Patent Publication Number: US-7916336-B2

Title: Printer for produce cooling and storage apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. application Ser. No. 11/149,160 filed on Jun. 10, 2005 which is a continuation of U.S. application Ser. No. 9/722,171 filed on Nov. 25, 2000, now issued U.S. Pat. No. 6,927,871, the entire contents of which are herein incorporated by reference 
    
    
     FIELD OF INVENTION 
     The present invention relates to an apparatus enabling interaction with a network computer system. It has particular application to a system employing a printer for printing an interface onto a surface to produce an interface surface. 
     The invention has been developed primarily to produce interface surfaces which allow users to interact with networked information and to obtain interactive printed matter on demand via high-speed networked color printers. Although the invention will largely be described herein with reference to this use, it will be appreciated that the invention is not limited to use in this field. 
     CO-PENDING APPLICATIONS 
     Various methods, systems and apparatus relating to the present invention are disclosed in the following co-pending applications/granted patents filed by the applicant or assignee of the present invention on Nov. 25, 2000: 
                                                    6,530,339   6,631,897   7,295,839   09/722,174   7,175,079   7,064,851       6,826,547   6,741,871   6,927,871   6,980,306   6,965,439   6,788,982       7,263,270   6,788,293   6,946,672   7,091,960   6,792,165   7,105,753       7,182,247                    
The disclosures of these co-pending applications are incorporated herein by cross-reference.
 
     Various methods, systems and apparatus relating to the present invention are disclosed in the following co-pending applications/granted patents filed by the applicant or assignee of the present invention on Oct. 20, 2000: 
                                                    7,190,474   7,110,126   6,813,558   6,965,454   6,847,883   7,131,058       09/693,690   6,474,888   6,627,870   6,724,374   7,369,265   6,454,482       6,808,330   6,527,365   6,474,773   6,550,997                    
The disclosures of these co-pending applications are incorporated herein by cross-reference.
 
     Various methods, systems and apparatus relating to the present invention are disclosed in the following co-pending applications/granted patents filed by the applicant or assignee of the present invention on Sep. 15, 2000: 
                                                        6,679,420   6,963,845   6,995,859   6,720,985                        
The disclosures of these co-pending applications are incorporated herein by cross-reference.
 
     Various methods, systems and apparatus relating to the present invention are disclosed in the following co-pending applications/granted patents filed by the applicant or assignee of the present invention on Jun. 30, 2000: 
                                                    6,824,044   6,678,499   6,976,220   6,976,035   6,766,942   7,286,113       6,922,779   6,978,019   7,406,445   6,959,298   6,973,450   7,150,404       6,965,882   7,233,924   7,007,851   6,957,921   6,457,883   6,831,682       6,977,751   6,398,332   6,394,573   6,622,923                    
The disclosures of these co-pending applications are incorporated herein by cross-reference.
 
     Various methods, systems and apparatus relating to the present invention are disclosed in the following co-pending applications/granted patents filed by the applicant or assignee of the present invention on 23 May 2000: 
                                                6,428,133   6,526,658   6,315,399   6,338,548   6,540,319       6,328,431   6,328,425   6,991,320   6,383,833   6,464,332       6,390,591   7,018,016   6,328,417   09/575,197   7,079,712       6,825,945   7,330,974   6,813,039   6,987,506   6,824,044       7,038,797   6,980,318   6,816,274   7,350,236   6,681,045       6,728,000   7,173,722   7,088,459   09/575,181   7,068,382       7,062,651   6,789,194   6,789,191   6,644,642   6,502,614       6,622,999   6,669,385   6,549,935   6,987,573   6,727,996       6,591,884   6,439,706   6,760,119   7,295,332   6,290,349       6,428,155   6,785,016   6,870,966   6,822,639   6,737,591       7,055,739   7,233,320   6,830,196   6,832,717   6,957,768       7,456,820   7,170,499   7,106,888   7,123,239   6,409,323       6,281,912   6,604,810   6,318,920   6,488,422   6,795,215       7,154,638   6,859,289                    
The disclosures of these co-pending applications are incorporated herein by cross-reference.
 
     BACKGROUND 
     Presently, a user of a networked computer system typically interacts with the system via a local computer terminal (e.g. a personal computer) using a monitor for displaying information and a control device, such as a keyboard, mouse, trackball, joystick, etc, for inputting information and interacting with the computer system. Whilst such an interface is powerful, it is relatively bulky and non-portable. Information printed on paper can be easier to read and more portable than information displayed on a computer monitor. However, unlike a keyboard or mouse, a pen on paper generally lacks the ability to interact with computer software. 
     For many applications, the typical type of terminal arrangement imposes a number of limitations. In the home environment, for example, one encumbrance is brought about by the fact that terminals and ancillary equipment (printers, scanners, etc) are usually, by necessity, installed in a facility specifically organized to accommodate the network terminal, such as the home office. Hence, to access the terminal, the operator must make a deliberate act to make use of the dedicated facility, e.g. to enter the home office. This does not lead to a natural integration of computer system use into other more general functions of the domestic arrangement. It would be more desirable if a user was able to interact with the network terminal incidentally rather than deliberately. 
     Furthermore, conventional network terminals and items of computer ancillary equipment are typically designed and packaged in a form which is tailored towards desktop type applications, and which can tend therefore to consume valuable space. 
     SUMMARY OF INVENTION 
     According to the invention in a first aspect, there is provided an apparatus enabling interaction with a network computer system, the apparatus including: 
     an appliance for storing and cooling produce for use by an appliance user; and 
     a printer device integrated into said appliance, the printer device being operatively interconnectable with said network computer system, the printer device including a printer module operable to print at least one form delivered from said network computer system, and the printer device being configured to receive indicating data from a sensing device operated by an appliance user, the sensing device, when placed in an operative position relative to said at least one form, sensing the indicating data. 
     According to the invention in a second aspect, there is provided an appliance for storing and cooling produce, the appliance including an integral printer. 
     The invention, then, provides a means whereby the network terminal capability can be incorporated into a commonly used host appliance, such as a domestic refrigerator. Such an appliance is generally accessed routinely and frequently as a part of the day-to-day activities of members of the household. 
     In the domestic environment, the kitchen, and more particularly the kitchen refrigerator, can be regarded as a focal point for activity. The invention therefore affords the capability to take advantage of the physical size and centralized location of this appliance. 
     Accordingly, the present invention involves the use of a system and a method which utilizes one or more forms capable of interacting with a computer system. Whilst this method and system may be used in conjunction with a single computer system, in a particularly preferred form it is designed to operate over a computer network, such as the Internet. 
     As a physical entity, the interactive form is disposed on a surface medium of any suitable structure. However, in a preferred arrangement, the form comprises sheet material such as paper or the like which has the coded data printed on it and which allows it to interact with the computer system. The coded data is detectable preferably, but not exclusively, outside the visible spectrum, thereby enabling it to be machine-readable but substantially invisible to the human eye. The form may also include visible material which provides information to a user, such as the application or purpose of the form, and which visible information may be registered or correlate in position with the relevant hidden coded data. 
     The system also includes a sensing device to convey data from the form to the computer system, and in some instances, to contribute additional data. Again, the sensing device may take a variety of forms but is preferably compact and easily portable. In a particularly preferred arrangement, the sensing device is configured as a pen which is designed to be able to physically mark the interactive form as well as to selectively enable the coded data from the form to be read and transmitted to the computer system. The coded data then provides control information, configured such that designation thereof by a user causes instructions to be applied to the software running on the computer system or network. 
     The nature of the interaction between the form and the sensing device and the data that each contributes to the computer system may vary. In one arrangement, the coded data on the form is indicative of the identity of the form and of at least one reference point on that form. In another embodiment, the interactive form includes coded data which is indicative of a parameter of the form, whereas the sensing device is operative to provide data regarding its own movement relative to that form to the computer system together with coded data from the form. In yet another arrangement, the form includes the coded data which at least identifies the form, and the sensing device is designed to provide, to the computer system, data based on the form coded data, and also on data which identifies the user of the device. 
     In a preferred arrangement, then, the system and method employs specially designed printers to print the interactive form. Further these printers constitute or form part of the computer system and are designed to receive data from the sensing device. As indicated above, the system and method of the invention is ideally suited to operate over a network. In this arrangement, the printers are fully integrated into the network and allow for printing of the interactive forms on demand and also for distributing of the forms using a mixture of multi-cast and point-cast communication protocols. 
     Accordingly, in a preferred form, the present invention provides methods and systems which use a paper and pen based interface for a computer system. This provides many significant benefits over traditional computer systems. The advantage of paper is that it is widely used to display and record information. Further, printed information is easier to read than information displayed on a computer screen. Moreover, paper does not run on batteries, can be read in bright light, robustly accepts coffee spills or the like, and is portable and disposable. Furthermore, the system allows for hand-drawing and hand-writing to be captured which affords greater richness of expression than input via a computer keyboard and mouse. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Preferred and other embodiments of the invention will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic of a the relationship between a sample printed netpage and its online page description; 
         FIG. 2  is a schematic view of a interaction between a netpage pen, a netpage printer, a netpage page server, and a netpage application server; 
         FIG. 3  illustrates a collection of netpage servers and printers interconnected via a network; 
         FIG. 4  is a schematic view of a high-level structure of a printed netpage and its online page description; 
         FIG. 5   a  is a plan view showing a structure of a netpage tag; 
         FIG. 5   b  is a plan view showing a relationship between a set of the tags shown in  FIG. 5   a  and a field of view of a netpage sensing device in the form of a netpage pen; 
         FIG. 6   a  is a plan view showing an alternative structure of a netpage tag; 
         FIG. 6   b  is a plan view showing a relationship between a set of the tags shown in  FIG. 6   a  and a field of view of a netpage sensing device in the form of a netpage pen; 
         FIG. 6   c  is a plan view showing an arrangement of nine of the tags shown in  FIG. 6   a  where targets are shared between adjacent tags; 
         FIG. 6   d  is a plan view showing the interleaving and rotation of the symbols of the four codewords of the tag shown in  FIG. 6   a;    
         FIG. 7  is a flowchart of a tag image processing and decoding algorithm; 
         FIG. 8  is a perspective view of a netpage pen and its associated tag-sensing field-of-view cone; 
         FIG. 9  is a perspective exploded view of the netpage pen shown in  FIG. 8 ; 
         FIG. 10  is a schematic block diagram of a pen controller for the netpage pen shown in  FIGS. 8 and 9 ; 
         FIG. 11  is a perspective view of a wall-mounted netpage printer; 
         FIG. 12  is a section through the length of the netpage printer of  FIG. 11 ; 
         FIG. 12   a  is an enlarged portion of  FIG. 12  showing a section of the duplexed print engines and glue wheel assembly; 
         FIG. 13  is a detailed view of the ink cartridge, ink, air and glue paths, and print engines of the netpage printer of  FIGS. 11 and 12 ; 
         FIG. 14  is a schematic block diagram of a printer controller for the netpage printer shown in  FIGS. 11 and 12 ; 
         FIG. 15  is a schematic block diagram of duplexed print engine controllers and Memjet™ printheads associated with the printer controller shown in  FIG. 14 ; 
         FIG. 16  is a schematic block diagram of the print engine controller shown in  FIGS. 14 and 15 ; 
         FIG. 17  is a perspective view of a single Memjet™ printing element, as used in, for example, the netpage printer of  FIGS. 10 to 12 ; 
         FIG. 18  is a perspective view of a small part of an array of Memjet™ printing elements; 
         FIG. 19  is a series of perspective views illustrating the operating cycle of the Memjet™ printing element shown in  FIG. 13 ; 
         FIG. 20  is a perspective view of a short segment of a pagewidth Memjet™ printhead; 
         FIG. 21  is a simple exploded view of the wallprinter; 
         FIG. 22  is an exploded view of the ink cartridge; 
         FIG. 23  is a front three-quarter view of the open media tray; 
         FIG. 24  is a front three-quarter view of the electrical system of the printer; 
         FIG. 25  is a rear three-quarter view of the electrical system; 
         FIG. 26  is a section through the binder assembly; 
         FIG. 27  is a rear three-quarter view of the open glue wheel assembly; 
         FIG. 28  is a section through the binding assembly and the exit hatch; 
         FIG. 29  is a top three-quarter view of the media tray; 
         FIG. 30  is a section through the top part of the printer; 
         FIG. 31  is a perspective view of a refrigerator incorporating a netpage printer, in accordance with the present invention; 
         FIG. 32  is a perspective view of the refrigerator of  FIG. 31  with an open consumables access hatch; 
         FIG. 33  is a perspective view of the refrigerator of  FIG. 31  showing a thermal sensor, and network and power connections; and 
         FIG. 34  is a cross-sectional view of the refrigerator of  FIG. 31 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED AND OTHER EMBODIMENTS 
     Note: Memjet™ is a trade mark of Silverbrook Research Pty Ltd, Australia. 
