Anti-aliasing for digital printing

Details Anti-aliasing for digital printing Anti-aliasing for digital printing

1999-07-06

2003-11-25

Lee, Thomas D. (Department: 2624)

Facsimile and static presentation processing

Static presentation processing

Attribute control

C358S003200

Reexamination Certificate

active

06654147

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the field of digital printing and particularly to the performance of anti-aliasing for quality enhancement of the printed picture in the application of variable printing.
BACKGROUND OF THE INVENTION
In the scope of the present invention, anti-aliasing (AA) is a method for solving quality problems resulting from printing graphic elements in relatively low resolutions. Printing resolutions that are low enough to benefit from this enhancement are in the range of 240-600 DPI.
Printing systems relevant for the suggested enhancement are those with color depth per pixel greater than 2 levels per color plane. For example, suppose a printing machine uses 4 color planes—cyan, magenta, yellow & black (CMYK). If for each pixel, any of C, M, Y & K components can only be printed as either 0% color or 100% color, then this machine is not relevant for the anti-aliasing method suggested in the present invention. If, however, any of the color components can have more than 2 values (for example 16 or 256 different values), then anti-aliasing as suggested can be applied.
In general, anti-aliasing is a process aimed at overcoming the resolution limitation imposed by the printing machine, by getting more spatial graphic information than what is allowed by the printing resolution. The excess spatial information is then transformed into the color space, in a way that fools the eye to perceive more details. A simpler definition of anti-aliasing is reducing the undesirable stair-step pattern along the edges of images printed on low-resolution printing device. Typically, anti-aliasing involves reducing the intensity of intermediate pixels, to give the appearance of a smooth line or edge.
A better understanding of the anti-aliasing process may be attained by the following detailed description and examples in FIGS.
1
and
2
:
When rasterizing graphic data for the purpose of printing, the rasterization is done to a resolution that is higher than the printing resolution, for example, 4 times higher at each direction x & y). That is, if printing is done at 300 psi, rasterization will be done to 1200 dpi. Each printed pixel will thus be represented by a matrix of 4×4 pixels in the image raster (e.g. matrix
14
in FIG.
2
).
Following that, a pass is made over the entire detailed raster data (1200 dpi). Each 4×4 matrix is averaged to a single color value. The averaging may be a simple sum of all 16 values divided by 16, or any other, more sophisticated known filter. Today, the simple approach is mostly used. The exact averaging algorithm used does not have much influence on the implementation of the present invention.
When the element is not monochromatic, but has several color components (like RGB or CMYK) the process takes place for each color component in the picture (Cyan, Magenta etc.), independently. The present invention will be described only in reference to the monochromatic printing. However, anything said could easily be expanded to the polychromatic case, where each pixel is defined by several color components like: RGB, CMYK, etc.
FIG. 1
is an example of an upper case letter ‘I’
10
, printed with tint color (100%) surrounded by gray background of 20%
12
. No anti-aliasing has been performed.
FIG. 2
shows the same example, but here, anti-aliasing has been performed, creating gray levels that have not existed in the original image. For example, the value of 10% tint in the lower right pixel
16
has been derived by averaging the {fraction (4/16)} pixels having 100% tint with the {fraction (12/16)} pixels having 20% tint. Thus: 100
30
¼+20
+
¾=40. As a result, the letter ‘I’ in
FIG. 2
will have a smoother perceived outline
18
.
The process of preparing a page for printing is the pre-press process that includes, amongst others, the RIP (Raster Image Processing) stage that creates a raster representation of the page. The raster representation is the definition of the color of each pixel in the printing resolution.
In the process of RIPping, a hardware or software module called ‘RIP’ gets a non-raster representation of the complete page. A page description file containing such a representation is created by an application that assembles a page from its components: text, graphics and scanned images. Such an application could be QuarkXpress, available from Quark, Inc. of Denver, Colo. for example. The page description file describes a page in a Page description Language (PDL) such as PostScript, available from Adobe Systems Inc. of San Jose, Calif. The raster generated by the RIP can be stored in any raster format file, such as Scitex CT & I.W, as used by Scitex Corporation of Herzlia, Israel. Alternately, it can be held in memory buffers from which it is printed without being stored on disk.
It is important to not that time needed for RIPping a complex page will usually be much longer than the time it takes to print a single copy of the same page by the printing machine. In the case of conventional (non-variable) printing, this is not significant, since a single (lengthy) RIP operation will yield the printing of many (hundreds, thousands, or more) copies of the page.
With the rising of digital technology, where pages are printed directly from a computer data stream, without involving a pre-set medium such as film or plate, the printing device has no overhead in having each printed page different from its predecessor. This provides the opportunity for variable printing. The ultimate implementation of this technology enables the printing of a unique copy for each recipient of the printed material, tailored according to his/her measurements.
For example: A motor company, that manufactures dozens of models, wants to send advertising brochures in direct mail. Instead of sending a comprehensive and expensive brochure containing a lot of irrelevant information to a certain recipient (like too expensive car, station wagon to a bachelor, etc.), a unique brochure will be assembled for each recipient. For this, available personal information can be used, like place of residence, sex, education level, economic status, marital status and even hobbies and more. Part of this information can be attained from various database owners. Thus, a brochure will be assembled for each recipient, containing only relevant information, as well as additional specific information like a map illustrating the location of the dealers nearest to the recipient. In order to increase the impact, some text may be addressed to the recipient by name.
It is evident that in a case like the one described, where an advertiser in the USA might wish to print several millions of copies, the model introduced above for RIPping may be very problematic. RIPping each of those pages separately can take an unreasonable amount of time, even using a very fast computer. For this purpose, an enhanced model was developed to enable the processing of pages to keep pace with the speed of the digital-printing machine.
This model assumes that even though each page is unique, it may contain graphic elements that are used by other pages.
In the example of the car brochures, many recipients, though not all, will get the picture of a specific car, along with the text accompanying it. All recipients from a certain area will get the same map for the dealers, etc.
The collection of all instances of a document to be printed (brochure in the above example) is called a Printing Job, and is described by a Job Description file that comprises information about all the pages and all the graphical elements participating in all the instances.
FIG. 3
describes schematically a prior-art system designed to perform a more efficient processing of pages for digital printing.
First, Job description file
100
is fed into RIP
110
. RIP
110
identifies all the elements participating in all the pages contained in the Job. Each element is RIPped separately and its raster representation stored on disk
160
or, in rare cases, in memory. Scitex CT & LW raster formats are suitable for

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