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This document presents known discrepancies between Artifex's PCL XL
interpreter and the output of the Genoa LaserJet 5 PCL6 FTS and CET.

		Discrepancies from Genoa LaserJet 5 PCL6 Output

Introduction
============

This document presents discrepancies between our interpreter and the output
of individual Genoa tests due to any of the following:

	A) Residual problems in our code.

	B) Differences in halftone screen.

	C) Differences in TrueType font rasterizer.

	D) Different implementation choices that we believe will be fully
	acceptable to customers.

	E) [2] Implementation shortcuts that are visible to customers but
	that are too much work to change unless we receive an explicit
	request to do so.

	F) Likely H-P firmware bugs.

We will fix category (A) to the best of our ability in the initial product
code release, with some additional fixes likely to be required in later
releases.  [4] We will fix category (B) when we have been able to analyze
the H-P screen in complete detail, which we have not yet been able to do.
We do not plan to do anything about (C) or (D).  We are willing to discuss
items in (E) individually.

Some discrepancies are inevitable given that our interpreter is designed to
meet the following not always consistent specifications:

	1) The published "PCL XL Feature Reference Protocol Class 1.1"
	document from Hewlett-Packard.

	2) The printed output provided by Genoa with their LaserJet 5
	PCL6 test suites.

[4]	3) The printed output we obtained from a LaserJet 6MP.

	4) In a few cases, primarily where #1 is silent or ambiguous
	or where #1 and #2 disagree, our best guess as to H-P's
	intentions.

In general, when we could find an interpretation of the H-P document that
was consistent with the Genoa output, we have followed it in our
implementation, even if that interpretation is not the most obvious one.
[4] In the few cases of discrepancies between the LJ5 and the LJ6MP, we have
followed the latter, since in all such cases the 6MP agrees with our
understanding of the specification.

Changes in a given revision of this document are marked with the revision
number in [brackets].  Revision history:
		first issued January 1, 1997
		rev. [1] January 11, 1997
		rev. [2] January 15, 1997
		rev. [3] January 23, 1997
		rev. [4] February 9, 1997

Code problems
=============

Page shifting
-------------

We do not currently handle discrepancies between the printer's (0,0) point
and the physical page corner correctly.  For example, when printing to the
LaserJet 4/5/6, all pages are shifted down and to the right by approximately
1/4".  (This should not affect OEM customers.)

Memory management
-----------------

If an error occurs in the middle of downloading an object like a font or a
pattern, the current code does not free the partially-built structure.

We have not attempted to exhaustively test graceful recovery from
out-of-memory conditions; it is likely that such conditions may, at least
under some circumstances, require reinitializing the interpreter.  [4] The
interpreter does not store any state in global variables, so simply
releasing all memory acquired by the interpreter and reinitializing the
interpreter is always safe.

Miscellaneous
-------------

Our current implementation of pseudo-bold characters requires a large amount
of buffer space, which causes a memory overflow error on the following
files:
	CET c309

[4] Our current implementation of elliptical arcs has a slight rounding
inaccuracy that produces an inappropriate visible hairline in 4 of the 5
images in the rightmost column of the bottom half of
	CET c404 p. 25
This inaccuracy may affect other CET pages where the closing lines of two
chords whose bounding boxes have different aspect ratios are supposed to
coincide.

[4] In our current implementation, reading back the clipping path as the
current path may produce a path that covers a region as much as 1 pixel
larger all the way around as the original path that was specified for
clipping.  This produces an inappropriate hairline border around the objects
in image 02 of
	FTS t324

Halftone screen
===============

We have not yet been able to duplicate the behavior of H-P's carefully
designed halftone screen, which produces striking and unusual effects when
ORing together different source and paint gray shades using RasterOp.  Many
of the Genoa tests do this, even though this is not something that we expect
to occur frequently (or perhaps at all) in application output.
Unfortunately, the visible effects are not subtle (they arise from something
similar to Moire' patterns).  We can't give an exhaustive list of the test
pages that this affects, since many pages on many tests do this.  [4] One
unobvious result of this is that certain objects simply disappear: for
example, some gray-shaded rectangles in
	CET c420 pp. 5-14

TrueType rasterizer
===================

Our current TrueType font rasterizer is not of production quality: it
disregards the hinting instructions and does not handle correctly multi-part
characters where one part is offset with respect to another.  This causes
many differences in the output, mostly minor.  We plan to offer a
commercial-quality rasterizer at some time in the future; however, our PCL
XL interpreter is an OEM product for which our customers are expected to
provide their own rasterizers.

Implementation choices
======================

Range of coordinates
--------------------

Our interpreter uses a fixed-point representation for device coordinates (20
bits of integer, 12 bits of fraction) and keeps all coordinates in this
form, including the cursor position.  Thus, any operation that sends the
cursor outside this range will cause an InternalOverflow error.  At 600 dpi,
this corresponds to positioning the cursor more than about 72 feet outside
the page.  A number of the CET files do this, to exercise the full range of
possible parameter values.  We recognize this is a limitation, but we do not
currently intend to do anything about it, since it is extremely unlikely to
cause problems in practice.  The files on which this causes an error are:
	CET c309, c310

Line Printer font implementation
--------------------------------

Our interpreter implements the Line Printer fonts as aliases for Courier,
unlike the H-P printers which apparently implement them using bitmaps.  As a
result, attempting to scale, rotate, or shear them does not produce an error
in our implementation.  This causes an output difference in:
	CET e104

Graphics state storage management
---------------------------------

Our interpreter allocates graphics states from the main storage pool, rather
than imposing a fixed limit on the number of PushGS levels.  As a result,
our implementation does not report an error, even though the H-P printer
produces one, in:
	CET e112

Downloaded font checking
------------------------

Our interpreter defers almost all of its checking of downloaded font headers
until it receives the EndFontHeader command.  Because of this, many of the
errors associated with font downloading occur on an EndFontHeader instead of
a ReadFontHeader, at a slightly later position in the file.  This affects
the output from:
	CET e114
Also, our implementation does not produce the very first error from this
file (the "InternalError 0x50").