     In the preferred embodiment, the invention is configured to work with the netpage networked computer system, a detailed overview of which follows. It will be appreciated that not every implementation will necessarily embody all or even most of the specific details and extensions discussed below in relation to the basic system. However, the system is described in its most complete form to reduce the need for external reference when attempting to understand the context in which the preferred embodiments and aspects of the present invention operate. 
     In brief summary, the preferred form of the netpage system employs a computer interface in the form of a mapped surface, that is, a physical surface which contains references to a map of the surface maintained in a computer system. The map references can be queried by an appropriate sensing device. Depending upon the specific implementation, the map references may be encoded visibly or invisibly, and defined in such a way that a local query on the mapped surface yields an unambiguous map reference both within the map and among different maps. The computer system can contain information about features on the mapped surface, and such information can be retrieved based on map references supplied by a sensing device used with the mapped surface. The information thus retrieved can take the form of actions which are initiated by the computer system on behalf of the operator in response to the operator&#39;s interaction with the surface features. 
     In its preferred form, the netpage system relies on the production of, and human interaction with, netpages. These are pages of text, graphics and images printed on ordinary paper, but which work like interactive web pages. Information is encoded on each page using ink which is substantially invisible to the unaided human eye. The ink, however, and thereby the coded data, can be sensed by an optically imaging pen and transmitted to the netpage system. Substrates other than paper may be used. The encoded information in the preferred embodiment is an infrared absorptive ink and so an infrared sensitive optical sensor may be used. If desired other wavelengths may be used or sensing techniques other than optical sensing; one alternative is to use magnetic inks and sensors. 
     In the preferred form, active buttons and hyperlinks on each page can be clicked with the pen to request information from the network or to signal preferences to a network server. In one embodiment, text written by hand on a netpage is automatically recognized and converted to computer text in the netpage system, allowing forms to be filled in. In other embodiments, signatures recorded on a netpage are automatically verified, allowing e-commerce transactions to be securely authorized. 
     As illustrated in  FIG. 1 , a printed netpage  1  can represent a interactive form which can be filled in by the user both physically, on the printed page, and “electronically”, via communication between the pen and the netpage system. The example shows a “Request” form containing name and address fields and a submit button. The netpage consists of graphic data  2  printed using visible ink, and coded data  3  printed as a collection of tags  4  using invisible ink. The corresponding page description  5 , stored on the netpage network, describes the individual elements of the netpage. In particular it describes the type and spatial extent (zone) of each interactive element (i.e. text field or button in the example), to allow the netpage system to correctly interpret input via the netpage. The submit button  6 , for example, has a zone  7  which corresponds to the spatial extent of the corresponding graphic  8 . 
     As illustrated in  FIG. 2 , the netpage pen  101 , a preferred form of which is shown in  FIGS. 8 and 9  and described in more detail below, works in conjunction with a netpage printer  601 , an Internet-connected printing appliance for home, office or mobile use. The pen is wireless and communicates securely with the netpage printer via a short-range radio link  9 . If desired the pen may be connected to the system utilizing wires or an infrared transmitter, although both alternatives limit usability. 
     The netpage printer  601 , a preferred form of which is shown in  FIGS. 11 to 13  and described in more detail below, is able to deliver, periodically or on demand, personalized newspapers, magazines, catalogs, brochures and other publications, all printed at high quality as interactive netpages. Unlike a personal computer, the netpage printer is an appliance which can be, for example, wall-mounted adjacent to an area where the morning news is first consumed, such as in a user&#39;s kitchen, near a breakfast table, or near the household&#39;s point of departure for the day. It also comes in tabletop, desktop, portable and miniature versions. 
     Netpages printed at their point of consumption combine the ease-of-use of paper with the timeliness and interactivity of an interactive medium. 
     As shown in  FIG. 2 , the netpage pen  101  interacts with the coded data on a printed netpage  1  and communicates, via a short-range radio link  9 , the interaction to a netpage printer. The printer  601  sends the interaction to the relevant netpage page server  10  for interpretation. In appropriate circumstances, the page server sends a corresponding message to application computer software running on a netpage application server  13 . The application server may in turn send a response which is printed on the originating printer. 
     The netpage system is made considerably more convenient in the preferred embodiment by being used in conjunction with high-speed microelectromechanical system (MEMS) based inkjet (Memjet™) printers. In the preferred form of this technology, relatively high-speed and high-quality printing is made more affordable to consumers. In its preferred form, a netpage publication has the physical characteristics of a traditional newsmagazine, such as a set of letter-size glossy pages printed in full color on both sides, bound together for easy navigation and comfortable handling. 
     The netpage printer exploits the growing availability of broadband Internet access. Cable service is available to 95% of households in the United States, and cable modem service offering broadband Internet access is already available to 20% of these. The netpage printer can also operate with slower connections, but either with longer delivery times or lower image quality or both. Indeed, the netpage system can be enabled using existing consumer inkjet and laser printers, although the system will operate more slowly and will therefore be less acceptable from a consumer&#39;s point of view. In other embodiments, the netpage system is hosted on a private intranet. In still other embodiments, the netpage system is hosted on a single computer or computer-enabled device, such as a printer. 
     Netpage publication servers  14  on the netpage network are configured to deliver print-quality publications to netpage printers. Periodical publications are delivered automatically to subscribing netpage printers via pointcasting and multicasting Internet protocols. Personalized publications are filtered and formatted according to individual user profiles. 
     A netpage printer can be configured to support any number of pens, and a pen can work with any number of netpage printers. In the preferred implementation, each netpage pen has a unique identifier. A household may have a collection of colored netpage pens, one assigned to each member of the family. This allows each user to maintain a distinct profile with respect to a netpage publication server or application server, assuming that the assigned pen is only used by the respective family member. However, as explained below, other means may be used to identify a user. 
     A netpage pen can also be registered with a netpage registration server  11  and linked to one or more payment card accounts. This allows e-commerce payments to be securely authorized using the netpage pen. The netpage registration server compares the signature captured by the netpage pen with a previously registered signature, allowing it to authenticate the user&#39;s identity to an e-commerce server. Other biometrics can also be used to verify identity. A version of the netpage pen includes fingerprint scanning, verified in a similar way by the netpage registration server. 
     Although a netpage printer may deliver periodicals such as the morning newspaper without user intervention, it can be configured never to deliver unsolicited junk mail. In its preferred form, it only delivers periodicals from subscribed or otherwise authorized sources. In this respect, the netpage printer is unlike a fax machine or e-mail account which is visible to any junk mailer who knows the telephone number or email address. Alternatively the entire system may be made visible to outside users or each user may be provided with the ability to expose their printer(s) to outside users. This may be by way of selecting outside users allowed too send junk mail. 
     1 Netpage System Architecture 
     Each object model in the system is described using a Unified Modeling Language (UML) class diagram. A class diagram consists of a set of object classes connected by relationships, and two kinds of relationships are of interest here: associations and generalizations. An association represents some kind of relationship between objects, i.e. between instances of classes. A generalization relates actual classes, and can be understood in the following way: if a class is thought of as the set of all objects of that class, and class A is a generalization of class B, then B is simply a subset of A. 
     Each class is drawn as a rectangle labeled with the name of the class. It contains a list of the attributes of the class, separated from the name by a horizontal line, and a list of the operations of the class, separated from the attribute list by a horizontal line. In the class diagrams which follow, however, operations are never modeled. 
     An association is drawn as a line joining two classes, optionally labeled at either end with the multiplicity of the association. The default multiplicity is one. An asterisk (*) indicates a multiplicity of “many”, i.e. zero or more. Each association is optionally labeled with its name, and is also optionally labeled at either end with the role of the corresponding class. An open diamond indicates an aggregation association (“is-part-of”), and is drawn at the aggregator end of the association line. 
     A generalization relationship (“is-a”) is drawn as a solid line joining two classes, with an arrow (in the form of an open triangle) at the generalization end. 
     When a class diagram is broken up into multiple diagrams, any class which is duplicated is shown with a dashed outline in all but the main diagram which defines it. It is shown with attributes only where it is defined. 
     1.1 Netpages 
     Netpages are the foundation on which a netpage network is built. They provide a paper-based user interface to published information and interactive services. 
     A netpage consists of a printed page (or other surface region) invisibly tagged with references to an online description of the page. The tags may be printed on or into the surface of the page, may be in or on a sub-layer of the page or may be otherwise incorporated into the page. The online page description is maintained persistently by a netpage page server. The page description describes the visible layout and content of the page, including text, graphics and images. It also describes the input elements on the page, including buttons, hyperlinks, and input fields. The page descriptions of different netpages may share components, such as an image, although the netpages (and the associated page descriptions) are visibly different. The page description for each netpage may include references to these common components. A netpage allows markings made with a netpage pen on its surface to be simultaneously captured and processed by the netpage system. 
