BACKGROUND OF THE INVENTION Many digital pen and paper systems have been proposed. Few have been used on any commercial scale. One that is in use is the Anoto system, with their Logitech IOTM pen, available from Anoto Group AB. The invention will be described in relation to that kind of technology, and is especially, but not exclusively, applicable to arrangements where the pen sees a position-determining pattern that has been printed onto the page and in which an evaluation of the pen's position, and movements, is made using data collected by the pen. A discussion of that kind of technology follows later.

SUMMARY OF THE INVENTION According to a first aspect the invention comprises a digital pen adapted to acquire information from a position-determining pattern printed on digital paper to enable the position of the pen to be determined, the pen having a writing stylus adapted to write handwritten markings onto the paper and being adapted to capture those handwritten markings digitally, and the pen also having an eraser adapted to erase markings on an area of the paper and being adapted to capture the position of the erased area digitally.

Using such a pen is more natural and intuitive than actually making additional ink markings on the paper but knowing that they will be interpreted electronically as erasing. What You See Is What You Get

(WYSIWYG) is a powerful advantage. Having a user physically erase markings in the physical world in order to electronically erase markings in the digital world is an intuitive way of a user inputting data into a processor.

The writing stylus is usually adapted to apply ink to the paper, but pencil is also envisaged, or indeed any marking medium.

The eraser may be a device that applies an erasing covering material, e. g. an erasing liquid, to the paper, e. g. a white liquid. The eraser may for example comprise an eraser stylus, but it could be a device that physically removes written material (e. g. ink) from the paper, or that changes the optical properties of the hand-written ink (e. g. renders the ink invisible, or much less so than normal). Applying an erasing, or covering, material that covers up unwanted markings may be technically easier to perform.

In many embodiments the pattern is detected by the pen using a detector at a non-visible wavelength, e. g. in an infra-red wavelength. The erasing covering material may be transparent, or substantially so, at said non- visible wavelength (so that the pen can still see the pattern). The pen's marking ink, or other marking medium, is preferably transparent at said non-visible wavelength.

Of course, the pen may not be of the kind that looks for an optically detected pattern, having the pen's ink be transparent at a wavelength may not matter. For example, the pen may determine its position in some other way, for example by using a known starting point, or way marker, on the page/document and using internal gyroscopes and/or accelerometers to determine its position; or the pen may triangulate its position using triangulation-signals emitted by emitters.

The pen may have an eraser projection and retraction mechanism adapted to project the eraser from a body of the pen to a position of use, and adapted to enable the eraser to be retracted back to a position of non-use at which it does not project from the body so far. The eraser may be completely or substantially completely retracted into the body when in its position of non- use. Retraction of the eraser may protect the eraser within the body of the pen and keep it safe from damage. Also, it may hinder evaporation of an erasing/covering fluid. A finger-operated eraser projection mechanism may be provided, which may comprise a push-button, or a lever, or slide, or dial, or knob: some control member which can be moved by a user to a position at which the eraser is moved to a position of use. The same, or a different, control mechanism may be used to move the eraser to a position of non-use.

The writing stylus may remain in a fixed position relative to a body of the pen whilst the eraser is projected/retracted, or it also may move.

A fixed position writing stylus may be less complex to engineer, but one that is retracted when the eraser is projected may avoid a user writing with the writing stylus with the eraser in its operative position of use which could confuse the processor that operates on writing/erasing signals.

The writing stylus may be retracted to a position of non-use when the eraser is projected to a position of use, and projected to a position of use when the eraser is retracted to a position of non-use. A mechanical linkage may be provided to cause the writing stylus and the eraser to move in antiphase.

The eraser and writing stylus may be provided at the same, forward, end region of the pen. This, in a pen with an image capture camera, allows the same camera to capture both writing and erasing operation data.

The eraser may be provided at the rear end region of the pen. An additional erasing operation data capture camera may be required if the eraser's movements are to be captured optically.

The eraser may have an exposed configuration and a covered configuration, possibly by being advanced and retracted relative to a body of the pen, or possibly in some other way, for example by the pen having a cap or covering that is moved or removed to reveal the eraser. The eraser may be permanently in an exposed, position of use, configuration.

The pen may have a single control member which is moved by the user to put alternately the writing stylus and the eraser into respective positions of use. Using a single control member may avoid a user simultaneously trying to advance both the writing stylus and the eraser to a position of use, which could conceivably damage components of a pen, depending upon its construction.

The writing stylus and eraser may be adapted to exchange positions at a position of use at the forward end of the pen. This may facilitate a single camera observing both of them in use.

The pen preferably has a clock and uses the clock to determine which was the later activity on a particular region of the paper: writing or erasing.