Byte order support
------------------

The H-P implementation apparently supports only the little-endian binary
binding; our interpreter supports both big- and little-endian binary
bindings.  The only file that tests this is:
	CET e120
Because our interpreter supports both bindings, this file should produce at
least one fewer error than the H-P printers.  However, there is an error in
the file itself: at byte position 101 in the big-endian section (counting
the first byte of PCL XL data after the stream header as byte 0), the length
of the font name "Courier" is incorrectly coded as (16,0) (little-endian)
rather than (0,16) (big-endian).  The resulting 4096-byte count causes the
entire remainder of the file to be interpreted as a data stream; since this
exceeds the length of the file, the test terminates.

[2] Implementation shortcuts
============================

[1] Negative dash pattern elements
----------------------------------

SetLineDash allows negative dash pattern elements, but the H-P documentation
gives no clue about what these are supposed to do.  The H-P printers
apparently interpret it as drawing a line backwards in the current direction
(which may extend outside the original subpath) with no visible caps; a dash
pattern with a negative total length crashed the LJ 6MP firmware so badly
the printer had to be power cycled!  We have chosen to take a different
approach: we compensate for negative elements by propagating them to
adjacent positive ones.  This doesn't produce quite the same output as the
H-P printers do (segments drawn in the reverse direction do have caps, and
if the line is shorter than one complete cycle of the pattern, some parts of
the line may be stroked that the H-P printers skip), but this is such an
obscure feature that we don't think it's worth the trouble to emulate
exactly.  The difference affects:
	CET c318

Rounding
--------

On the page:
	FTS t421 p. 4
our implementation shows faint vertical "seams" on the two left-hand
patterns in image 09.  While the Genoa output doesn't have a seam, a test we
made using H-P's default halftone does show a seam.  We conclude that our
implementation and H-P's both have some rounding sensitivities, and they
simply happen to show up on slightly different input values.  [2] Matching
H-P's implementation at this level of detail would be extremely onerous.

[3] A similar seam occurs in the lower image on the page:
	CET c422 p. 38

[4] For a similar reason, in image 06 of
	FTS t315 p. 2
our implementation does not produce either bevels or miters.  The reason is
that the line join point coincides exactly with the end of an "ink off" dash
segment: H-P's implementation has a rounding inaccuracy that causes it to
produce a vertical hairline as the last segment of the horizontal line
(which is drawn in the -X direction), and then join this hairline with the
first segment of the diagonal line.

H-P firmware bugs
=================

[4] Except for the instances mentioned here, our implementation emulates
behavior of the H-P printers that we have stated elsewhere we believe result
from firmware bugs.

Changing destination size of raster patterns is permanent
---------------------------------------------------------

Specifying a NewDestinationSize for a raster pattern in the SetBrushSource
or SetPenSource command apparently changes the destination size permanently,
or at least for a subsequent SetBrush/PenSource command that doesn't specify
a NewDestinationSize.  This is tested explicitly (and the H-P printers fail
the test) in:
	CET c306 p. 43, c307 p. 44

Mysterious missing character
----------------------------

In:
	CET c330 last page
a partial 'B' should appear in portrait orientation to the left of the
partial 'A'.  The input data read as follows (slightly abridged):

	0 6600 usp @PageOrigin SetPageOrigin
	90 ss @PageAngle SetPageRotation
	PushGS
	-2300 1900 ssp @PageOrigin SetPageOrigin
	2.23 1.1 rp @PageScale SetPageScale
	(Courier       Bd) ba @FontName 1000 us @CharSize
	  629 us @SymbolSet SetFont
	0 b @NullBrush SetBrushSource
	NewPath
	1500 0 3500 1200 usq @BoundingBox
	2100 0 ssp @StartPoint
	2600 0 ssp @EndPoint
	PiePath PaintPath
	0 b @ClipRegion SetClipReplace
	<79 7b 7d> ba @RGBColor SetBrushSource
	2500 900 ssp @PageOrigin SetPageOrigin
	0 0 usp @Point SetCursor
	-180 ss @PageAngle SetPageRotation
	(A) ba @TextData Text
	SetPageDefaultCTM
	2100 3600 usp @Point SetCursor
	(B) ba @TextData Text

Even when we modified the test data to paint an entire page of 'B' at this
point, no characters appeared.  We assume this results from a firmware bug,
but we have no theories as to its nature.

[2] Characters transformed in wrong order
-----------------------------------------

In
	CET c333 p. 16	
in the "Effect on CharAngle" and "Effect on CharShear" lines, the character
appears to be scaled by the page scale before being rotated by the character
angle or sheared, rather than vice versa.  We believe this is a firmware bug
because we verified, by testing all 125 possible sequences of 3 operations
chosen from the set {SetCharAngle, SetCharScale, SetCharShear,
SetPageRotation, SetPageScale}, that the character transformations uniformly
occur before the page transformations.  (We have identified two other
firmware bugs relating to character transformations, one of which is present
in the LJ5 but not the LJ6MP: see our separate report on the CET for
details.)