     Multiple netpages can share the same page description. However, to allow input through otherwise identical pages to be distinguished, each netpage is assigned a unique page identifier. This page ID has sufficient precision to distinguish between all netpages envisaged to be used in the environment of use. If the environment is small then the precision need not be as great as where the environment is large. 
     Each reference to the page description is encoded in a printed tag. The tag identifies the unique page on which it appears, and thereby indirectly identifies the page description. In the preferred embodiments the tag also identifies its own position on the page. Characteristics of the tags are described in more detail below. 
     Tags are printed in infrared-absorptive ink on any substrate which is infrared-reflective, such as ordinary paper. Near-infrared wavelengths are invisible to the human eye but are easily sensed by a solid-state image sensor with an appropriate filter. A sensor sensitive to the relative wavelength or wavelengths may be used, in which case no filters are required. Other wavelengths may be used, with appropriate substrates and sensors. 
     A tag is sensed by an area image sensor in the netpage pen, decoded and the data encoded by the tag is transmitted to the netpage system, preferably via the nearest netpage printer. The pen is wireless and communicates with the netpage printer via a short-range radio link. Tags are sufficiently small and densely arranged that the pen can reliably image at least one tag even on a single click on the page. It is important that the pen recognize the tag and extract the page ID and position on every interaction with the page, since the interaction is stateless. Tags are error-correctably encoded to make them partially tolerant to surface damage. 
     The netpage page server maintains a unique page instance for each printed netpage, allowing it to maintain a distinct set of user-supplied values for input fields in the page description for each printed netpage. 
     The relationship between the page description, the page instance, and the printed netpage is shown in  FIG. 4 . In the preferred embodiment the page instance is associated with both the netpage printer which printed it and, if known, the netpage user who requested it. It is not essential to the working of the invention in its basic form that the page instance be associated with either the netpage printer which printed the corresponding physical page or the netpage user who requested it or for whom the page was printed. 
     1.2 Netpage Tags 
     1.2.1 Tag Data Content 
     In a preferred form, each tag identifies the region in which it appears, and the location of that tag within the region. A tag may also contain flags which relate to the region as a whole or to the tag. One or more flag bits may, for example, signal a tag sensing device to provide feedback indicative of a function associated with the immediate area of the tag, without the sensing device having to refer to a description of the region. A netpage pen may, for example, illuminate an “active area” LED when in the zone of a hyperlink. 
     As will be more clearly explained below, in a preferred embodiment, each tag contains an easily recognized invariant structure which aids initial detection, and which assists in minimizing the effect of any warp induced by the surface or by the sensing process. The tags preferably tile the entire page, and are sufficiently small and densely arranged that the pen can reliably image at least one tag even on a single click on the page. It is important that the pen recognize the page ID and position on every interaction with the page, since the interaction is stateless. 
     In a preferred embodiment, the region to which a tag refers coincides with an entire page, and the region ID encoded in the tag is therefore synonymous with the page ID of the page on which the tag appears. In other embodiments, the region to which a tag refers can be an arbitrary subregion of a page or other surface. For example, it can coincide with the zone of an interactive element, in which case the region ID can directly identify the interactive element. 
     Each tag typically contains 16 bits of tag ID, at least 90 bits of region ID, and a number of flag bits. Assuming a maximum tag density of 64 per square inch, a 16-bit tag ID supports a region size of up to 1024 square inches. Larger regions can be mapped continuously without increasing the tag ID precision simply by using abutting regions and maps. The distinction between a region ID and a tag ID is mostly one of convenience. For most purposes the concatenation of the two can be considered as a globally unique tag ID. Conversely, it may also be convenient to introduce structure into the tag ID, for example to define the x and y coordinates of the tag. A 90-bit region ID allows 2 90  (˜10 27  or a thousand trillion trillion) different regions to be uniquely identified. Tags may also contain type information, and a region may be tagged with a mixture of tag types. For example, a region may be tagged with one set of tags encoding x coordinates and another set, interleaved with the first, encoding y coordinates. It will be appreciated the region ID and tag ID precision may be more or less than just described depending on the environment in which the system will be used. 
     1.2.2 Tag Data Encoding 
     In one embodiment each tag contains 120 bits of information. The 120 bits of tag data are redundantly encoded using a (15, 5) Reed-Solomon code. This yields 360 encoded bits consisting of 6 codewords of 15 4-bit symbols each. The (15, 5) code allows up to 5 symbol errors to be corrected per codeword, i.e. it is tolerant of a symbol error rate of up to 33% per codeword. 
     Each 4-bit symbol is represented in a spatially coherent way in the tag, and the symbols of the six codewords are interleaved spatially within the tag. This ensures that a burst error (an error affecting multiple spatially adjacent bits) damages a minimum number of symbols overall and a minimum number of symbols in any one codeword, thus maximizing the likelihood that the burst error can be fully corrected. 
     Any suitable error-correcting code can be used in place of a (15, 5) Reed-Solomon code, for example a Reed-Solomon code with more or less redundancy, with the same or different symbol and codeword sizes; another block code; or a different kind of code, such as a convolutional code (see, for example, Stephen B. Wicker, Error Control Systems for Digital Communication and Storage, Prentice-Hall 1995, the contents of which a herein incorporated by cross-reference). 
     1.2.3 Physical Tag Structure 
     The physical representation of the tag, shown in  FIG. 5 , includes fixed target structures  15 ,  16 ,  17  and variable data areas  18 . The fixed target structures allow a sensing device such as the netpage pen to detect the tag and infer its three-dimensional orientation relative to the sensor. The data areas contain representations of the individual bits of the encoded tag data. 
     To achieve proper tag reproduction, the tag is rendered at a resolution of 256×256 dots. When printed at 1600 dots per inch this yields a tag with a diameter of about 4 mm. At this resolution the tag is designed to be surrounded by a “quiet area” of radius 16 dots. Since the quiet area is also contributed by adjacent tags, it only adds 16 dots to the effective diameter of the tag. 
     The tag includes six target structures. A detection ring  15  allows the sensing device to initially detect the tag. The ring is easy to detect because it is rotationally invariant and because a simple correction of its aspect ratio removes most of the effects of perspective distortion. An orientation axis  16  allows the sensing device to determine the approximate planar orientation of the tag due to the yaw of the sensor. The orientation axis is skewed to yield a unique orientation. Four perspective targets  17  allow the sensing device to infer an accurate two-dimensional perspective transform of the tag and hence an accurate three-dimensional position and orientation of the tag relative to the sensor. 
     All target structures are redundantly large to improve their immunity to noise. 
     The overall tag shape is circular. This supports, amongst other things, optimal tag packing on an irregular triangular grid, such as is required to tile an arbitrary non-planar surface. The tags may, however, be arranged at the apexes of any polygon having n apexes, where n ranges from 3 to infinity, as desired. In combination with the circular detection ring  15 , this makes a circular arrangement of data bits within the tag optimal. To maximize its size, each data bit is represented by a radial wedge in the form of an area bounded by two radial lines, a radially inner arc and a radially outer arc. Each wedge has a minimum dimension of 8 dots at 1600 dpi and is designed so that its base (i.e. its inner arc), is at least equal to this minimum dimension. The radial height of the wedge is always equal to the minimum dimension. Each 4-bit data symbol is represented by an array  518  of 2×2 wedges. 
     The 15 4-bit data symbols of each of the six codewords are allocated to the four concentric symbol rings  18   a  to  18   d , shown in  FIG. 5 , in interleaved fashion. Symbols of first to sixth codewords  520 - 525  are allocated alternately in circular progression around the tag. 
     The interleaving is designed to maximize the average spatial distance between any two symbols of the same codeword. Other arrangements of the codewords or their data symbols may be utilized. 
     The physical layout of the tags or the shape and/or arrangement of data symbols within each tag are nor essential to the working of the invention. It is merely necessary that each tag encode sufficient information for the intended use. The use of redundancy in the tag is preferred but, at its basic level, not truly essential to the working of the invention. As such other tag arrangements may be utilized. Examples of other tag structures are described in U.S. Pat. Nos. 5,625,412, 5,661,506, 5,477,012 and 5,852,434, and PCT application PCT/US98/20597, the contents of each of which are incorporated herein by reference. 
     In order to support “single-click” interaction with a tagged region via a sensing device, the sensing device must be able to see at least one entire tag in its field of view no matter where in the region or at what orientation the sensing device is positioned. The required diameter of the field of view of the sensing device is therefore a function of the size and spacing of the tags. 
     Assuming a circular tag shape, the minimum diameter of the sensor field of view is obtained when the tags are tiled on a equilateral triangular grid, as shown in  FIG. 6 . 
     1.2.4 Tag Image Processing and Decoding 
     The tag image processing and decoding of a tag of  FIG. 5  performed by a sensing device such as the netpage pen is shown in  FIG. 7 . While a captured image is being acquired from the image sensor, the dynamic range of the image is determined (at  20 ). The center of the range is then chosen as the binary threshold for the image  21 . The image is then thresholded and segmented into connected pixel regions (i.e. shapes  23 ) (at  22 ). Shapes which are too small to represent tag target structures are discarded. The size and centroid of each shape is also computed. 
     Binary shape moments  25  are then computed (at  24 ) for each shape, and these provide the basis for subsequently locating target structures. Central shape moments are by their nature invariant of position, and can be easily made invariant of scale, aspect ratio and rotation. 
     The ring target structure  15  is the first to be located (at  26 ). A ring has the advantage of being very well behaved when perspective-distorted. Matching proceeds by aspect-normalizing and rotation-normalizing each shape&#39;s moments. Once its second-order moments are normalized the ring is easy to recognize even if the perspective distortion was significant. The ring&#39;s original aspect and rotation  27  together provide a useful approximation of the perspective transform. 
     The axis target structure  16  is the next to be located (at  28 ). Matching proceeds by applying the ring&#39;s normalizations to each shape&#39;s moments, and rotation-normalizing the resulting moments. Once its second-order moments are normalized the axis target is easily recognized. Note that one third order moment is required to disambiguate the two possible orientations of the axis. The shape is deliberately skewed to one side to make this possible. Note also that it is only possible to rotation-normalize the axis target after it has had the ring&#39;s normalizations applied, since the perspective distortion can hide the axis target&#39;s axis. The axis target&#39;s original rotation provides a useful approximation of the tag&#39;s rotation due to pen yaw  29 . 
     The four perspective target structures  17  are the last to be located (at  30 ). Good estimates of their positions are computed based on their known spatial relationships to the ring and axis targets, the aspect and rotation of the ring, and the rotation of the axis. Matching proceeds by applying the ring&#39;s normalizations to each shape&#39;s moments. Once their second-order moments are normalized the circular perspective targets are easy to recognize, and the target closest to each estimated position is taken as a match. The original centroids of the four perspective targets are then taken to be the perspective-distorted corners  31  of a square of known size in tag space, and an eight-degree-of-freedom perspective transform  33  is inferred (at  32 ) based on solving the well-understood equations relating the four tag-space and image-space point pairs (see Heckbert, P., Fundamentals of Texture Mapping and Image Warping, Masters Thesis, Dept. of EECS, U. of California at Berkeley, Technical Report No. UCB/CSD 89/516, June 1989, the contents of which are herein incorporated by cross-reference). 