The pen may also have a way of determining which of the eraser and writing stylus is in use when a user applies the pen to the paper and moves

it/marks the paper (marks in the sense of either with pen ink or writing material or eraser material).

Some other way of determining which was the latter action: erasing or writing, may be provided. For example, if pen-acquired data is held sequentially, or its sequence can be determined, a clock as such may not be necessary: it will still be possible to determine what happened last from the sequence.

The pen may have a pressure sensor to determine when the stylus is writing. The pen may have a pressure sensor to determine when the eraser is erasing. They may be the same or different pressure sensors. This will enable a distinction to be made between holding the eraser over a marking and actually erasing a marking.

The pen may have a sensor to determine which of the writing stylus or the eraser is in its position of use, and/or which is in use.

The pen may have a sensor to determine that there is erasing material in an eraser unit, or eraser reservoir, available for dispensing by the eraser.

The eraser may be arranged to erase physically on the page a line of a predetermined width, a first width. The pen, or a processor that receives data from the pen, may be adapted to erase from a digital version of the page a line of a particular width, a second width.

The first and second widths may be the same or substantially so, or they may be set to different widths. The second width may be wider than the first width, for example at least 10% wider, or at least 50%, or at least 100% wider, for example from 10% to 100% wider on each side of the line.

More likely is for the second width to be about 50% wider than the first width on each side of the first width (100% wider altogether). A user may be able to select the second line width to be used, possibly from a plurality of available options.

Thus, a user may not have the problem of having to make repeated, closely spaced, traverses of a document with the eraser to erase a line digitally: the eraser may be a little wider in the digital world than in the physical world.

The digital erasing of markings may be handled by a processor in the pen, the pen being adapted to convey pen-acquired marking data to an external computer after an erasing function has been performed on the digital pen- acquired data by the pen's processor. This is advantageous since it does not require any changes to existing software in the external computer to erase written markings. Some changes to existing software to allow the erasing of printed markings may be necessary.

Alternatively, the pen may transmit to an external computer data relating to writing and erasing operations, with the external computer processing the pen-acquired data with a program application to compute a resultant net digital record of the pen's activity. This can be advantageous because the pen's software requires little modification (possibly the setting of an "erase"or"write"flag), and there is in the external computer a record of what the user had previously written (if it is ever desirable to know that).

So far, we have concentrated on a single pen having both write and erase capabilities. We may provide a separate eraser, possibly as a separate "pen" (elongate pen-sized, pen-shaped, eraser that writes in erasing liquid or removes material). We seek protection for a kit having a write digital pen and an erase digital eraser, possibly provided in the form of a pen.

However, a combined write and erase digital pen is seen as better at present.

A further functional feature of many embodiments of our pen, not provided to users with existing systems, is that not only do we envisage erasing handwritten markings on the document/paper, we also (instead of, or in addition to that) envisage erasing printed markings.

As an example, a user may have a printed document and may erase a printed word, and possibly replace it with a handwritten word. A user may make editorial changes to a pre-existing document using the eraser.

Drawings may be edited, possibly using the combined physical (paper) and digital (computer record) eraser.

When an electronic/digital version of a document is known, it is possible to apply (e. g. superimpose) digital eraser markings onto the electronic version in order to change the electronic version of the document, as long as the correct digital eraser data can be synchronised with, spatially, the correct document electronic record.

The pen may therefore be adapted to simultaneously physically erase printed content on a document and create a digital record of the erasing of that content.

Thus a user may print out a digital document, for example a draft letter, edit it using a digital pen and eraser, and have a digital version of the edited document created automatically. The printed document may have printed on it a key for determining the identity of the electronic record that was printed. That key may be the allocation of a specified pattern. For

example, there may be a pattern-document allocation table created by a computer associated with the printing.

Depending upon where the digital version of the printed content of the document is held, it may, or may not, be possible for the pen to create a digital version of the amended document having the amendments/erasures to the printed content included in the digital version. If the digital version of the printed content is stored off-pen, then the pen may not have access to it. A digital record of the printed content of the document may be transferable to the pen, or it may not, depending upon the capabilities of the pen.

According to another aspect the invention comprises using a digital eraser device on a printed document having markings on it to erase markings both physically on the document and also to cause equivalent erasing in a digital version of the document.

The erasing in the digital document may be a facsimile (in the sense of a faithful or nearly-faithful copy) of the erasing on the printed document.

The user may write over an erased region on the printed document, such writing being incorporated into the digital version of the document. By "writing"is meant is either of, or both of, the writing of words and the writing of lines, markings, drawings, etc.

The method may comprise using a single pen to write handwritten markings on the printed document and to erase markings, and to acquire both handwritten marking and erasure data.

