lines for consistent rendering.
With automatic stroke adjustment the interpreter performs this
adjustment to ensure consistent widths.  Doing it in the interpreter
rather than in the PostScript language program is 20 - 30% faster.
=> Performance, convenience, improved quality.

Optimized text operators
------------------------

+ The xyshow operator provides a more natural way for applications
to deal with individual character positioning.  Allows simultaneous
track kerning, pair kerning, and justification. => Performance,
convenience.

+ The selectfont operator optimizes switching between fonts.  It
does the work of 3 Level 1 operators:  findfont, scalefont, and
setfont and has been optimized by using a caching mechanism.
=> Performance, convenience.

Forms
-----

+ A form is a self-contained description of any arbitrary graphics,
text, and sampled images that are to be painted multiple times on
each of several pages or several times at different locations on a
single page.

+ With the new forms feature, you can define a base form  whose
representation stays cached between pages, so only information that
changes between forms will need to be interpreted for each page. The
representation used to cache the form may vary from device to device
depending on the available resources, such as memory and/or hard
disk space. In some cases, the actual rasterized form will be saved,
in other cases, an intermediate representation (such as a display
list) may be saved. => End-users will benefit by improved performance.

+ This makes forms processing faster and provide a natural
framework for ISVs implementing a forms functionality in their
application. => Convenience for ISVs.

+ Besides the traditional concept of "forms," some other examples
of forms include:  Letterhead, stationary, overhead presentation
backgrounds, repetitive symbols in a CAD drawing such as screws
(mechanical drawing) or windows (architectural drawing), complex
background blends in 35mm slides. => Enhanced functionality and
application of PostScript printers in a variety of different environments.

Patterns
--------

+ The new pattern color space provides the ability to establish a
pattern as the current color. Subsequent use of operators such as
fill, stroke, and show apply "paint" that is produced by replicating
(or tiling) a small graphical figure called a pattern cell at fixed
intervals in x and  y to cover the areas to be painted. The
appearance of a pattern cell is defined by a PostScript language
procedure, which can include any arbitrary graphics, text, and
sampled images. The shape of the pattern cell need not be
rectangular, and the spacing of tiles can differ from the size of
the pattern cell. => Enhanced functionality, performance, convenience.

+ For efficiency, the representation of the pattern cell may be
cached. When cached, the execution of the procedure that defines the
pattern need be done only once for the current pattern.  The pattern
cache is similar to the font cache. => Performance.

+ Multiple colors can be specified in the pattern or the pattern can
be used as a mask to paint a color defined in some other color space.
=> Enhanced functionality

+ For display environments, this feature will allow patterns to be
represented in a resolution independent manner. Until now, patterns
have typically been represented by arrangements of pixels. This
resolution-dependent representation does not work well when trying
to image the pattern at a variety of different resolutions.

Images
------

+ There are several enhancements to the facilities for painting
sampled images: use of any color space, 12-bit component values,
direct use of files as data sources, and additional decoding and
rendering options. => Convenience, performance, quality.

Composite Fonts
---------------

+ Provides the basic machinery for non-Roman character sets. Enables
the encoding of very large character sets and non-horizontal writing
modes. => Enhanced functionality.

+ Provides a page description language for international business.
Composite font technology makes printers more international. The
same font technology can be used worldwide, and will provide support
for companies that must work in today's international business
environment. => Enhanced functionality.

+ Advantages not limited to foreign languages - also useful for
strictly Roman printers: allows the creation of a single composite
font that combines two or more fonts.  For example, you may wish to
combine a textual font (such as Times-Roman) with a graphical font
(such as Zapf-Dingbats), and have all  characters at their disposal
within a single font. Other uses of composite fonts:  IBM extended
character set, and expert sets (such as Adobe Garamond). => Enhanced
functionality and increased performance by minimizing
switching between fonts.

New Color Spaces
----------------

+ CMYK color model and support for color images. Enhanced
functionality. This will encourage more ISVs to use the color
operators, because the operators will be widely available (The
printer itself may not be able to print in color, but the PostScript
language program won't generate errors when the operators for CMYK
color are used.)

+ PostScript Level 2 supports several device-independent color
spaces based on the CIE 1931 (XYZ)-space. CIE-based color
specification enables a page description to specify color in a way
that is related to human visual perception. The goal of the CIE
standard is that a given CIE-based color specification should
produce consistent results on different color output devices,
independent of variations in marking technology, ink colorants, or
screen phosphors. True device-independent color specification.
Improved color matching between devices.

+ PostScript Level 2 supports three classes of color spaces:  device
independent, special, and device dependent.