     The inferred tag-space to image-space perspective transform is used to project (at  36 ) each known data bit position in tag space into image space where the real-valued position is used to bilinearly interpolate (at  36 ) the four relevant adjacent pixels in the input image. The previously computed image threshold  21  is used to threshold the result to produce the final bit value  37 . 
     Once all 360 data bits  37  have been obtained in this way, each of the six 60-bit Reed-Solomon codewords is decoded (at  38 ) to yield 20 decoded bits  39 , or 120 decoded bits in total. Note that the codeword symbols are sampled in codeword order, so that codewords are implicitly de-interleaved during the sampling process. 
     As mentioned above, the physical tag structure or encoding system is not essential to the invention and other physical arrangements of each tag may be used. It will be understood that the process for recognizing and decoding the tag image to retrieve the data encoded depends on the physical structure of the tag and the system used for redundantly encoding the data. 
     The ring target  15  is only sought in a subarea of the image whose relationship to the image guarantees that the ring, if found, is part of a complete tag. If a complete tag is not found and successfully decoded, then no pen position is recorded for the current frame. Given adequate processing power and ideally a non-minimal field of view  193 , an alternative strategy involves seeking another tag in the current image. 
     The obtained tag data indicates the identity of the region containing the tag and the position of the tag within the region. An accurate position  35  of the pen nib in the region, as well as the overall orientation  35  of the pen, is then inferred (at  34 ) from the perspective transform  33  observed on the tag and the known spatial relationship between the pen&#39;s physical axis and the pen&#39;s optical axis. 
     1.2.5 Alternative Tag Structures 
     The tag structure just described is designed to allow both regular tilings of planar surfaces and irregular tilings of non-planar surfaces. Regular tilings are not, in general, possible on non-planar surfaces. In the more usual case of planar surfaces where regular tilings of tags are possible, i.e. surfaces such as sheets of paper and the like, more efficient tag structures can be used which exploit the regular nature of the tiling. 
     An alternative tag structure more suited to a regular tiling is shown in  FIG. 6   a . The alternative tag  4  is square and has four perspective targets  17 . It is similar in structure to tags described by Bennett et al. in U.S. Pat. No. 5,051,746. The tag represents sixty 4-bit Reed-Solomon symbols  47 , for a total of 240 bits. The tag represents each one bit as a dot  48 , and each zero bit by the absence of the corresponding dot. The perspective targets are designed to be shared between adjacent tags, as shown in  FIGS. 6   b  and  6   c .  FIG. 6   b  shows a square tiling of 16 tags and the corresponding minimum field of view  193 , which must span the diagonals of two tags.  FIG. 6   c  shows a square tiling of nine tags, containing all one bits for illustration purposes. 
     Using a (15, 7) Reed-Solomon code, 112 bits of tag data are redundantly encoded to produce 240 encoded bits. The four codewords are interleaved spatially within the tag to maximize resilience to burst errors. Assuming a 16-bit tag ID as before, this allows a region ID of up to 92 bits. 
     The data-bearing dots  48  of the tag are designed to not overlap their neighbors, so that groups of tags cannot produce structures which resemble targets. This also saves ink. The perspective targets therefore allow detection of the tag, so further targets are not required. Tag image processing proceeds as described in section 1.2.4 above, with the exception that steps  26  and  28  are omitted. 
     Although the tag may contain an orientation feature to allow disambiguation of the four possible orientations of the tag relative to the sensor, it is also possible to embed orientation data in the tag data. For example, the four codewords can be arranged so that each tag orientation contains one codeword placed at that orientation, as shown in  FIG. 6   d , where each symbol is labelled with the number of its codeword (1-4) and the position of the symbol within the codeword (A-O). Tag decoding then consists of decoding one codeword at each orientation. Each codeword can either contain a single bit indicating whether it is the first codeword, or two bits indicating which codeword it is. The latter approach has the advantage that if, say, the data content of only one codeword is required, then at most two codewords need to be decoded to obtain the desired data. This may be the case if the region ID is not expected to change within a stroke and is thus only decoded at the start of a stroke. Within a stroke only the codeword containing the tag ID is then desired. Furthermore, since the rotation of the sensing device changes slowly and predictably within a stroke, only one codeword typically needs to be decoded per frame. 
     It is possible to dispense with perspective targets altogether and instead rely on the data representation being self-registering. In this case each bit value (or multi-bit value) is typically represented by an explicit glyph, i.e. no bit value is represented by the absence of a glyph. This ensures that the data grid is well-populated, and thus allows the grid to be reliably identified and its perspective distortion detected and subsequently corrected during data sampling. To allow tag boundaries to be detected, each tag data must contain a marker pattern, and these must be redundantly encoded to allow reliable detection. The overhead of such marker patterns is similar to the overhead of explicit perspective targets. One such scheme uses dots positioned a various points relative to grid vertices to represent different glyphs and hence different multi-bit values (see Anoto Technology Description, Anoto April 2000). 
     1.2.6 Tag Map 
     Decoding a tag results in a region ID, a tag ID, and a tag-relative pen transform. Before the tag ID and the tag-relative pen location can be translated into an absolute location within the tagged region, the location of the tag within the region must be known. This is given by a tag map, a function which maps each tag ID in a tagged region to a corresponding location. 
     A tag map reflects the scheme used to tile the surface region with tags, and this can vary according to surface type. When multiple tagged regions share the same tiling scheme and the same tag numbering scheme, they can also share the same tag map. 
     The tag map for a region must be retrievable via the region ID. Thus, given a region ID, a tag ID and a pen transform, the tag map can be retrieved, the tag ID can be translated into an absolute tag location within the region, and the tag-relative pen location can be added to the tag location to yield an absolute pen location within the region. 
     1.2.7 Tagging Schemes 
     Two distinct surface coding schemes are of interest, both of which use the tag structure described earlier in this section. The preferred coding scheme uses “location-indicating” tags as already discussed. An alternative coding scheme uses “object-indicating” tags. 
     A location-indicating tag contains a tag ID which, when translated through the tag map associated with the tagged region, yields a unique tag location within the region. The tag-relative location of the pen is added to this tag location to yield the location of the pen within the region. This in turn is used to determine the location of the pen relative to a user interface element in the page description associated with the region. Not only is the user interface element itself identified, but a location relative to the user interface element is identified. Location-indicating tags therefore trivially support the capture of an absolute pen path in the zone of a particular user interface element. 
     An object-indicating tag contains a tag ID which directly identifies a user interface element in the page description associated with the region. All the tags in the zone of the user interface element identify the user interface element, making them all identical and therefore indistinguishable. Object-indicating tags do not, therefore, support the capture of an absolute pen path. They do, however, support the capture of a relative pen path. So long as the position sampling frequency exceeds twice the encountered tag frequency, the displacement from one sampled pen position to the next within a stroke can be unambiguously determined. 
     With either tagging scheme, the tags function in cooperation with associated visual elements on the netpage as user interactive elements in that a user can interact with the printed page using an appropriate sensing device in order for tag data to be read by the sensing device and for an appropriate response to be generated in the netpage system. 
     1.3 The Netpage Network 
     In a preferred embodiment, a netpage network consists of a distributed set of netpage page servers  10 , netpage registration servers  11 , netpage ID servers  12 , netpage application servers  13 , netpage publication servers  14 , and netpage printers  601  connected via a network  19  such as the Internet, as shown in  FIG. 3 . 
     The netpage registration server  11  is a server which records relationships between users, pens, printers, applications and publications, and thereby authorizes various network activities. It authenticates users and acts as a signing proxy on behalf of authenticated users in application transactions. It also provides handwriting recognition services if desired. As described above, a netpage page server  10  maintains persistent information about page descriptions and page instances. The netpage network includes any number of page servers, each handling a subset of page instances. Since a page server also maintains user input values for each page instance, clients such as netpage printers send netpage input directly to the appropriate page server. The page server interprets any such input relative to the description of the corresponding page. 
     A netpage ID server  12  allocates document IDs  51  on demand, and provides load-balancing of page servers via its ID allocation scheme. 
     A netpage printer uses the Internet Distributed Name System (DNS), or similar, to resolve a netpage page ID  50  into the network address of the netpage page server handling the corresponding page instance. 
     A netpage application server  13  is a server which hosts interactive netpage applications. A netpage publication server  14  is an application server which publishes netpage documents to netpage printers. 
     Netpage servers can be hosted on a variety of network server platforms from manufacturers such as IBM, Hewlett-Packard, and Sun. Multiple netpage servers can run concurrently on a single host, and a single server can be distributed over a number of hosts. Some or all of the functionality provided by netpage servers, and in particular the functionality provided by the ID server and the page server, can also be provided directly in a netpage appliance such as a netpage printer, in a computer workstation, or on a local network. 
     1.4 The Netpage Printer 
     The netpage printer  601  is an appliance which is registered with the netpage system and prints netpage documents on demand and via subscription. Each printer has a unique printer ID  62 , and is connected to the netpage network via a network such as the Internet, ideally via a broadband connection. 
     Apart from identity and security settings in non-volatile memory, the netpage printer need not contain any persistent storage. As far as a user is concerned, “the network is the computer”. Netpages function interactively across space and time with the help of the distributed netpage page servers  10 , independently of particular netpage printers. 
     The netpage printer receives subscribed netpage documents from netpage publication servers  14 . Each document is distributed in two parts: the page layouts, and the actual text and image objects which populate the pages. Because of personalization, page layouts are typically specific to a particular subscriber and so are pointcast to the subscriber&#39;s printer via the appropriate page server. Text and image objects, on the other hand, are typically shared with other subscribers, and so are multicast to all subscribers&#39; printers and the appropriate page servers. 
     The netpage publication server optimizes the segmentation of document content into pointcasts and multicasts. After receiving the pointcast of a document&#39;s page layouts, the printer knows which multicasts, if any, to listen to. 
     Once the printer has received the complete page layouts and objects that define the document to be printed, it can print the document. 
     The printer rasterizes and prints odd and even pages simultaneously on both sides of the sheet. It contains duplexed print engine controllers  760  and print engines utilizing Memjet™ printheads  350  for this purpose. 
     The printing process consists of two decoupled stages: rasterization of page descriptions, and expansion and printing of page images. The raster image processor (RIP) consists of one or more standard DSPs  757  running in parallel. The duplexed print engine controllers consist of custom processors which expand, dither and print page images in real time, synchronized with the operation of the printheads in the print engines. 