The method may comprise using a different pen or eraser to erase from the pen that is used to write.

The method may comprise erasing, both physically and digitally, handwritten markings on the document. The method may comprise erasing, both physically and digitally, printed markings on the document.

According to another aspect the invention comprises a digital pen and paper system comprising: (i) a digital pen; (ii) an-off-pen computer; the pen being preferably adapted for use with digital paper having a pattern printed on it, and the pen being adapted to enable the pen to determine its position on the paper, and the pen having a writing nib and an erasing nib or other eraser, and being adapted to write on the paper using its writing nib and erase written markings on the paper and/or printed markings on the paper using its eraser, and being adapted to determine its position and movements relative to the pattern on the paper, and the pen being adapted to acquire data relating to pen writing and pen erasing activity digitally and to communicate data representative of, or derived from, such activity to said off-pen computer.

The system may also include said digital paper.

The pen may have a processor adapted to process data representative of said pen writing and pen erasing activity and to communicate to said off- pen processor a record of said pen writing activity and a record of said pen erasing activity. Alternatively, the processor in the pen may be adapted to operate a pen-writing and an pen-erasing activity data to create

consolidated document-altering activity and be adapted to transmit to said off-pen processor data representative of said consolidated activity.

The off-pen processor, or the pen processor, may be provided with access to a digital record of printed content of a document printed on the paper and may be adapted to use the digital record of the printed content and the pen-acquired pen-writing and pen-erasing activity to create a modified digital record of the document incorporating the changes made to the printed document using the pen.

Handwritten markings may be incorporated into the digital version of the printed document as facsimile copies of the physical handwritten markings, or at least some handwritten markings may be interpreted by character recognition software and reproduced in the digital version as typeset characters.

According to another aspect of the present invention there is provided an eraser device and erasing software, said software being stored upon a data carrier and suitable for execution upon a processor remote from the eraser device: the device comprising an erasing stylus arranged to apply a substance to a document, or adapted to remove material from the surface of a document, position sensing means arranged to enable the position of the device relative to positions within the document to be determined, an eraser activity sensor arranged to indicate when the eraser device is in contact with the document, data acquisition means arranged to obtain data from the position sensing means and/or from the activity sensor relating to areas of the document that have been erased by the device, and a transmitter arranged to communicate device activity data to the remote processor; and

the erasing software being arranged to run on said remote processor and when running on said remote processor to use said device activity data in combination with a digital version of said document to cause a modified digital document to be displayable, said modified document incorporating a digital version of erasing activity performed by the eraser device upon the physical document.

Preferably the software uses writing data from the eraser device, or any other writing device, in combination with the device activity data to produce said modified document.

We may provide no eraser device which erases but does not write markings in ink onto a document. We may provide a kit having a separate writing instrument and eraser device.

According to another aspect of the present invention, there is provided a computer program product capable of being run on a computer processor, the computer program product when running upon a computer processor being adapted to use inputs representative of the position of an eraser device on the surface of a document to cause the electronic erasing of electronic markings in an electronic version of the document to create an erase-amended electronic version of the document.

According to a further aspect of the present invention there is provided a digital pen adapted to acquire information from a position-determining pattern printed on digital paper to enable the position of the pen to be determined, the pen having a writing stylus adapted to write handwritten markings onto the paper and being adapted to capture those handwritten markings digitally, and the pen also having an eraser adapted to erase markings on an area of the paper and being adapted to capture the position

of the erased area digitally and wherein the eraser is adapted to apply an erasing material to the paper, adapted to erase said markings on said paper.

The erasing material may comprise an erasing covering material adapted to cover said markings. The erasing material may comprise a marking- changing material adapted to change optical properties of said markings so as to make them less visible to human users. The erasing material may comprise a liquid adapted to be dispensed onto said paper by said eraser.

The pen may comprise a marking ink, or other marking medium, that is transparent at said non-visible wavelength. The pen may be adapted to contain erasing covering material which is transparent, or substantially so, at a non-visible wavelength.

The stylus may be arranged make an indelible mark on the digital paper.

The erasing material may be arranged to alter the optical properties of the mark or marking such that the mark or marking is made substantially transparent at operating wavelengths of the sensor. The erasing may be arranged to alter the optical properties of the mark or marking such that the mark or marking is substantially transparent to a user of the pen.

