Concepts printing

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本文适用于 Linux 下 CUPS 版本高达 2.x，以及那里的传统过滤系统和后端。

如今的无驱动程序打印工作流程则大不相同。

Common printing data formats

PostScript: The traditional printing data format

Linux uses a common data format for printing: The PostScript format. This means that application programs usually produce PostScript output when printing.

Some printers accept PostScript natively, i.e. PostScript printers can print application's printing output directly.

Other printers need Ghostscript plus a printer driver which convert PostScript into the "language" (i.e. printer-model-specific data) that the particular printer accepts.

In general "driver" means device-model-specific software and data.

更多信息

PDF: The recent printing data format

There is a general move away from PostScript to PDF as the standard print job format. This change is advocated by the OpenPrinting workgroup of the Linux Foundation and the CUPS author.

On http://www.linuxfoundation.org/collaborate/workgroups/openprinting there was the following

PDF is the standard print job format from CUPS 1.6.x on

All important desktop applications (GTK/GNOME, Qt/KDE,
LibreOffice/OpenOffice.org, Firefox, Thunderbird, ...)
send print jobs in PDF and not in PostScript any more
by default. In addition, a complete CUPS filter chain
to process print jobs in PDF is available and used.

CUPS author Mike Sweet/Apple have decided to
not include the Linux-specific CUPS filters in the
upstream CUPS source any more and we have agreed
to maintain them at OpenPrinting.
Here we have done some clean-up and have discontinued
the filters for the PostScript-centric workflow
in favor of the PDF workflow, meaning that the upstream
standard for CUPS under Linux (using CUPS plus our
cups-filters package) is the PDF-based job processing,
letting every non-PDF input be converted to PDF first,
page management options being applied by a pdftopdf filter
and Ghostscript being called with PDF as input.

Only exception is if the input data is PostScript and
the printer is a PostScript printer or the printer driver
requires PostScript as input. The [sic] we do not turn
PostScript into PDF and back into PostScript but pass
the PostScript through the pstops filter as before.

Having this workflow we ask all driver developers kindly
to not create any PPDs/drivers for non-Postscript printers
which require exclusively PostScript. PPD files should
at least accept PDF or CUPS Raster now. See also our driver
design/packaging page
http://www.linuxfoundation.org/collaborate/workgroups/openprinting/writingandpackagingprinterdrivers

More info on our page about the PDF printing workflow
http://www.linuxfoundation.org/collaborate/workgroups/openprinting/pdf_as_standard_print_job_format
and on the cups-filters page
https://www.linuxfoundation.org/collaborate/workgroups/openprinting/cups-filters.

PostScript versus PDF as standard print job format

In contrast to PostScript the PDF format is a de facto standard that is commonly used for worldwide document interchange which makes the PDF format most suitable to be also used as standard print job format regardless that in particular related to printing the PDF format has drawbacks compared to PostScript.

With PDF as the standard print job format traditional PostScript printers can no longer print application's printing output directly so that a conversion step in the printing workflow is required that converts PDF into PostScript (usually via Ghostscript). But there are also PostScript+PDF printers that can print both PostScript and PDF directly.

For PostScript there are three versions (called "levels"): Level 1, level 2, level 3 where each is a superset of the former (see https://en.wikipedia.org/wiki/PostScript). Additionally there is the DSC specification (see http://en.wikipedia.org/wiki/Document_Structuring_Conventions).

There is no such thing as THE PDF format.

For PDF there are various versions and many different subsets where each subset again can have various different versions (e.g. PDF/A-1, PDF/A-2, PDF/A-3, PDF/X-1, PDF/X-1a, PDF/X-3, PDF/X-4, PDF/X-4p, PDF/X-5, PDF/X-5g, PDF/X-5pg, PDF/X-5n, PDF/VT-1, PDF/VT-2, ... and so on ad nauseam) see http://en.wikipedia.org/wiki/Portable_Document_Format and follow the links therein.

PDF is only partially an open standard. There are still some technologies that could be used in PDF files, that are defined only by Adobe and remain proprietary (see http://en.wikipedia.org/wiki/Portable_Document_Format).

Accordingly free software applications and tools that support PDF in general cannot support all the possible features that are supported by original Adobe programs. Adobe is inventing more and more new features that take time to go into the open standards and then additional time until they get supported by free software applications and tools.