     Printers not enabled for invisible IR printing have the option to print tags using IR-absorptive black ink, although this restricts tags to otherwise empty areas of the page. Although such pages have more limited functionality than invisible IR-printed pages, they are still classed as netpages. 
     A normal netpage printer prints netpages on sheets of paper. More specialized netpage printers may print onto more specialized surfaces, such as globes or sheets of plastics. Each printer supports at least one surface type, and supports at least one tag tiling scheme, and hence tag map, for each surface type. The tag map  811  which describes the tag tiling scheme actually used to print a document becomes associated with that document so that the document&#39;s tags can be correctly interpreted. 
       FIG. 2  shows the netpage printer class diagram, reflecting printer-related information maintained by a registration server  11  on the netpage network. 
     A preferred embodiment of the netpage printer is described in greater detail in Section 6 below, with reference to  FIGS. 11 to 16 . 
     1.4.1 Memjet™ Printheads 
     The netpage system can operate using printers made with a wide range of digital printing technologies, including thermal inkjet, piezoelectric inkjet, laser electrophotographic, and others. However, for wide consumer acceptance, it is desirable that a netpage printer have the following characteristics:
         photographic quality color printing   high quality text printing   high reliability   low printer cost   low ink cost   low paper cost   simple operation   nearly silent printing   high printing speed   simultaneous double sided printing   compact form factor   low power consumption       

     No currently commercially available printing technology has all of these characteristics. 
     To enable production of printers with these characteristics, the present applicant has invented a new print technology, referred to as Memjet™ technology. Memjet™ is a drop-on-demand inkjet technology that incorporates pagewidth printheads fabricated using microelectromechanical systems (MEMS) technology.  FIG. 17  shows a single printing element  300  of a Memjet™ printhead. The netpage wallprinter incorporates 168960 printing elements  300  to form a 1600 dpi pagewidth duplex printer. This printer simultaneously prints cyan, magenta, yellow, black, and infrared inks as well as paper conditioner and ink fixative. 
     The printing element  300  is approximately 110 microns long by 32 microns wide. Arrays of these printing elements are formed on a silicon substrate  301  that incorporates CMOS logic, data transfer, timing, and drive circuits (not shown). 
     Major elements of the printing element  300  are the nozzle  302 , the nozzle rim  303 , the nozzle chamber  304 , the fluidic seal  305 , the ink channel rim  306 , the lever arm  307 , the active actuator beam pair  308 , the passive actuator beam pair  309 , the active actuator anchor  310 , the passive actuator anchor  311 , and the ink inlet  312 . 
     The active actuator beam pair  308  is mechanically joined to the passive actuator beam pair  309  at the join  319 . Both beams pairs are anchored at their respective anchor points  310  and  311 . The combination of elements  308 ,  309 ,  310 ,  311 , and  319  form a cantilevered electrothermal bend actuator  320 . 
       FIG. 18  shows a small part of an array of printing elements  300 , including a cross section  315  of a printing element  300 . The cross section  315  is shown without ink, to clearly show the ink inlet  312  that passes through the silicon wafer  301 . 
       FIGS. 19(   a ),  19 ( b ) and  19 ( c ) show the operating cycle of a Memjet™ printing element  300 . 
       FIG. 19(   a ) shows the quiescent position of the ink meniscus  316  prior to printing an ink droplet. Ink is retained in the nozzle chamber by surface tension at the ink meniscus  316  and at the fluidic seal  305  formed between the nozzle chamber  304  and the ink channel rim  306 . 
     While printing, the printhead CMOS circuitry distributes data from the print engine controller to the correct printing element, latches the data, and buffers the data to drive the electrodes  318  of the active actuator beam pair  308 . This causes an electrical current to pass through the beam pair  308  for about one microsecond, resulting in Joule heating. The temperature increase resulting from Joule heating causes the beam pair  308  to expand. As the passive actuator beam pair  309  is not heated, it does not expand, resulting in a stress difference between the two beam pairs. This stress difference is partially resolved by the cantilevered end of the electrothermal bend actuator  320  bending towards the substrate  301 . The lever arm  307  transmits this movement to the nozzle chamber  304 . The nozzle chamber  304  moves about two microns to the position shown in  FIG. 19(   b ). This increases the ink pressure, forcing ink  321  out of the nozzle  302 , and causing the ink meniscus  316  to bulge. The nozzle rim  303  prevents the ink meniscus  316  from spreading across the surface of the nozzle chamber  304 . 
     As the temperature of the beam pairs  308  and  309  equalizes, the actuator  320  returns to its original position. This aids in the break-off of the ink droplet  317  from the ink  321  in the nozzle chamber, as shown in  FIG. 19(   c ). The nozzle chamber is refilled by the action of the surface tension at the meniscus  316 . 
       FIG. 20  shows a segment of a printhead  350 . In a netpage printer, the length of the printhead is the full width of the paper (typically 210 mm) in the direction  351 . The segment shown is 0.4 mm long (about 0.2% of a complete printhead). When printing, the paper is moved past the fixed printhead in the direction  352 . The printhead has 6 rows of interdigitated printing elements  300 , printing the six colors or types of ink supplied by the ink inlets  312 . 
     To protect the fragile surface of the printhead during operation, a nozzle guard wafer  330  is attached to the printhead substrate  301 . For each nozzle  302  there is a corresponding nozzle guard hole  331  through which the ink droplets are fired. To prevent the nozzle guard holes  331  from becoming blocked by paper fibers or other debris, filtered air is pumped through the air inlets  332  and out of the nozzle guard holes during printing. To prevent ink  321  from drying, the nozzle guard is sealed while the printer is idle. 
     1.5 The Netpage Pen 
     The active sensing device of the netpage system is typically a pen  101 , which, using its embedded controller  134 , is able to capture and decode IR position tags from a page via an image sensor. The image sensor is a solid-state device provided with an appropriate filter to permit sensing at only near-infrared wavelengths. As described in more detail below, the system is able to sense when the nib is in contact with the surface, and the pen is able to sense tags at a sufficient rate to capture human handwriting (i.e. at 200 dpi or greater and 100 Hz or faster). Information captured by the pen is encrypted and wirelessly transmitted to the printer (or base station), the printer or base station interpreting the data with respect to the (known) page, or, in the preferred embodiment, transmitting the information to a netpage server for interpretation. 
     The preferred embodiment of the netpage pen operates both as a marking ink pen and as a non-marking stylus. The marking aspect, however, is not necessary for using the netpage system as a browsing system, such as when it is used as an Internet interface. Each netpage pen is registered with the netpage system and has a unique pen ID  61 . 
     When either nib is in contact with a netpage, the pen determines its position and orientation relative to the page. The nib is attached to a force sensor, and the force on the nib is interpreted relative to a threshold to indicate whether the pen is “up” or “down”. This allows a interactive element on the page to be ‘clicked’ by pressing with the pen nib, in order to request, say, information from a network. Furthermore, the force is captured as a continuous value to allow, say, the full dynamics of a signature to be verified. The nib may be movable when subject to a specified force which is greater than that normally applied when writing. To “click” the user applies a force sufficient to move the nib. This may provide more desirable feedback to the user compared to that provided by a non-moving nib. 
     The pen determines the position and orientation of its nib on the netpage by imaging, in the infrared spectrum, an area  193  of the page in the vicinity of the nib. It decodes the nearest tag and computes the position of the nib relative to the tag from the observed perspective distortion on the imaged tag and the known geometry of the pen optics. Although the position resolution of the tag may be low, because the tag density on the page is inversely proportional to the tag size, the adjusted position resolution is quite high, exceeding the minimum resolution required for accurate handwriting recognition. 
     Pen actions relative to a netpage are captured as a series of strokes. A stroke consists of a sequence of time-stamped pen positions on the page, initiated by a pen-down event and completed by the subsequent pen-up event. A stroke is also tagged with the page ID  50  of the netpage whenever the page ID changes, which, under normal circumstances, is at the commencement of the stroke. 
     Each netpage pen has a current selection  826  associated with it, allowing the user to perform copy and paste operations etc. The selection is time-stamped to allow the system to discard it after a defined time period. The current selection describes a region of a page instance. It consists of the most recent digital ink stroke captured through the pen relative to the background area of the page. It is interpreted in an application-specific manner once it is submitted to an application via a selection hyperlink activation. 
     Each pen has a current nib  824 . This is the nib last notified by the pen to the system. In the case of the default netpage pen described above, either the marking ink nib or the non-marking stylus nib is current. Each pen also has a current nib style  825 . This is the nib style last associated with the pen by an application, e.g. in response to the user selecting a color from a palette. The default nib style is the nib style associated with the current nib. Strokes captured through a pen are tagged with the current nib style. When the strokes are subsequently reproduced, they are reproduced in the nib style with which they are tagged. 
     Whenever the pen is within range of a printer with which it can communicate, the pen slowly flashes its “online” LED. When the pen fails to decode a stroke relative to the page, it momentarily activates its “error” LED. When the pen succeeds in decoding a stroke relative to the page, it momentarily activates its “ok” LED. 
     A sequence of captured strokes is referred to as digital ink. Digital ink forms the basis for the digital exchange of drawings and handwriting, for online recognition of handwriting, and for online verification of signatures. 
     The pen is wireless and transmits digital ink to the netpage printer via a short-range radio link. The transmitted digital ink is encrypted for privacy and security and packetized for efficient transmission, but is always flushed on a pen-up event to ensure timely handling in the printer. 
     When the pen is out of range of a printer it buffers digital ink in internal memory, which has a capacity of over ten minutes of continuous handwriting. When the pen is once again within range of a printer, it transfers any buffered digital ink. The buffer may provide more or less buffer capacity. 
     A pen can be registered with any number of printers, but because all state data resides in netpages both on paper and on the network, it is largely immaterial which printer a pen is communicating with at any particular time. 
     A preferred embodiment of the pen is described in greater detail in Section 6 below, with reference to  FIGS. 8 to 10 . 
     1.6 Netpage Interaction 
     The netpage printer  601  receives data relating to a stroke from the pen  101  when the pen is used to interact with a netpage  1 . The coded data  3  of the tags  4  is read by the pen when it is used to execute a movement, such as a stroke. The data allows the identity of the particular page and associated interactive element to be determined and an indication of the relative positioning of the pen relative to the page to be obtained. The indicating data is transmitted to the printer, where it resolves, via the DNS, the page ID  50  of the stroke into the network address of the netpage page server  10  which maintains the corresponding page instance  830 . It then transmits the stroke to the page server. If the page was recently identified in an earlier stroke, then the printer may already have the address of the relevant page server in its cache. Each netpage consists of a compact page layout maintained persistently by a netpage page server (see below). The page layout refers to objects such as images, fonts and pieces of text, typically stored elsewhere on the netpage network. 
     When the page server receives the stroke from the pen, it retrieves the page description to which the stroke applies, and determines which element of the page description the stroke intersects. It is then able to interpret the stroke in the context of the type of the relevant element. 