According to a still further aspect of the present invention there is provided a digital pen adapted to acquire information from a position-determining pattern printed on digital paper to enable the position of the pen to be determined, the pen having a writing stylus adapted to write handwritten markings onto the paper and being adapted to capture those handwritten markings digitally, and the pen also having an eraser adapted to erase markings on an area of the paper and being adapted to capture the position of the erased area digitally, wherein a processor in the pen is arranged to handle digital erasing of markings, the pen being adapted to convey pen-

acquired marking data to an external processor after an erasing function has been performed on the digital pen-acquired data by the pen's processor; and wherein the eraser is adapted to erase physically a line on the paper of a first width, and wherein said pen's processor is configured to erase a digital line of a significantly different width.

According to another aspect of the present invention there is provided a digital pen adapted to acquire information from a position-determining pattern printed on digital paper to enable the position of the pen to be determined, the pen having a writing stylus adapted to write handwritten markings onto the paper and being adapted to capture those handwritten markings digitally, and the pen also having an eraser adapted to erase markings on an area of the paper and being adapted to capture the position of the erased area digitally, the pen being adapted to convey pen-acquired marking and erasing data to an external processor, said external processor being arranged to perform an erasing function on the digital pen-acquired data; and wherein the eraser is adapted to erase physically a line on the paper of a first width, and wherein said external processor is configured to erase a digital line of a significantly different width.

According to a still further aspect of the present invention there is provided method of changing a digital version of a printed document comprising using a digital eraser device on a printed document having markings on it to erase said markings physically on the document and also to cause equivalent erasing in a digital version of the document, wherein said digital erased line is significantly wider than said physical erased line.

According to another aspect of the present invention there is provided a digital pen having an elongate pen housing containing a first end a mounting for a nib adapted to write handwritten markings onto paper and

being adapted to capture those handwritten markings digitally, and having said nib projecting, generally longitudinally of said pen and at one end an eraser mounting having an eraser extending longitudinally of said pen, adapted to erase markings on an area of the paper and being adapted to capture the position of the erased area digitally, said nib and eraser being within a common envelope defined by said housing so to provide both said nib and said eraser extending generally along axes passing through said housing, wherein the eraser is adapted to apply an erasing material to the paper adapted to erase said marking on said paper.

According to a further aspect of the present invention there is provided a digital eraser device adapted to simultaneously physically erase written or printed content on a document and to create a digital record of the erasing of that content by applying an erasing liquid to cover content on said document.

According to a yet further aspect of the present invention there is provided a method of changing a digital version of a printed document comprising using a digital eraser device on a printed document having markings on it to erase said markings physically on the document and also to cause equivalent erasing in a digital version of the document by applying an erasing liquid to cover content on said document.

The method may comprise causing the erasing in the digital document to be a facsimile of the erasing on the printed document. The method may comprise erasing, both physically and digitally, printed markings on the document.

According to still another aspect of the present invention there is provided a processor of a digital pen arranged to receive marking data indicative of strokes of a stylus of the pen from a sensor of the pen, and also arranged to receive erasure data indicative of strokes of a liquid dispensing eraser of the pen from a sensor of the pen, the processor being arranged to combine the marking data and the erasure data to produce a composite data set indicative of areas of a document having writing upon them following erasure of at least one mark associated with a fraction of the marking data, the processor being arranged to initiate the transfer of the composite data set to an off pen processor.

According to a yet further aspect of the present invention there is provided a processor of a digital pen arranged to receive marking data indicative of strokes of a stylus of the pen from a sensor of the pen, and also arranged to receive erasure data indicative of strokes of a liquid dispensing eraser of the pen, the processor further being arranged to initiate the transfer of the marking data and the erasure data to an off pen processor.

According to another aspect of the present invention there is provided a method of changing a digital version of a printed document comprising using a digital eraser device on a printed document having pre-printed markings on it to erase said pre-printed markings both physically on the document and also to cause equivalent erasing in a digital version of the document by applying an erasing liquid to cover said pre-printed markings on said document.

According to another aspect of the present invention there is provided a digital pen adapted to acquire information from a position-determining means printed on digital paper to enable the position of the pen to be determined, the pen having a marking means for writing handwritten

markings onto the paper and the pen having capture means for capturing those handwritten markings digitally, and the pen also having erasing means for erasing markings on an area of the paper by depositing an erasing liquid on the paper, and said capture means being adapted to capture the position of the erased area digitally.

According to another aspect of the present invention there is provided a digital pen adapted to acquire information from a position-determining pattern comprising infra-red absorbing dots printed on digital paper to enable the position of the pen to be determined by means of an infra-red LED and a infra-red sensitive camera, the pen having a writing stylus adapted to write handwritten markings onto the paper and the camera being adapted to capture those handwritten markings digitally, and the pen also having an eraser adapted to erase markings on an area of the paper by depositing an erasing liquid on the paper, and the camera being adapted to capture the position of the erased area digitally.

