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In the current LaserWriter 8 driver,
the Chooser lists printers which are available on the AppleTalk network
(either LocalTalk or AppleTalk) with an NBP type of "LaserWriter".
When the user selects one of these printers, LaserWriter 8 creates a desktop
printer by sending an Apple Event to the Finder. In the past, this mechanism
of setting up printers has served the needs of most users. However, with
LaserWriter 8.5.1, Apple is responding to several requests for more flexible
desktop printing functionality. To this end, Apple has introduced a new
application called Desktop Printer Utility which enables users to create
additional types of desktop printers. Users can now create desktop printers
which use the Unix lpr protocol for printing, in addition to the regular
AppleTalk PAP printers. There are also "hold" printers which represent
local print queues and "virtual" printers which represent printers
which are not available on the network.
Along with the expanded capabilities, Desktop Printer Utility is also
customizable. LaserWriter 8.5.1 and Desktop Printer Utility (together)
support the creation and use of desktop printers known as Custom DTPs.
When printed to, Custom DTPs cause the LaserWriter 8.5.1 driver to create
a PostScript file and to launch an application that can post-process the
PostScript. This post-processing application can do anything it likes with
the PostScript file, such as converting the PostScript into another file format,
transferring the file to another location using a modem or a network connection,
or displaying the PostScript file to the user. This Technote describes how a
developer might customize Desktop Printer Utility for use with an application.
Updated: [Feb 06 1998]
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Transparent Mode
One transfer mode previously unsupported by the LaserWriter
driver is the transparent transfer mode. This mode allows drawing
of images other than 1-bit deep so that any sample in the source
which is the transparent color (defined as the QuickDraw background
color) will not be painted to the destination. The advantage of this
mode is that it lets a deep (> 1-bit) image have holes in it by coloring
the holes in the image with the transparent color. Whereas previously the
LaserWriter driver would paint these images as if the transfer mode were
srcCopy, LaserWriter 8.5.1's new imaging code now handles the
transparent mode.
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maskRgn Clipping
Another previously unsupported feature of CopyBits() was the ability
to pass in a mask (via the maskRgn field) to clip out portions of the
destination image. Previous LaserWriter drivers completely ignored the
maskRgn passed to CopyBits(). The new imaging code
in LaserWriter
8.5.1 now supports the supplied maskRgn.
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Note:
While the LaserWriter 8.5.1 driver now
handles a supplied maskRgn
to a CopyBits call, we do not do any special handling of the clip provided
in the grafPort. GrafPort clipping is still handled by clipping
only to the
bounding rectangle of the port's clipRgn, both for CopyBits calls as well as
line art and text drawing. Apple considered more generalized support for the
port clipRgn but has rejected it to date mainly due to compatibility concerns.
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Limitations
There are some known limitations with the added CopyBits() support
that you
should be aware of. They are:
- Extremely complex clipping regions may occasionally fail on Level 1 or
Level 2 devices in low printer memory conditions. The result is that the
output will not preserve the clip, and the image will print as if the
maskRgn
parameter is ignored.
Specifically, for PostScript Level 1 printers, there is a hard coded limit to how
complex a clip can be, regardless of the printer's installed memory. If the clip is
more complex than this limit, the LaserWriter driver ignores the maskRgn
parameter.
For Level 2 printers, the limits are completely memory-based and generally much more
flexible. However, once again, in low-memory conditions, the clip may still fail.
On PostScript Level 3 printers, all clips should always print correctly regardless
of the complexity of the clip since the driver uses the PostScript Level 3 masked
image to support this feature.
- Since the LaserWriter driver implements the transparent mode via clipping,
complex transparent regions may occasionally fail on Level 1 or Level 2 devices
in low-memory conditions as described above. The result is that the output does
not preserve the transparency, and the pixels image with the background color instead.
- For 2, 4, and 8-bit images with a color look-up table (CLUT), the LaserWriter
8.5.1 driver only supports one index which maps to the background color. Although
the CLUT may contain many index values which have the background color and are
therefore, in principle, transparent when displayed onscreen, the driver searches
the look-up table for the first index which corresponds to the background color.
Index samples with that index value are printed as transparent. Other index
values which correspond to the background color are not treated as transparent
but are printed as the background color.
- In order to maintain compatibility with many applications, the
LaserWriter 8.5.1 driver will ignore a
maskRgn which is an empty region
under some circumstances (e.g. some rotated text and graphics). Some
applications have used the fact that previous drivers always ignored the maskRgn
parameter to clip out certain data when printing to QuickDraw printers.
Relying on this 'feature' is not recommended, and the fact that there are some
edge cases where a zero clip is ignored for compatibility reasons may be a
temporary 'feature' of the driver. To avoid accidentally encountering these
cases, when you really want a zero clip, either do not draw the image (preferred)
or use grafPort clipping to clip it.
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Summary
New imaging code in the LaserWriter has opened up some new
functionality for the QuickDraw CopyBits() call. We encourage you
to try LaserWriter 8.5.1 with your application and any PostScript device.
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References
Inside Macintosh: Imaging With QuickDraw, Chapter 3 and Appendix B.
Apple's Technote web site
PostScript Level 3 documentation from Adobe Systems Incorporated.
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Downloadables
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Acrobat version of this Note (64K).
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