Facsimile and static presentation processing – Static presentation processing – Memory
Reexamination Certificate
2000-06-12
2004-08-17
Lamb, Twyler (Department: 2722)
Facsimile and static presentation processing
Static presentation processing
Memory
C358S001150, C358S001170
Reexamination Certificate
active
06778291
ABSTRACT:
TECHNICAL FIELD
This invention generally relates to printing devices with memory. More particularly, this invention relates to printing devices that compress rendered data of a to-be-printed page and decompress it before printing.
BACKGROUND
Page printers typically capture an entire page before any image is placed on paper. A typical page printer is a laser printer. A page printed by a laser printer may include one or more of the following elements: “text”, “graphics”, “halftone” images, or “natural” images.
Data Types
“Text” typically consists of letters, numbers, words, sentences, and paragraphs. Text normally has a font associated with it. The symbols in text are typically represented by codes that direct the printer to generate a rendered version over a given area on a page.
“Graphics” are typically computer-generated images. They usually have sharp edges and sharp color transitions. Graphics may have subtle shadings, but typically include blocks of solid colors. Line-art is a type of graphic consisting entirely of lines, without any shading.
A “halftone” image is typically a printed reproduction of a photograph, using evenly spaced spots of varying diameter to produce apparent shades of gray or color. The darker the shade at a particular point in the image, the larger the corresponding spot (i.e., cell) in the halftone. Newspapers typically print photographs using halftones. With a desktop laser printer, each halftone spot is represented by an area containing a collection of dots.
“Natural” images are digitized images typically captured from the real world. These images are typically captured by a scanner, a digital camera, or frame-grabs of a video signal. They are often digitized representations of photographs. They may be a digitized representation of a document. Unlike halftone images with variable cell sizes, natural images use actual shades of gray and shades of color.
Rendering an Entire Page
In laser printers, either a host computer or the printer itself formats pages containing text, graphics, halftone images, and/or natural image. Since a laser printer's print engine operates at a constant speed, new rendered (i.e., “rasterized”) data must be supplied to the print engine at a rate that keeps up with the engine's operating speed.
Typically, a laser printer buffers a full raster bitmap of an entire page so that the print engine always has rendered data awaiting action. Alternatively, a laser printer may store only portions of a page and print them. While the printer is printing portions of a page, it is rendering the next portions of the page.
Mopying
It is desirable for a laser printer to store the rendered data of an entire page before actually printing. One advantage of storing the fully rendered page is the ability to efficiently print multiple original copies (“mopying”).
A printer mopies by storing one copy of a page, but printing multiple copies of it. The printer does this without receiving additional copies of the page from the host computer. Dataquest, a San Jose, Calif.-based research group, estimates that fourty-three percent of laser-printer users are already producing multiple original prints with their printers. Printer manufacturers prefer to produce printers with mopying capability because it provides a cost-effective, efficient, and timesaving alternative to copying.
Color Conversion
Another advantage of storing the fully rendered page is efficient color conversion. The color standard for incoming data is RGB (Red-Green-Blue). RGB color data is typically represented by twenty-four bits consisting of three color components with eight bits devoted to each. Since each color is represented by one byte, each color may have 256 discrete levels. Mixing levels of RGB results in 16.7 million possible color combinations for each “dot”.
A “dot” is the smallest addressable and printable element. Herein, the terms “dot” and “pixel” are used interchangeable. Each dot has value that is associated with a color. The size of that value represents the number of possible colors of the dot. This is called “color depth.” Examples of color depth include:
monochrome (1 bit of information per dot)
grayscale (8 bits of information per dot)
color (RGB) (8 or 16 bits of information per dot)
true color (RGB) (24 or 32 bits of information per dot)
The standard for physically printing actual color on paper is not RGB. Rather, it is CMYK (Cyan-Magenta-Yellow-Black). CMYK is a color model in which all colors are described as a mixture of these four process colors. CMYK is the standard color model used in offset printing for full-color documents and in typical desktop color laser printers. Because such printing uses pigments (such as, ink or toner) of these four basic colors, it is often called four-color printing.
Most desktop color laser printers produce their best output when receiving RGB data, rather than CMYK data. The color laser printers include mature technology that automatically, effectively, and accurately converts RGB data to CMYK print results.
The rendered page for one of the color components is called a “color plane.” For each RGB color, it is desirable for a color laser printer to store the three color plane of each page. For example, a page where only the red component of the RGB data is rendered is the red color plane of that page.
Unlike color ink-jet printers, the print engine of a color laser printer cannot “pause” and wait for data while the printer is processing it. When this happens in a laser printer, a “laser underrun” error is generated. To ensure that the color laser printer has fully processed to-be-printed color data, the printer converts the three color planes of the RGB data to CMYK data before sending the data to the print engine. By concurrently storing the rendered data of all three RGB color planes of a page, the printer quickly and efficiently converts RGB data into CMYK to send to the print engine for printing without pauses.
Printer Memory Requirements
Memory requirements of a laser printer increase as the dots-per-inch (dpi) resolution and the color depth increases. Black-and-white (b&w) laser printers typically have a one-bit color depth. Such printers from a few years ago had a resolution of three hundred (300) dpi. These printers needed approximately one megabyte (MB) of raster memory for each letter-sized (8.5″×11″) page. With a 600 dpi b&w printer having one-bit color depth, approximately 4 MB of memory is required. At one extreme, a color laser printer having a 1200 dpi resolution and a thirty-two (32) bit color depth requires approximately 540 MB of raster memory to store one entire letter-sized page.
It seems that each successive generation of color laser printers produce sharper and more colorful output. In a large part, this is a result of greater resolution and greater color depth. Therefore, there is an apparent need to have a large raster memory in a color laser printer.
In addition the above reasons, speed is another reason for more memory in a printer. A printer needs additional memory to print a series of pages as fast as possible. To avoid printer engine idle time and to run the print engine at its rated speed, printers need additional raster memory to rasterize and store successive pages. Without additional memory, composition of a subsequent page cannot begin until the present page has been printed.
Despite the technological reasons to maximize the raster memory on a color printer, manufacturers prefer to minimize the memory to remain cost competitive. Therefore, substantial effort is directed to reducing the amount of required memory in a laser printer. To reduce the amount of required memory in a laser printer, many conventional printers employ general-purpose data compression techniques.
Data Compression to Minimize Memory Requirements
Generally, data compression techniques encode a stream of digital data signals into compressed digital code signals and decode the compressed digital code signals back into the original data. Data compression refers to any process that attempts to convert da
Hewlett--Packard Development Company, L.P.
Lamb Twyler
LandOfFree
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