According to another aspect of the present invention there is provided a digital pen and paper system comprising: (i) marking means associated with the pen for producing marks upon said paper; (ii) processing means remote from the digital pen; the pen being adapted for use with a document having a position determining means printed thereupon and being adapted to determine its position on the document by a sensing means arranged to sense said position determining means, and the pen having erasing means, and being adapted to write on the document using the marking means and erase written markings on the document and/or printed markings, if provided, on the document using the erasing means by depositing an erasing liquid on the paper, and being adapted to determine the position and movements of

the pen relative to the document, and the pen being adapted to acquire data relating to pen writing and pen erasing activity digitally and to communicate data representative of, or derived from, such activity to said processing means.

According to another aspect of the present invention there is provided a digital pen and paper system comprising: (i) a digital pen; (ii) an off-pen digital processor; the pen being adapted for use with a document and being adapted to determine its position on the document by means of an infra-red absorbing position determining pattern printed on the paper, and an infra-red LED and camera arranged to sense said position determining pattern and to communicate data relating to said pattern to the processor, and the pen having a writing nib and an erasing nib, and being adapted to write on the document using the writing nib and erase written markings on the document and/or printed markings, if provided, on the document using the erasing nib by depositing an erasing liquid on the paper, and being adapted to determine its position and movements relative to the document, and the pen being adapted to acquire data relating to pen writing and pen erasing activity digitally and to communicate data representative of, or derived from, such activity to said off-pen processor.

According to another aspect there is provided a computer program product capable of being run on a computer processor, the computer program product when running upon a computer processor being adapted to use inputs representative of the position of an eraser device on the surface of a document and the application of an erasing material to the document to cause the electronic erasing of electronic markings in an electronic version

of the document to create an erase-amended electronic version of the document.

The computer program product may be adapted when running on a computer processor to cause the processor to combine signals indicative of the use of a writing device and signals indicative of the use of the erasing device to produce the erase amended version of the document. The computer program product may be adapted when running on a computer processor to use an input representative of a pressure sensor reading and a switch position to determine whether the erasing device or a writing device is being used. The computer program product may be adapted when running on. a computer processor to initiate the transfer of the erase-amended electronic version of the document to a remote processor. The computer program product may be adapted when running on a computer processor to apply a nominal width to the erasing device when creating the erase- amended electronic version of the document.

According to another aspect of the present invention there is provided a digital pen adapted to acquire information from a position-determining pattern printed on digital paper to enable the position of the pen to be determined, the pen having a writing stylus adapted to write handwritten markings onto the paper and being adapted to capture those handwritten markings digitally, and the pen also having an eraser adapted to erase markings on an area of the paper and being adapted to capture the position of the erased area digitally by means of an infra-red LED and infra-red sensor, and wherein the eraser is adapted to apply an erasing fluid to the paper adapted to erase said marking on said paper, the erasing fluid being substantially opaque to a user and substantially transparent at infra-red frequencies such that infra-red light emitted by the LED and reflected by the pattern can be received by the sensor through the erasing fluid.

It will be appreciated that any feature of any one of the preceding aspects of the present invention can be combined with any feature of another one of preceding aspects of the present invention within the scope and spirit of the invention.

PCT Application Number WO 01/16872 discloses a digital eraser for use with a wipeable, reusable board.

BRIEF DESCRIPTION OF THE DRAWINGS At least one embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, of which: Figure 1 shows a document suitable for use in a digital pen and paper system; Figure 2 shows in detail part of the document of Figure 1; Figure 3 shows a known digital pen suitable for use in a digital pen and paper system; Figure 4 shows a first embodiment of a digital pen according to at least an aspect of the present invention; Figure 5 shows a second embodiment of a digital pen according to at least an aspect of the present invention; Figure 6 shows a third embodiment of a digital pen according to at least an aspect of the present invention;

Figure 7 shows a fourth embodiment of a digital pen according to at least an aspect of the present invention; Figure 8 shows a fifth embodiment of a digital pen according to at least an aspect of the present invention; and Figure 9 shows a sixth embodiment of a digital pen according to at least an aspect of the present invention.