The following device independent color spaces are standard:

The CIEBasedABC color space is defined in terms of a two-stage, non-
linear transformation of the CIE 1931 (XYZ)-space. The formulation
of the CIEBasedABC color space models a simple zone theory of color
vision, consisting of a non-linear trichromatic first stage combined
with a non-linear opponent color second stage. This formulation
allows colors to be digitized with minimum loss of fidelity; this
is important in sample images.

Special cases of CIEBasedABC include a variety of interesting and
useful color spaces, such as the CIE 1931 (XYZ)-space, a class of
calibrated RGB spaces, a class of opponent color spaces such as the
CIE 1976 (L*a*b*)-space and the NTSC, SECAM, and PAL television
spaces.

The CIEBased A color space is a one-dimensional and usually
achromatic analog of CIEBasedABC.

The following special color spaces are standard:

The Pattern color space enables painting with a "color" defined as
a pattern, a graphical figure used repeatedly to cover the areas
that are to be painted. See the discussion of patterns for more
information.

The Indexed color space provides a way to map from small integers
to arbitrary colors in a different color space such as a device
independent color space.

The Separation color space provides control over either the
production of a color separation or the application of a device
colorant, depending on the nature and configuration of the device.

The following device dependent color spaces are standard:

The DeviceGray color space is equivalent to the existing PostScript
language's gray color model.

The DeviceRGB color space is equivalent to the existing PostScript
language's red-green-blue (RGB) color model.

The DeviceCMYK color space is equivalent to the existing PostScript
language's cyan-magenta-yellow-black (CMYK) color model.

New screening/halftoning technology
-----------------------------------

+ Improved algorithms for determining the angles and frequencies
used for halftone screens. The improvements fall into two primary
categories:  general improvements, and improvements specific to
color separations.

+ General improvements:   (1) The new algorithms yield a 10%
improvement in the speed of the setscreen and image operators; (2)
Earlier version of PostScript software could produce halftone
screens only for certain angle and frequency combinations. Enough
of these combinations were available so that any requested screen
could be fairly well approximated by one of the available angle and
frequencey combinations. In contrast, the improved halftoning
algorithms can provide as much as a ten-fold increase in the number
of angle-frequency combinations that are available, depending on the
device resolution and the available memory. => Increased performance
and higher quality halftone screens.

+ Improvements specific to color separations:   An additional
feature is available that enables PostScript software to generate
extremely accurate screen angles and frequencies. The screens
produced by this method can achieve an angular accuracy of within
05 degrees or better, depending on such parameters as exact screen
angle requested, device resolution, and memory available for use by
the algorithm. => Extremely high-quality color separations that
approach the quality that previously was available only from high-end,
color electronic pre-press systems.

Improved printer support features
---------------------------------

+ Page device setup provides a device independent framework for
specifying the requirements of a page description and for
controlling both standard features, such as the number of copies,
and optional features, such as duplex printing, paper trays, paper
sizes, and other peripheral features.

+ Applications developers will be able to write a single driver for
a variety of different PostScript printers. The same code can be
used to address printer specific features whether the features exist
in the printer or not. If the feature is not in the printer, the
application can decide how to best respond to the lack of the
feature. => Enhanced functionality. ISVs benefit by having a more uniform
method for accessing printer specific features. End users benefit
by having software that will take advantage of their printer's
features.

Interpreter parameters
----------------------

+ Administrative operations, such as system configuration and
changing input-output device parameters, are now organized in a more
systematic way. Allocation of memory and other resources for
specific purposes is under software control. For example, there are
parameters controlling the maximum amount of memory to be used for
VM, font cache, pattern cache, and halftone screens. => Flexibility.

Resources
---------

+ A resource is a collection of named objects that either reside in
VM or can be located and brought into VM on demand. There are
separate categories of resources with independent name spaces - for
example, fonts and forms are distinct resource categories.

+ The language includes convenient facilities for locating and
managing resources.

Dictionaries
------------

+ Many Level 2 operators expect a dictionary operand that contains
key-value pairs specifying parameters to the operator. Language
features controlled in this way include halftones, images, forms,
patterns, and device setup. This organization allows for optional
parameters and future extensibility. For convenience in using such
operators, the PostScript language syntax includes new tokens, << and
>>, to construct a dictionary containing the bracketed key-value
pairs. => Convenience, extensibility.

*** What's the feedback from Adobe's OEMs on PostScript Level 2?

The feedback has been overwhelmingly positive. We have always
believed that we are taking our OEMs, ISVs and end users best
interests into account in moving forward with the PostScript
language. The feedback we have received so far confirms that we are
doing the right thing on all fronts.