     A “click” is typically a stroke where the distance and time between the pen down position and the subsequent pen up position are both less than some small maximum. An object which is activated by a click typically requires a click to be activated, and accordingly, a longer stroke is ignored. The failure of a pen action, such as a “sloppy” click, to register is indicated by the lack of response from the pen&#39;s “ok” LED. However, where a netpage includes a button a “click” can be registered when both the pen down and pen up positions are both within the area of the button. 
     There are two kinds of input elements in a netpage page description: hyperlinks and form fields. Input through a form field can also trigger the activation of an associated hyperlink. 
     1.7 Standard Features of Netpages 
     In the preferred form, each netpage is printed with the netpage logo at the bottom to indicate that it is a netpage and therefore has interactive properties. The logo also acts as a copy button. In most cases “clicking” the logo produces a copy of the page. In the case of a form, the button produces a copy of the entire form. And in the case of a secure document, such as a ticket or coupon, the button elicits an explanatory note or advertising page. 
     The default single-page copy function is handled directly by the relevant netpage page server. Special copy functions are handled by linking the logo button to an application. 
     1.8 User Help System 
     In a preferred embodiment, the netpage printer has a single button labeled “Help”. When pressed it elicits a single page of information, including:
         status of printer connection   status of printer consumables   top-level help menu   document function menu   top-level netpage network directory       

     The help menu provides a hierarchical manual on how to use the netpage system. 
     The document function menu includes the following functions:
         print a copy of a document   print a clean copy of a form   print the status of a document       

     A document function is initiated by simply pressing the button and then touching any page of the document. The status of a document indicates who published it and when, to whom it was delivered, and to whom and when it was subsequently submitted as a form. 
     The netpage network directory allows the user to navigate the hierarchy of publications and services on the network. As an alternative, the user can call the netpage network “900” number “yellow pages” and speak to a human operator. The operator can locate the desired document and route it to the user&#39;s printer. Depending on the document type, the publisher or the user pays the small “yellow pages” service fee. 
     The help page is obviously unavailable if the printer is unable to print. In this case the “error” light is lit and the user can request remote diagnosis over the network. 
     2. Netpage Printer Description 
     2.1 Printer Mechanics 
     The vertically-mounted netpage wallprinter  601  is shown fully assembled in  FIGS. 11 and 12 . As best shown in  FIGS. 12 ,  12   a  and  30 , it prints netpages on A4 sized media using duplexed 8½″ Memjet™ print engines  602  and  603 . It uses a straight paper path with the paper  604  passing through duplexed print engines  602  and  603  which print both sides of a sheet simultaneously, in full color and with full bleed. A multi-DSP raster image processor (RIP) rasterizes pages to internal memory, and a pair of custom print engine controllers expand, dither and print page images to the duplexed printheads in real time. 
     An integral binding assembly  605  applies a strip of glue along one edge of each printed sheet, allowing it to adhere to the previous sheet when pressed against it. This creates a final bound document  618  which can range in thickness from one sheet to several hundred sheets. The binding assembly will be considered in close detail below with particular reference to  FIGS. 26 ,  27  and  28 . 
     Referring to  FIGS. 11 ,  12 ,  12   a ,  13  and  21  to  23 , the wallprinter  601  consists of a main chassis  606 , which accommodates all major components and assemblies. As best shown in  FIG. 23 , it has a pivoting media tray  607  on the front upper portion, which is covered by a front molding  608  and handle molding  609 . The front molding  608 , handle molding  609  and lower front molding  610  can vary in color, texture and finish to make the product more appealing to consumers. They simply clip onto the front of the wallprinter  601 . 
       FIGS. 24 and 25  show the wallprinter electrical system in isolation. A flexible printed circuit board (flex PCB)  611  runs from the media tray  607  to the main PCB  612 . It includes four different color LEDs  613 ,  614 ,  615  and  616  and a push button  617 . The LEDs show through the front molding and indicate “on”  613 , “ink out”  614 , “paper out”  615 , and “error”  616 . The push button  617  elicits printed “help” in the form of usage instructions, printer and consumable status information, and a directory of resources on the netpage network. 
     Printed, bound documents  618  exit through the base of the wallprinter  601  into a clear, plastic, removable collection tray  619 . This is discussed in greater detail below with specific reference to  FIG. 28 . 
     The wallprinter  601  is powered by an internal 110V/220V power supply  620  and has a metal mounting plate  621  that is secured to a wall or stable vertical surface by four screws. Plunged keyhole slot details  622  in the metal plate  621  allow for four spigots mounted on the rear of the printer to hook onto the plate. The wallprinter  601  is prevented from being lifted off by a screw that locates the chassis molding  606  to the plate  621  at one position behind the media tray  607 . 
     Referring to  FIG. 21 , the side of the wallprinter  601  includes a module bay  624  which accommodates a network interface module  625  which allows the printer to be connected to the netpage network and to a local computer or network. The interface module  625  can be selected and installed in the factory or in the field to provide the interfaces required by the user. The modules may have common connector options, such as: IEEE 1394 (Firewire) connection, standard Centronics printer port connection or a combined USB2 and Ethernet connection. This allows the consumer to connect the wallprinter  601  to a computer or use it as a network printer. Other types of connections may be used. The interface module PCB, (with gold contact edge strips) plugs directly into the main wallprinter PCB  612  via an edge connector. The different connector configurations are accommodated in the module design by use of a tool insert. Finger recesses on either side of the module  625  allow for easy manual insertion or removal. 
     Turning to  FIG. 30 , the main PCB  612  is attached to the rear of the chassis  606 . The board  612  interfaces through the chassis molding  606  to the interface module  625 . The PCB  612  also carries the necessary peripheral electronics to the Memjet™ printheads  705 . This includes a main CPU with volatile memory (presently two 32 MB DRAMs are used), flash memory, IEEE 1394 interface chip, motor controllers (presently six), various sensor connectors, interface module PCB edge connector, power management, internal/external data connectors and a QA chip. 
       FIG. 23  shows the front hatch access to the paper  604  and the ink cartridge  627 . Referring to  FIG. 29 , paper  604  is placed into a hinged top tray  607  and pressed down onto a sprung platen  666 . The tray  607  is mounted to the chassis  606  via hinges  700 . Each hinge has a base, a hinge lever and a hinge side. Pivots on the base and paper/media tray  607  engage the lever and side such that the paper/media tray  607  rotates in a manner that avoids kinking the supply hoses  646 . Other paper tray designs may be used. 
     The paper  604  is positioned under edge guides  667  before being closed and is automatically registered to one side of the tray  607  by action of a metal spring part  668 . An ink cartridge  627  connects into a pivoting ink connector molding  628  via a series of self-sealing connectors  629 . The connectors  629  transmit ink, air and glue to their separate locations. The ink connector molding  628  contains a sensor, which detects a QA chip on the ink cartridge and verifies identification prior to printing. When the front hatch is sensed closed, a release mechanism allows the sprung platen  666  to push the paper  604  against a motorized media pick-up roller assembly  626 . 
       FIG. 22 , shows the complete assembly of the replaceable ink cartridge  627 . It has bladders or chambers for storing fixative  644 , adhesive  630 , and cyan  631 , magenta  632 , yellow  633 , black  634  and infrared  635  inks. The cartridge  627  also contains a micro air filter  636  in a base molding  637 . As shown in  FIG. 13 , the micro air filter  636  interfaces with an air pump  638  inside the printer via a hose  639 . This provides filtered air to the printheads  705  to prevent ingress of micro particles into the Memjet™ printheads  705  which may clog the nozzles. By incorporating the air filter  636  within the cartridge  627 , the operational life of the filter is effectively linked to the life of the cartridge. This ensures that the filter is replaced together with the cartridge rather than relying on the user to clean or replace the filter at the required intervals. Furthermore, the adhesive and infrared ink are replenished together with the visible inks and air filter thereby reducing how frequently the printer operation is interrupted because of the depletion of a consumable material. 
     The cartridge  627  has a thin wall casing  640 . The ink bladders  631  to  635  and fixative bladder  644  are suspended within the casing by a pin  645  which hooks the cartridge together. The single glue bladder  630  is accommodated in the base molding  637 . This is a fully recyclable product with a capacity for printing and gluing 3000 pages (1500 sheets). 
     Referring to  FIGS. 12 ,  12   a ,  24 ,  25  and  30 , the motorized media pick-up roller assembly  626  pushes the top sheet directly from the media tray  607  past a paper sensor (not shown) on the first print engine  602  into the duplexed Memjet™ printhead assembly. 
     Two Memjet™ print engines  602  and  603  are mounted in an opposing in-line sequential configuration along the straight paper path. The paper  604  is drawn into the first print engine  602  by integral, powered pick-up rollers  626 . The position and size of the paper  604  is sensed and full bleed printing commences. 
     Fixative is printed simultaneously to aid drying in the shortest possible time. 
     As best shown in  FIG. 12   a , the Memjet™ print engines  602  and  603  include a rotary capping, blotting and platen device  669 . The capping device seals the Memjet™ printheads  705  when not in use. It uncaps and rotates to produce an integral blotter, which is used for absorbing ink fired from the printheads  705  during routine printer startup maintenance. It simultaneously moves an internal capping device inside the Memjet™ printhead  705  that allows air to flow into the protective nozzle shield area. The third rotation of the device moves a platen surface into place, which supports one side of the sheet  604  during printing. 
     The paper exits the first Memjet™ print engine  602  through a set of powered exit spike wheels (aligned along the straight paper path), which acts against a rubberized roller. These spike wheels contact the ‘wet’ printed surface and continue to feed the sheet  604  into the second Memjet™ print engine  603 . 
     This second print engine  603  is mounted the opposite way up to the first in order to print the underside of the sheet  604 . 
     As shown in  FIGS. 12 ,  12   a ,  13 ,  26  and  27 , the paper  604  passes from the duplexed print engines  602  and  603 , into the binder assembly  605 . The printed page passes between a powered spike wheel axle  670  with a fibrous support roller and another movable axle with spike wheels and a momentary action glue wheel  673 . The movable axle/glue assembly  673  is mounted to a metal support bracket and it is transported forward to interface with the powered axle  670  by action of a camshaft  642 . A separate motor powers  675  this camshaft. Both motors  676  are controlled by the Memjet™ printheads. 
     The glue wheel assembly  673  consists of a partially hollow axle  679  with a rotating coupling  680  for the glue supply hose  641  from the ink cartridge  627 . This axle  679  connects to a glue wheel  681 , which absorbs adhesive by capillary action through radial holes. A molded housing surrounds the glue wheel  681 , with an opening at the front. Pivoting side moldings  683  and sprung outer doors  684  are attached to the metal support bracket and hinge out sideways when the rest of the assembly  673  is thrust forward. This action exposes the glue wheel  681  through the front of the molded housing. Tension springs  685  close the assembly and effectively cap the glue wheel  681  during periods of inactivity. 