DETAILED DESCRIPTION OF SOME EMBODIMENTS OF THE INVENTION Referring to Figure 1 a document 100 for use in a digital pen and paper system comprises a carrier 102 in the form of a single sheet of paper 104 with position identifying markings 106 printed on some parts of it to form areas 107 of a position identifying pattern 108. Also printed on the paper 104 are further markings 109 which are clearly visible to a human user of the form, and which make up the content of the document 100. The content 109 will obviously depend entirely on the intended use of the document. In this case an example of a very simple two page questionnaire is shown, and the content includes a number of boxes 110,112 which can be pre-printed with user specific information such as the user's name 114 and a document identification number 116. The content further comprises a number of check boxes 118 any one of which is to be marked by the user, and two larger boxes 120,121 in which the user can write comments. The form content also includes a send box 122 to be checked by the user when he has completed the questionnaire to initiate a document completion process by which pen stroke data is forwarded for processing, and typographical information on the form such as the headings or labels 124 for the various boxes 110,112, 118,120. The position identifying pattern 108 is only printed onto the parts of the form which the user is expected to write on or

mark, that is within the check boxes 118, the comments boxes 120,121 and the send box 122.

Referring to Figure 2, the position identifying pattern 108 is made up of a number of dots 130 arranged on an imaginary grid 132. The grid 132 can be considered as being made up of horizontal and vertical lines 134,136 defining a number of intersections 140 where they cross. The intersections 140 are of the order of 0.3mm apart. One dot 130 is provided at each intersection 140, but offset slightly in one of four possible directions up, down, left or right, from the actual intersection 140. The dot offsets, which are typically about 50? m from the notional centre of the crossing grid "lines", are arranged to vary in a systematic way so that any group of a sufficient number of dots 130, for example any group of 36 dots arranged in a six by six square, will be unique within a very large area of the pattern.

This large area is defined as a total imaginary pattern space, and only a small part of the pattern space is taken up by the pattern on the document 100. By allocating a known area of the pattern space to the document 100, for example by means of a co-ordinate reference, the document and any position on the patterned parts of it can be identified from the pattern printed on it. An example of this type of pattern is described in WO 01/26033. It will be appreciated that other position identifying patterns can equally be used. Some examples of other suitable patterns are described in WO 00/73983 and WO 01/71643.

Referring to Figure 3, a pen 300 comprises a writing stylus 310, and a camera 312 made up of an infra red (IR) LED 314 and an IR sensor 316.

The camera 312 is arranged to image an area adjacent to the tip 311 of the pen stylus 310. A processor 318 processes images from the camera 312. A pressure sensor 320 detects when the stylus 310 is in contact with the document 100 and triggers operation of the camera 312. Whenever the pen

is being used on a patterned area of the document 100, the processor 318 can therefore determine from the pattern 108 the position of the stylus of the pen whenever it is in contact with the document 100. From this it can determine the position and shape of any marks made on the patterned areas of the document 100. This information is stored in a memory 322 in the pen as it is being used. When the user has finished marking the document, in this case when the questionnaire is completed, this is recorded in a document completion process, for example by making a mark with the pen in the send box 122. The pen is arranged to recognise the pattern in the send box 122 and determine from that pattern the identity of the document 100. The information stored regarding the location of a user's pen strokes in the memory 320 is transferred to a suitable processor upon the user placing the pen 302 in the send box 122.

The above described system is known.

In a variant the document may comprise simply a sheet of paper with a position determining pattern printed on it (no printed human discernable meaningful content is visible), possibly with the whole area of the sheet of paper covered with pattern. Another alternative is a sheet of paper with pattern but not human discernable content apart from a"send"box or "work complete"box; it may have one or more instruction areas for the pen to read but not extra text associated with pen instructions Referring now to Figure 4, a pen 400 comprises an elongate housing 401, a writing stylus 402, typically an ink nib or a pencil nib, an eraser 404 and a camera 405 made up of an infra red (IR) LED 406 and an IR sensor 408.

The stylus 402 and the eraser 404 are located within the housing 401 and project from the housing 401 when in use. The stylus 402 and the eraser 404 lie substantially parallel to the longitudinal axis of the housing 401.

The location of the pen 400 upon the document 100 is determined from sensing of the pattern 108 by the camera 405, and the writing stylus 402 writes upon the document in the manner described in relation to Figures 1 to 3.

In some preferred embodiments, the stylus 402 is arranged to make an indelible mark upon a sheet of paper, or other medium, typically by ink soaking into pores of the paper. It will be understood that by an indelible mark it is meant that the mark cannot be wiped off or rubbed off the page.

For example where the mark is made by applying a marking material to the paper, that material cannot be removed. This is in contrast to the case of a whiteboard marker used to mark a whiteboard, where the mark is readily rubbed out using a whiteboard rubber or a piece of cloth.

Both the writing stylus 402 and the eraser 404 are operable to project at an end of the pen 400 from which the IR LED 406 illuminates the document 100. The writing stylus 402 is retractable and biasable forwards by means of a spring loaded catch mechanism 410 mounted within the body pen 400.