When printing "just PDF" it is basically more or less luck what that results as printout because it depends on how much built-in "intelligence" the programs have (usually Ghostscript) that convert the "just PDF" data into printer specific data.

For example transparency is by design problematic, see http://en.wikipedia.org/wiki/Portable_Document_Format#Transparency and http://en.wikipedia.org/wiki/Transparency_%28graphic%29#Transparency_in_PDF

In contrast to "just PDF" which has zillions of fancy features (also think about various kind of interactive elements and so on) PDF/A-1 is a subset of the whole PDF specification that is intentionally meant to have only a minimal feature set to work stable over the time.

But contradictorily even PDF/A gets enhanced and enhanced with new features over the time, see http://en.wikipedia.org/wiki/PDF/A

This means when one wants to ensure that the printout looks as intended, one should let the application produce PDF/A-1.

Unfortunately most applications do not support any PDF/A. Usually applications only support "just PDF".

Alternatively there is also PDF/X in various flavours (PDF/X-1a,... PDF/X-4, PDF/X-4p, PDF/X-5pg,...), see http://en.wikipedia.org/wiki/PDF/X

PPD files: Printer-specific options

PostScript Printer Description (PPD) files list printer-specific values and options and the possible choices for each option like

  • Page size: A4, Letter, Legal
  • Paper input slot: Default, Envelope, ManualFeed
  • Printing resolution: 300x300dpi, 600x600dpi

Application programs use PPD files to show the printer-specific options in their printing dialogs and to get printer-specific values that are of interest when applications produce PostScript or PDF output for printing like

  • Monochrome or color printer
  • Printable area values for each page size

Printer setup tools (e.g. the YaST Printer module) use the following values from PPD files to list printer models

  • Printer manufacturer
  • Model name

For PostScript printers the manufacturers provide a PPD file for each particular printer model.

For other printers (i.e. non-PostScript printers) the printer driver software package usually contains PPD files for the printer models which are supported by the particular printer driver. For non-PostScript printers, the PPD file contains the information which printer driver program must be used and the printer-specific options that are available for the particular driver.

更多信息

CUPS: The server between user and printer

The Common Unix Printing System (CUPS) is the base printing system developed by Apple Inc. for Mac OS X and other UNIX-like operating systems, in particular Linux. CUPS is now developed at OpenPrinting https://openprinting.github.io/cups/

A printer device is not used directly by user application programs but via a print queue.

When various application programs submit print jobs simultaneously, these jobs are put in a queue and sent one after the other to the printer device. Even when a single user with one application program submits a single print job, the print queue holds the job so that the application program can continue and the user does not need to wait until the printer device had finished to actually print it.

CUPS is the software which must run on a computer so that application programs can send their printing output to CUPS which queues the print jobs and processes them one by one.

For each print queue there is a PPD file which CUPS provides to application programs so that the printer-specific options are available in the printing dialogs.

In particular for non-PostScript printers CUPS runs the printer driver and sends its result to the printer device. To do this, CUPS must know which printer driver belongs to the printer device and how the printer device is connected to the computer so that CUPS can send printing data to the printer device.

因此，“设置打印机”意味着“设置打印队列”并告诉 CUPS 针对特定队列

  • How the printer device is connected to the computer
  • Which printer driver belongs to the printer device (via the PPD file)

Strictly speaking from the computer's internal point of view it usually doesn't know about the actual printer devices but only about print queues. What CUPS and application programs call "printers" are usually not the actual printer devices but only their associated print queues. This sketchy usage of the word "printer" for the computer's internal representation can lead to confusion when it is not clear for the user what it actually means when CUPS or application programs show that everything is o.k. with the "printer". Usually this means that the state of the print queue is o.k. (e.g. "ready") but the state of actual printer device could be different at the same time (e.g. "out of paper").

更多信息

Printer drivers: Make the printer print

In general "driver" means device-model-specific software and data.

The printer driver produces the final printer-specific data which actually lets the printer print.

Each driver accepts only certain printing data formats and produces data compatible with only certain printer models. If a wrong driver is used, wrong data is sent to the printer which produces bad looking printout, chaotic printout, or no printout at all.