     As the sheet  604  passes into the glue wheel assembly  673 , adhesive is applied to one vertical edge on the front side (apart from the first sheet of a document) as it is transported down into the binding assembly  605 . It will be appreciated that this arrangement applies adhesive to each page during printing so that the paper movement through the printer is not interrupted or stopped at a separate gluing station. This increases the printer speed, however, it requires that the pages move through the printer in “portrait” configuration (that is, in a direction parallel to the long edges). This in turn requires the paper tray, binding station and collection station to be in portrait configuration. This may make the overall length of the printer too great to conveniently fit into areas having limited space. In these situations, the media tray, binding station and collection station can be arranged in “landscape” orientation (short sides parallel to paper movement) to shorten the length of the printer. However, the gluing assembly must still be able to apply glue along the long side of the pages. In this version of wallprinter (not shown), the adhesive is applied to the longitudinal edge of each page with a reciprocating glue strip. 
     The “portrait” binder assembly  605  is best shown in  FIG. 26 . It has a metal support chassis  686 , a sprung molded binding platen  687  that runs on four traverser rods, a molded angled platen  689  which supports the document  618  after the sheet  604  has been moved across, and an exit hatch  690  with support bracket  691 . The printed page  604  is fed in until it rests on the exit hatch  690 . The binding platen  687  is propelled forward at high speed via a looped system of wheels  692  and a sprung steel cable  693  that attaches to a powered cable winder shaft  694 . As the cable winder shaft  694  is rotated, the cable loop  693  shortens and transports the binding platen  687  forward. This powered shaft  694  has a slip clutch mechanism and provides the necessary speed to push the sheet  604  forward onto the rear of a previous sheet, glue/bind it then return under the action of return springs  699  to the home position to accept the next printed sheet. A single operating cycle of the reciprocating platen takes less than 2 seconds. 
     The binding assembly  605  binds pages one by one into a bound document, thereby producing bound documents without significantly adding to the time taken to print the separate pages of the document. Furthermore it applies the adhesive directly prior to pressing it against the previous page. This is more effective than applying adhesive to the rear of each page and sequentially pressing each page to the subsequent page because any interruption in the printing process such as replenishing the paper supply may allow the adhesive applied to the last adhered page to deteriorate and become less effective. 
     The cable  693  is sprung to allow for positive pressure to be applied to the previous sheet to aid binding. Furthermore, the angled platen  689  is shallower at the top than at the base in order to support the document  618  in an over axis configuration. 
     A sensor (not shown) operatively connected to the control of the stepper motor, may be used to determine the position of the last page bound to the document to allow the platen to accurately adhere the next page to it. 
     A paper tapper  643  knocks the sheet  604  to one side of the binder  605  as it is transported across to the angled platen  689 . The main PCB  612  controls motors  695 ,  696  and  697  for the cable winder shaft  694 , the tapper  643  and the exit hatch  690  respectively. 
     When a document  618  is bound and finished, the powered exit hatch  690  opens. A tamper sensor (not shown) is provided to detect document jams or other interferences acting to prevent the exit hatch  690  from closing. The tapper  643  also tap aligns the printed document  618  during ejection out of the binder  605  into the collection tray  619 . Plastic foils  698  on the lower front molding  610  work together with the hatch  690  to direct the finished document  618  to the back of the collection tray  619  and feed any further documents into the tray without hitting existing ones. A plurality the flexible foils may be provided, each having different lengths to accommodate documents having different page sizes. The collection tray  619  is molded in clear plastic and pulls out of its socket under a certain loading. Access for removing documents is provided on three sides. 
     2.2 Memjet-Based Printing 
     A Memjet™ printhead produces 1600 dpi bi-level CMYK. On low-diffusion paper, each ejected drop forms an almost perfectly circular 22.5 □m diameter dot. Dots are easily produced in isolation, allowing dispersed-dot dithering to be exploited to its fullest. 
     A page layout may contain a mixture of images, graphics and text. Continuous-tone (contone) images and graphics are reproduced using a stochastic dispersed-dot dither. Unlike a clustered-dot (or amplitude-modulated) dither, a dispersed-dot (or frequency-modulated) dither reproduces high spatial frequencies (i.e. image detail) almost to the limits of the dot resolution, while simultaneously reproducing lower spatial frequencies to their full color depth, when spatially integrated by the eye. A stochastic dither matrix is carefully designed to be free of objectionable low-frequency patterns when tiled across the image. As such its size typically exceeds the minimum size required to support a particular number of intensity levels (e.g. 16□16□8 bits for 257 intensity levels). 
     Human contrast sensitivity peaks at a spatial frequency of about 3 cycles per degree of visual field and then falls off logarithmically, decreasing by a factor of 100 beyond about 40 cycles per degree and becoming immeasurable beyond 60 cycles per degree. At a normal viewing distance of 12 inches (about 300 mm), this translates roughly to 200-300 cycles per inch (cpi) on the printed page, or 400-600 samples per inch according to Nyquist&#39;s theorem. 
     In practice, contone resolution above about 300 ppi is of limited utility outside special applications such as medical imaging. Offset printing of magazines, for example, uses contone resolutions in the range 150 to 300 ppi. Higher resolutions contribute slightly to color error through the dither. 
     Black text and graphics are reproduced directly using bi-level black dots, and are therefore not anti-aliased (i.e. low-pass filtered) before being printed. Text is therefore super-sampled beyond the perceptual limits discussed above, to produce smoother edges when spatially integrated by the eye. Text resolution up to about 1200 dpi continues to contribute to perceived text sharpness (assuming low-diffusion paper, of course). 
     The netpage printer uses a contone resolution of 267 ppi (i.e. 1600 dpi/6), and a black text and graphics resolution of 800 dpi. 
     2.3 Document Data Flow 
     Because of the pagewidth nature of the Memjet™ printhead, each page must be printed at a constant speed to avoid creating visible artifacts. This means that the printing speed can&#39;t be varied to match the input data rate. Document rasterization and document printing are therefore decoupled to ensure the printhead has a constant supply of data. A page is never printed until it is fully rasterized. This is achieved by storing a compressed version of each rasterized page image in memory. 
     This decoupling also allows the raster image processor (RIP) to run ahead of the printer when rasterizing simple pages, buying time to rasterize more complex pages. 
     Because contone color images are reproduced by stochastic dithering, but black text and line graphics are reproduced directly using dots, the compressed page image format contains a separate foreground bi-level black layer and background contone color layer. The black layer is composited over the contone layer after the contone layer is dithered. 
     Netpage tags are rendered to a separate layer and are ultimately printed using infrared-absorptive ink. 
     At 267 ppi, a Letter size page of contone CMYK data has a size of 25 MB. Using lossy contone compression algorithms such as JPEG (ISO/IEC 19018-1:1994, Information technology—Digital compression and coding of continuous-tone still images: Requirements and guidelines, 1994, the contents of which are herein incorporated by cross-reference), contone images compress with a ratio up to 10:1 without noticeable loss of quality, giving a compressed page size of 2.5 MB. Lossless compression algorithms may be used but these do not usually result in as high compression ratios compared to lossy compression algorithms. 
     At 800 dpi, a Letter size page of bi-level data has a size of 7 MB. Coherent data such as text compresses very well. Using lossless bi-level compression algorithms such as Group 4 Facsimile (ANSI/EIA 538-1988, Facsimile Coding Schemes and Coding Control Functions for Group 4 Facsimile Equipment, August 1988, the contents of which are herein incorporated by cross-reference), ten-point text compresses with a ratio of about 10:1, giving a compressed page size of 0.8 MB. 
     Once dithered, a Letter size page of CMYK contone image data consists of 114 MB of bi-level data. Using lossless bi-level compression algorithms on this data is pointless precisely because the optimal dither is stochastic—i.e. since it introduces hard-to-compress disorder. 
     The two-layer compressed page image format therefore exploits the relative strengths of lossy JPEG contone image compression and lossless bi-level text compression. The format is compact enough to be storage-efficient, and simple enough to allow straightforward real-time expansion during printing. 
     Since text and images normally don&#39;t overlap, the normal worst-case page image size is 2.5 MB (i.e. image only), while the normal best-case page image size is 0.8 MB (i.e. text only). The absolute worst-case page image size is 3.3 MB (i.e. text over image). Assuming a quarter of an average page contains images, the average page image size is 1.2 MB. 
     2.4 Printer Controller Architecture 
     The netpage printer controller consists of a controlling processor  750 , a factory-installed or field-installed network interface module  625 , a radio transceiver (transceiver controller  753 , baseband circuit  754 , RF circuit  755 , and RF resonators and inductors  756 ), dual raster image processor (RIP) DSPs  757 , duplexed print engine controllers  760   a  and  760   b , flash memory  658 , and DRAM  657  (presently 64 MB), as illustrated in the block diagram in  FIG. 14 . 
     The controlling processor handles communication with the network  19  and with local wireless netpage pens  101 , senses the help button  617 , controls the user interface LEDs  613 - 616 , and feeds and synchronizes the RIP DSPs  757  and print engine controllers  760 . It consists of a medium-performance general-purpose microprocessor. The controlling processor  750  communicates with the print engine controllers  760  via a high-speed serial bus  659 . 
     The RIP DSPs rasterize and compress page descriptions to the netpage printer&#39;s compressed page format. Each print engine controller expands, dithers and prints page images to its associated Memjet™ printhead  350  in real time (i.e. at over 30 pages per minute). The duplexed print engine controllers print both sides of a sheet simultaneously. 
     The master print engine controller  760   a  controls the paper transport and monitors ink usage in conjunction with the master QA chip  665  and the ink cartridge QA chip  761 . 
     The printer controller&#39;s flash memory  658  holds the software for both the processor  750  and the DSPs  757 , as well as configuration data. This is copied to main memory  657  at boot time. 
     The processor  750 , DSPs  757 , and digital transceiver components (transceiver controller  753  and baseband circuit  754 ) are integrated in a single controller ASIC  656 . Analog RF components (RF circuit  755  and RF resonators and inductors  756 ) are provided in a separate RF chip  762 . The network interface module  625  is separate, since netpage printers allow the network connection to be factory-selected or field-selected. Flash memory  658  and the 2□256 Mbit (64 MB) DRAM  657  is also off-chip. The print engine controllers  760  are provided in separate ASICs. 
     A variety of network interface modules  625  are provided, each providing a netpage network interface  751  and optionally a local computer or network interface (not shown). Netpage network Internet interfaces include POTS modems, Hybrid Fiber-Coax (HFC) cable modems, ISDN modems, DSL modems, satellite transceivers, current and next-generation cellular telephone transceivers, and wireless local loop (WLL) transceivers. Local interfaces include IEEE 1284 (parallel port), 10Base-T and 100Base-T Ethernet, USB and USB 2.0, IEEE 1394 (Firewire), and various emerging home networking interfaces. If an Internet connection is available on the local network, then the local network interface can be used as the netpage network interface. 