The eraser 404 is also retractable and biasable forwards by a second spring loaded catch mechanism 412, such that either of the writing stylus 402 or the eraser 404 can project from beyond a forward end of a body of the pen 400 and can be in contact with the document 100 at any one time. The retraction of either the stylus 402 or the eraser 404 results in a flag being set in a processor 413 within the pen 400 such that the status of stylus 402 and the eraser 404 are typically recorded in a memory 414. A sensor, or sensors (not shown) are provided to determine whether the stylus or the eraser is projecting from the pen 400. The recording of the status of the stylus 402 and the eraser 404 takes place either in a sequential temporal time frame by the use of a clock within the processor to produce a history of pen strokes, or by determining the sum effect of the pen strokes upon the

document at each point within the document (e. g. by each pixel in the electronic version of the document having a"written or", or not, value, and the pen altering digital pixel values as it physically marks the page-the eraser function re-setting a pixel value to"not written on").

The eraser 404 comprises an erasing fluid reservoir 415 connected to a feed tube 416, and a valve member 418 arranged to selectively open and close an open end of the feed tube 416 in response to the application of pressure to the valve member 418. The pressure is applied to the valve member 418 when the eraser is in contact with the document 100. The reservoir 415 is adapted to hold an erasing fluid such as a white liquid which dries and sets on the document, for example a liquid such as TippexTM or SnowpaqueTM, or something conceptually similar. Usually, the erasing fluid is substantially opaque to a user of the pen 400 when dry. The erasing fluid may, in some embodiments, be at least partially transparent at the operating wavelengths of the LED 406 and sensor 408. The erasing fluid may be either solvent based, or water based, and may be arranged to dry in a few seconds.

In the case of the stylus 402 ink indelibly marking a sheet of paper the erasing fluid may react with the ink. This may cause the ink to, for example, change colour or become transparent to a user of the pen, i. e. the erasing fluid may change the optical properties of the ink rather than overlay an ink mark on a sheet of paper.

The reservoir 415 can take the form of a replaceable, exchangeable plug in cartridge that is arranged to plug into the front of the pen 400 and there may be an optional holder for a spare cartridge in the rear of the pen 400.

A sensor (not shown) may be adapted to check that there is still usable

erasing liquid in the eraser 404-so that it will dispense erasing liquid when pressed against a document/page.

A pressure sensor 420 determines if either the stylus 402 or the eraser 404 are in contact with the document 100. In an alternative embodiment each of the stylus 402 and the eraser 404 have respective pressure sensors and these pressure sensors can be used to determine which of the stylus 402 and the eraser 404 is in contact with the document. The valve member 418 may form part of the pressure sensor associated with the eraser 404 in this instance.

The location of the pen 400 is determined from the pattern 108. A marking, e. g. an earlier marking pen stroke or printed marking upon the document 100, can be overlain by erasing liquid stored within the reservoir 415 when the eraser 404 is in contact with the document 100. The erasing liquid is typically opaque at visible wavelengths but transparent at the non-visible, IR, wavelengths sensed by the sensor 408. Alternatively, the fluid may alter the composition of ink deposited upon the document 100 such that it becomes transparent at visible wavelengths.

It will be usual for the processor 413 to record an erasing line as being approximately 150% the width of the actual physical erased line on the document. In another embodiment the digital width of an erased line is extended 50% to either side of the physical erased line (100% wider in total). Typically, the width of the erasing line or stroke recorded by the processor 413 can be varied as a parameter of the digital pen 400, typically entered at the pen 400 by means of a dial, slider or other manual switch, or at a terminal that is linked to the pen 400, or by software in a pen marking software application (e. g. via a GUI).

A transmitter 422, typically an IR transmitter employing a data transmission protocol such as for example Bluetooth, IEEE 802.11 or HiPerLAN, within the pen 400 transmits positional and status data to a remote processor 424, typically a personal computer (PC) which will usually be running Microsoft Windows@. The status data relates to which of the stylus 402 or the eraser 404 is in operation at any point in time. The transmitter 422 may be arranged to transmit the data in real time directly from the pen or, alternatively, it may be arranged to transmit stored data from the memory 414 intermittently, in bursts, or when triggered by an event, for example when a user of the pen 400 moves the pen 400 into the send box 122, or when a user docks the pen 400 with the remote processor 424 to allow communication between them.

Referring now to Figure 5, in an alternative embodiment of the present invention a pen 500 is substantially similar to that described hereinbefore with reference to Figure 4 and similar parts will be accorded the similar reference numerals in the five hundred series. The pen 500 has a stylus 502 that is fixed and a retractable eraser 504. The eraser 504 operates in the same manner as that described in relation to Figure 4 with a spring loaded catch mechanism 512 causing a flag to be switched within a processor 513 upon the projection or retraction of the eraser 504 and a pressure sensor 520 determining whether either the stylus 502 or the eraser 504 are in contact with the document 100. Activation by hand of the same manual control may cause both the physical advancement/retraction of the eraser and/or writing stylus and the setting of the electronic flag: movement of the same component triggers both in this embodiment.