*** How much ROM/RAM will it take for a Level 2 printer?

As is true with our current implementations, RAM/ROM requirements
will vary from one device to the next depending on the specific
capabilities of each device.  However, our estimates put the code
size at approximately 1.5 Mb of ROM (for CISC processors), and 1.5
Mb of RAM, minimum.

*** When will Level 2 products be available?

The first Level 2 products should be available in early 1991. Exact
product delivery dates will be announced by our OEMs as usual.

*** What about existing PostScript printers? Are they obsolete?

The current generation of PostScript printers (which you could think
of as PostScript Level 1) will not become obsolete because of Level
2 products. Think of Level 1 and Level 2 printers as a family of
products, each having its own set of features to suit the needs of
a particular customer. While we will continue to support and build
Level 1 products (based on our OEM's demands) we think that over the
next 12-18 months most of our OEMs will begin providing PostScript
Level 2 products.

*** Are Level 1 and Level 2 implementations compatible?

All existing programs that run on today's PostScript printers will
run on a Level 2 device. That is, PostScript Level 2 is upward
compatible with the existing installed base of printers and print
drivers. However, it is not 100 % backward compatible. A file
written specifically to take advantage of some Level 2 features will
not run on a Level 1 printer because some functionality cannot be
emulated. Most Level 2 features can be emulated on a Level 1 printer
and an intelligent driver can conditionally use Level 2 features
when available, and fall back on Level 1 operators when not. The new
red book will include an appendix that will help ISVs deal
specifically with compatibility issues.

*** When will the new red book be available?

A new version of the red book, called the PostScript Language
Reference Manual, Second Edition, will be published by Addison-
Wesley in December 1990.

*** How is Adobe positioning PostScript Level 2?

Adobe is positioning PostScript Level 2 as an integral part of a
total system solution for printing and display environments.
PostScript Level 2 software provides the foundation for Adobe's OEMs
to implement an entire spectrum of products from low-cost desktop
laser printers for office-automation to high-resolution
imagesetters for producing color separations.

Let's put PostScript Level 2 in perspective with respect to the
overall printing solution. The effectiveness and performance of any
particular printing solution is affected by four main elements:

+ Driver:  Each major system software environment (Macintosh,
Windows, OS/2 Presentation Manager, NeXT) has a built-in PostScript
language driver. These system level drivers ensure that all
applications running in the environment can output to PostScript
printers. These drivers do not always produce the most efficient
PostScript language programs, and may not support the wide variety
of features available in the language or specific hardware features
in a PostScript printer.

+ Language:  The PostScript language as defined in the PostScript
Language Reference Manual (the "red book") is the standard today.

+ Communications:  AppleTalk, parallel, and serial communications
are  the most commonly used interfaces with PostScript printers
today.

+ Controller:  Today, most Adobe PostScript printers are based on a
variety of controllers:  Scout (68000), Atlas (68020), and Atlas
Plus (68030). In addition, there are a number of custom controller
solutions offered by our OEMs.

Total system throughput is a function of all four elements. An
efficient driver can produce PostScript page descriptions that print
much faster; speed increases of 2-3x over an inefficient driver are
not uncommon. Communications bottlenecks can account for a majority
of the time it takes to print a page; a very large scanned image can
take minutes to transmit to the printer, even using AppleTalk. And
of course, the speed of the controller itself has a direct impact
on the time it can take to print a page. However, the limiting factor
is ultimately the rated engine speed of the output device.

PostScript Level 2 is one componto exTha total systems solution
being assembled by Adobe:

+ Adobe is developing drivers for the Macintosh, Windows 3.0, and
OS/2 Presentation Manager environments. These drivers will take full
advantage of the features and performance enhancements in PostScript
Level 2 printers as well as existing PostScript printers.

+ PostScript Level 2 extends the PostScript language with new
operators to improve performance and provide additional
functionality to address the need of end users and ISVs.

+ PostScript Level 2 includes a variety of file compression
techniques that can be used to reduce the amount of information sent
(and hence the time to do so) to the PostScript printer.

+ Adobe is developing new controllers based on the latest RISC
technology which are up to 22 times faster than current controllers.
In addition, these controllers provide  our OEMs the potential for
providing direct SCSI input and Ethernet connections for increased
throughput.

(C) 1990 Adobe Systems Incorporated. All rights reserved.
PostScript, Display PostScript, and Adobe are trademarks of Adobe
Systems Incorporated registered in the U.S. All other product names
are trademarks or registered trademarks of their respective holders.