     The radio transceiver  753  communicates in the unlicensed 900 MHz band normally used by cordless telephones, or alternatively in the unlicensed 2.4 GHz industrial, scientific and medical (ISM) band, and uses frequency hopping and collision detection to provide interference-free communication. 
     The printer controller optionally incorporates an Infrared Data Association (IrDA) interface for receiving data “squirted” from devices such as netpage cameras. In an alternative embodiment, the printer uses the IrDA interface for short-range communication with suitably configured netpage pens. 
     2.4.1 Rasterization and Printing 
     Once the main processor  750  has received and verified (at  550 ) the document&#39;s page layouts and page objects into memory  657  (at  551 ), it runs the appropriate RIP software on the DSPs  757 . 
     The DSPs  757  rasterize (at  552 ) each page description and compress (at  553 ) the rasterized page image. The main processor stores each compressed page image in memory  657  (at  554 ). The simplest way to load-balance multiple DSPs is to let each DSP rasterize a separate page. The DSPs can always be kept busy since an arbitrary number of rasterized pages can, in general, be stored in memory. This strategy only leads to potentially poor DSP utilization when rasterizing short documents. 
     Watermark regions in the page description are rasterized to a contone-resolution bi-level bitmap which is losslessly compressed to negligible size and which forms part of the compressed page image. 
     The infrared (IR) layer of the printed page contains coded netpage tags at a density of about six per inch. Each tag encodes the page ID, tag ID, and control bits, and the data content of each tag is generated during rasterization and stored in the compressed page image. 
     The main processor  750  passes back-to-back page images to the duplexed print engine controllers  760 . Each print engine controller  760  stores the compressed page image in its local memory  769 , and starts the page expansion and printing pipeline. Page expansion and printing is pipelined because it is impractical to store an entire 114 MB bi-level CMYK+IR page image in memory. 
     The print engine controller expands the compressed page image (at  555 ), dithers the expanded contone color data to bi-level dots (at  556 ), composites the expanded bi-level black layer over the dithered contone layer (at  557 ), renders the expanded netpage tag data (at  558 ), and finally prints the fully-rendered page (at  559 ) to produce a printed netpage  1 . 
     2.4.2 Print Engine Controller 
     The page expansion and printing pipeline of the print engine controller  760  consists of a high speed IEEE 1394 serial interface  659 , a standard JPEG decoder  763 , a standard Group 4 Fax decoder  764 , a custom halftoner/compositor unit  765 , a custom tag encoder  766 , a line loader/formatter unit  767 , and a custom interface  768  to the Memjet™ printhead  350 , as illustrated in the block diagram in  FIG. 16 . 
     The print engine controller  360  operates in a double buffered manner. While one page is loaded into DRAM  769  via the high speed serial interface  659 , the previously loaded page is read from DRAM  769  and passed through the print engine controller pipeline. Once the page has finished printing, the page just loaded is printed while another page is loaded. 
     The first stage of the pipeline expands (at  763 ) the JPEG-compressed contone CMYK layer, expands (at  764 ) the Group 4 Fax-compressed bi-level black layer, and renders (at  766 ) the bi-level netpage tag layer according to the tag format defined in section 1.2, all in parallel. The second stage dithers (at  765 ) the contone CMYK layer and composites (at  765 ) the bi-level black layer over the resulting bi-level CMYK layer. The resultant bi-level CMYK+IR dot data is buffered and formatted (at  767 ) for printing on the Memjet™ printhead  350  via a set of line buffers. Most of these line buffers are stored in the off-chip DRAM. The final stage prints the six channels of bi-level dot data (including fixative) to the Memjet™ printhead  350  via the printhead interface  768 . 
     When several print engine controllers  760  are used in unison, such as in a duplexed configuration, they are synchronized via a shared line sync signal  770 . Only one print engine  760 , selected via the external master/slave pin  771 , generates the line sync signal  770  onto the shared line. 
     The print engine controller  760  contains a low-speed processor  772  for synchronizing the page expansion and rendering pipeline, configuring the printhead  350  via a low-speed serial bus  773 , and controlling the stepper motors  675 ,  676 . 
     In the 8½″ versions of the netpage printer, the two print engines each prints 30 Letter pages per minute along the long dimension of the page (11″), giving a line rate of 8.8 kHz at 1600 dpi. In the 12″ versions of the netpage printer, the two print engines each prints 45 Letter pages per minute along the short dimension of the page (8½″), giving a line rate of 10.2 kHz. These line rates are well within the operating frequency of the Memjet™ printhead, which in the current design exceeds 30 kHz. 
     2.5 The Netpage Refrigerator 
     The particular apparatus of the invention is a system, for domestic or industrial application, which provides the combined capability of an appliance such as a refrigerator, and an interactive printer device. For simplicity, the apparatus is referred to below as a “netpage fridge” and described in the context of the netpage system. However, it will be understood that other appropriate domestic appliances may be equipped in a similar manner, and that alternative computer systems may be employed to operatively interact with the refrigerator or other appliance. 
     Since the netpage printer of the invention is remotely interactive (generally, through wireless transmission from netpage pen to printer device), there is little danger of conflict between the use of the printer and that of the refrigerator itself. Further, a refrigerator is already powered and can readily be augmented with a network connection, and its door provides an ideal form factor for a netpage printer, being tall but shallow. 
     The netpage fridge  1001  is shown fully assembled in  FIGS. 31 ,  32  and  33 , and in cross section in  FIG. 34 . The netpage fridge  1001  provides an upper rectangular enclosure  1034  serving as a first environmental control chamber, having walls defining an interior compartment suitable for storage and refrigeration of produce, and a lower rectangular enclosure  1036  serving as a second environmental control chamber suitable for storage and freezing of produce. The cross sectional view of  FIG. 34  also illustrates the rear refrigerator heat exchange pipes  1030  and the system compressor pump and motor assembly  1032 . Access to the storage compartments  1034 ,  1036  of the appliance is provided via, respectively, an upper door  1012  and a lower door  1014 , which also serve to provide the mechanical structure for supporting the interactive printer device function. This mechanical arrangement allows for convenient access to the interactive printer function for operation and maintenance purposes and satisfies human factor engineering design principles. 
       FIG. 32  shows the accessibility to a consumables access hatch  1016 , i.e. the hatch by which access is gained to supplement those parts requiring periodic replacement, commonly paper and ink. This access hatch  1016  is provided as a hinged assembly in the upper door  1012  of the netpage fridge  1001 , mounted to fold downwardly and outwardly for ready access to a user. Those components of the interactive printer which do not require routine access are neatly concealed within the door assemblages, accessible by maintenance operators when required. The cross-sectional view of  FIG. 34  illustrates the relative positioning of the functional elements of the printer device, including a media tray  607 , a PCB  612 , and a paper supply compartment, print engines  602 ,  603  for producing both the visible and the invisibly coded printed information, and a binding assembly  605  closed at its lower end by an exit hatch  690 , to produce a bound multipage document  618  if required. In the lower door  1014  of the netpage fridge  1001  an outlet tray assembly is provided, enabling convenient access to the netpage printer&#39;s printed output. The outlet tray assembly includes a collection tray  619  accepting output fed by gravity from the exit hatch of the binding assembly  605 , the outlet tray readily accessible to allow removal of a printed, bound document  618 . 
     In the preferred embodiment, then, the netpage fridge  1001  is configured to work with the netpage networked computer system described in detail above and in previously and co-filed applications. The netpage fridge is thus registered with the netpage system and prints netpage documents on demand or via subscription in accordance with the process described above and detailed in our earlier application U.S. Ser. No. 09/722,142. Each netpage fridge has a unique ID and is connected to the netpage system via a network such as the Internet, ideally via a broadband connection. The network interconnection is supported via a network interface which is integral to the netpage fridge. A receiver for receiving signals from the netpage pen is built into the printer device in the upper door of the netpage fridge.  FIG. 33  schematically illustrates two leads  1022 ,  1028 , running through the upper fridge door  1012 , the upper fridge door hinge mechanism, and the side wall of the upper part of the fridge cabinet to emerge at the rear of the appliance, providing connection to, respectively, a mains power supply  1026  to supply power to the netpage printer device, and a network link  1024 , such as a residential telephone line providing a connection to the Internet. 
     In a preferred form of the netpage fridge, the refrigerator function and the interactive printer function are interdependent. That is, certain specific capabilities of the interactive printer function are inexorably linked to specific functional characteristics of the refrigerator function. Therefore, in addition to providing the inherent capabilities of a refrigerator the netpage fridge provides unique additional capabilities which supplement the inherent capability of the appliance. The additional functional capabilities include, but are not limited to:
     a. Automated stock monitoring and control;   b. Device control; and   c. Fault diagnostics and reporting.
 
2.5.1 Automated Stock Monitoring and Control
   

     In conjunction with an external sensing device, such as the netpage pen, the netpage fridge provides the capability to monitor stock as it is entered into and removed from the storage compartments. This function provides the appliance with the capability to assist with restocking, monitor use-by dates, and suggest recipes which utilise the available ingredients. A user&#39;s netpage pen can be readily augmented to support barcode scanning, and the netpage system can provide an application which converts barcode input into updates to an appropriate shopping cart. As contemplated, the netpage system itself is able to record a favorite application for each user for each of a set of product types. 
     2.5.2 Device Control 
     The netpage fridge provides the capability to control the fridge, eg. temperature control, as well as other network enabled appliances such as home theatre systems, air-conditioning units and security systems. The netpage fridge is able to produce printed control buttons in the form of a remote control device which can be subsequently used to control the normal functions of the appropriate appliances. A netpage fridge will typically be located in a central position which is readily accessible, and hence is an appropriate system to incorporate the device control capability. 
     2.5.3 Fault Diagnostics and Reporting 
     In conjunction with one or more thermal sensors  1018  which are located within the netpage fridge storage compartment and monitored via a sensor interface embedded within the interactive printer device function, the netpage fridge provides a fault diagnostics and reporting capability. For example, in the event that a temperature is detected which exceeds some threshold level, the netpage fridge produces a printed report to indicate that the temperature inside one or other storage compartment has exceeded that preset threshold level. Similarly, if a power outage has been detected in the electric power supply to the refrigerator the netpage fridge produces a report detailing the interruption in supply for the information of the appliance user. 
     CONCLUSION 
     The present invention has been described with reference to a preferred embodiment and number of specific alternative embodiments. However, it will be appreciated by those skilled in the relevant fields that a number of other embodiments, differing from those specifically described, will also fall within the spirit and scope of the present invention. Accordingly, it will be understood that the invention is not intended to be limited to the specific embodiments described in the present specification, including documents incorporated by cross-reference as appropriate. The scope of the invention is only limited by the attached claims.