Referring now to Figure 6, a pen 600 operates a comprises a writing stylus 602, typically an ink nib or a pencil nib, an eraser 604 and a camera 605 made up of an infra red (IR) LED 606 and an IR sensor 608. The location

of the pen 600 upon the document 100 is determined from sensing of the pattern 108 by the camera 605, and the. writing stylus 602 writes upon the document in the manner described in relation to Figures 1 to 3.

The retraction and projection of the stylus 602 and the eraser 604 are achieved, and recorded in the same manner as described with reference to Figure 4, as is transmission of data to a remote processor.

In this embodiment the eraser 604 is a device that physically removes markings, for example ink or pencil, from the document 100 and hence there is no need for a correction fluid reservoir, feed tube or valve member.

The eraser could be a"rubber"that is worn away, along with the surface of the document, by a rubbing action.

Referring now to Figure 7, a pen 700 comprises a writing stylus 702, a camera 704 made up of an infra red (IR) LED 706 and an IR sensor 708 at a first end thereof, and an eraser 710, a camera 712 made up of a second IR LED 714 and an IR sensor 716 at a second end thereof. The location of the pen 700 upon the document 700 is determined from the sensing of the pattern 108 by either of the respective cameras 704,712. The writing stylus 702 writes upon the document in the manner described in relation to Figures 1 to 3 and the eraser 710 operates either as a correction fluid depositing eraser as described with reference to Figures 4 and 5 or as a physical eraser as described with reference to Figure 6, it is shown here as a physical eraser for clarity.

In this embodiment a processor 717 receives signals from the cameras 704, 712 and determines whether the stylus 702 or the eraser 710 is in use by a signal received from pressure sensors 718,720, and also which of the cameras 704,712 is observing the pattern 108.

Also, the stylus 702 and the eraser 710 are protected when not in use by respective end caps 721,722 (shown in broken lines in Figure 7).

Referring now to Figure 8, a digital eraser device 800 comprises an eraser 802, a camera 804 made up of an infra red (IR) LED 806 and an IR sensor 808. The eraser 802 can be a correction fluid type eraser or it may be a physical eraser, it is shown here as a physical eraser for clarity.

Additionally, the eraser 802 may be retractable, in which case the eraser device 800 operates in substantially the same manner as the eraser of the pen described hereinbefore with reference to Figures 4 and 6, or the eraser 802 may project from the device 800 at all times, as shown here, in which case the device 800 operates in substantially the same manner as the eraser of the pen described hereinbefore with reference to Figure 5. The eraser device 800 does not have a pen nib or writing stylus, it can only erase, not write onto a document.

Referring now to Figure 9, a pen 900 comprises interchangeable writing stylus 902 and eraser 904 attachments. The eraser attachment 904 typically is a correction liquid containing cartridge or a physical eraser. The stylus 902 is located in an end of the pen 900 such that it abuts the pressure sensor 920 and thus the pen 900 is in a configuration suitable for writing upon the document 100. The eraser 904 is stowed in the body of the pen 900 when the pen 900 is configured for writing.

When the pen 900 is to be configured for erasing the stylus 902 is removed and the erasure 904 is located in the end of the pen 900. The pressure sensor 920, or another sensor, is configured such that it recognises which of the stylus 902 or eraser 904 is located in the end of the pen 900. Typical

recognition devices include selective projection-slot location arrangements, bar code readers or r. f. tags.

In one envisaged use, digitally recorded documents are printed out on demand by a user using a local printer (e. g. ink jet or laser printer, or LEP or other printer) and hand-edited using a pen with integral eraser (or both a separate pen and separate eraser device) and the writing and erasing activity (including any erasing of printed material such as words/text) is transmitted to a processor which modifies the digitally recorded document using input from the writing/erasing operations on the physical version of the document.

It will be appreciated that whilst our currently preferred embodiment of the invention uses a camera to capture position-determining markings printed on a sheet, typically paper, other embodiments may have their pen and/or eraser, or combined pen and eraser, determine their positions in some other way, for example using gyroscopes, accelerometers, and dead-reckoning from a start point on the document.

It will also be appreciated in a currently favoured embodiment we detect pattern at an infra-red wavelength (non-visible wavelength), but we could detect pattern at other non-visible wavelengths, or indeed at a visible wavelength. If pattern is detected at a visible wavelength the content would have to avoid that wavelength, at least in some embodiments.