Even if a printer is listed as "supported", there could be minor differences between an application's PostScript printing output and the actual printout on the paper when the driver is not specifically optimized for the particular printer model (see SDB:Purchasing a Printer and Compatibility).

PostScript printers do not need a printer driver because they do not need driver software which converts PostScript into another "language" that the printer actually "understands". For example such a printer language is PCL (Printer Command Language developed by Hewlett-Packard).

PostScript printers need only a PPD file which matches the particular model to make the printer-specific options available to application programs.

Other printers (i.e. non-PostScript printers) need printer driver software plus a PPD file which matches the particular driver.

Usually there is neither such a thing as "the one and only driver" nor is there such a thing as "the best driver" for a particular printer model. The better the printer hardware is, the more different drivers are available, where each driver is best only for a particular use case.

For example a PostScript+PCL color laser printer could be operated as

  • PostScript printer using a PostScript PPD
  • color PCL printer using a PPD for a color PCL driver
  • monochrome PCL printer using a PPD for a monochrome PCL driver

Printing via a PCL driver is usually faster but with less quality than printing in PostScript mode and using a monochrome PCL driver is probably the best way to enforce fast monochrome-only printing.

More information regarding PostScript+PCL printers

openSUSE printer driver software packages

See the "Driver" section in YaST Printer how to install printer driver software packages with the YaST printer setup module.

Some standard printer driver and PPD files packages

  • gutenprint: the Gutenprint driver and its matching PPDs
  • OpenPrintingPPDs-ghostscript: PPDs for Ghostscript built-in drivers
  • OpenPrintingPPDs-hpijs: PPDs for the HPIJS driver for non-HP printers
  • OpenPrintingPPDs-postscript: PPDs for PostScript printers

此外，惠普还提供了适用于惠普打印机和扫描仪的 Linux Imaging and Printing (HPLIP) 驱动解决方案，该方案包含在惠普一体机中。它包括 Ghostscript 打印机驱动程序 HPIJS 及其匹配的 PPD 文件，以及原生 CUPS 打印机驱动程序 HPCUPS 及其匹配的 PPD 文件，以及 SANE 扫描仪驱动程序 hpaio。openSUSE 在 hplip 和 hplip-hpijs 包中提供 HPLIP，并且从 openSUSE 12.1 开始，还在 hplip-sane 包中提供，请参阅 SDB:How to set-up a HP printer。

此外，从 openSUSE 12.1 开始，还提供了 适用于 Linux 的 Epson Inkjet Printer Driver (ESC/P-R)。openSUSE 在 epson-inkjet-printer-escpr 包中提供它。

最后，打印机制造商提供的更多 PostScript 打印机 PPD 文件包含在 manufacturer-PPDs 包中。

关于“打印机驱动程序包的版本升级”，请参阅 SDB:Installing a Printer。

更多信息

用户期望

打印应该在不引起注意的情况下工作。

最终用户期望

可用性专家 Peter Sikking 和 Jan Mühlig 对真实普通用户进行了一些研究。Peter Sikking 在 http://www.mmiworks.net/eng/publications/labels/openPrinting.html 上写道

1st rule of printing: printing does not exist

m+mi works’ http://mmiworks.net/ partner company
relevantive http://relevantive.de/ performed user research
at the beginning of this project.

It showed that for users there is no worthwhile, productive activity
between the moment they want to see something printed and
the moment it comes out of the printer.

I was curious to find out what users expectations are
apart from get it on paper when they print.
So Jan did some user testing.

And guess what: there is no such thing as printing.

It does not exist as a task, as a meaningful activity.
One moment you decide to see it on paper,
the next it rolls out of the printer.

这意味着最终用户通常对复杂的打印功能或以牺牲兼容性为代价的创新不感兴趣。最终用户想要的是打印在不引起注意的情况下工作。

系统管理员期望

经验丰富的系统管理员希望打印能够按照匹配的上游项目设计和记录的方式工作，而无需特别关注。

这意味着打印应该在不出现意外行为的情况下工作，特别是不要有 openSUSE 特有的东西，也不要突然出现行为变化。

因此，关于“打印”，openSUSE 完全符合各种上游项目（主要是 CUPS、cups-filters、Ghostscript 和打印机驱动程序项目）。

参见

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