Image authentication

Abstract

A system for managing the application of artwork to a substrate, the system comprising one or more processors configured to: receive an indication of a graphic; generate a set of candidate locations for markers visually differentiable from the graphic; transmit the candidate locations to a graphic applicator; receive from the applicator an image of a target substrate region having visually differentiable markers applied thereto; and compare the locations of the markers in the image with the candidate locations.

Description

[0001] IMAGE AUTHENTICATION

[0002] FIELD

[0003] The present description relates to authenticating images, for example manually drawn images.

[0004] BACKGROUND

[0005] It is possible to buy art images in the form of paintings or drawings on portable media such as paper or canvas. Art in this form can be made by a prominent artist and then transported to a buyer. Some forms of art are not transportable in this way. For example, tattoos and murals must be made on a specific substrate, so it is not feasible to have them made by a remote artist.

[0006] It is desirable to be able to authenticate images, for example so that it can be reliably determined that they have derived from a specific artist. Often an artist will sign a work, but authenticating a signature requires expert analysis.

[0007] Machine-readable codes such as barcodes are known. However, in general they are unsuitable for authenticating art, especially manually drawn art, because they are either so simple as to be easily reproduced or, in the case of typical two-dimensional barcodes, so complex that it is difficult for an artist to manually reproduce the barcode with sufficient precision that it can be reliably read. It is often undesired to have a machine-readable identifier incorporated into an artwork, as identifiers such as QR codes may detract from the visual style of the artwork. Equally, if a machine-readable identifier is removably applied to an artwork, for example an adhesive barcode applied to the back of a canvas, the identifier may be removed, altered, forged or otherwise be unfit for purpose.

[0008] There is a need for an improved system for authenticating images, including manually drawn images. SUMMARY OF THE INVENTION

[0009] According to one aspect there is provided A system for managing the application of artwork to a substrate, the system comprising one or more processors configured to: receive an indication of a graphic; generate a set of candidate locations for markers visually differentiable from the graphic; transmit the candidate locations to a graphic applicator; receive from the applicator an image of a target substrate region having visually differentiable markers applied thereto; and compare the locations of the markers in the image with the candidate locations.

[0010] The one or more processors may be configured to: if the comparison indicates conformity between the markers in the image and the candidate locations, transmit an instruction to the applicator to apply the graphic to the target substrate; and otherwise, transmit an instruction to the applicator to apply a further marker to the substrate.

[0011] The one or more processors may be configured to perform image processing on the received image of the target substrate region to identify the locations of markers in that image.

[0012] The one or more processors may be configured to store the set of locations of the markers in the image in association with data indicative of a transaction by which the graphic is authorised to be applied to the target substrate.

[0013] The one or more processors may be configured to, subsequent to the said storing step: receive from a user terminal a second image of the target substrate having the graphic and the markers thereon; process the second image to identify the locations of markers in that image; compare those identified marker locations with sets of stored marker locations to identify a match between the identified marker locations and a set of stored marker locations; and in response to identifying such a match, transmit associated data indicative of a transaction to the user terminal.

[0014] The system may store sets of marker locations for previously applied graphics, and the step of generating a set of candidate locations for markers may comprise: determining a target quantity (n) of candidate locations; and determining n candidate locations subject to the constraint that any set of n markers located such that each such marker is offset from a respective one of the candidate locations by not more than a predetermined deviation is distinct from all of the stored sets of marker locations.

[0015] The predetermined deviation may be a two-dimensional deviation. It may be a deviation in the plane or local plane of a substrate.

[0016] The marker may be circular or square.

[0017] The greatest dimension of each marker may be less than 5mm.

[0018] The graphic applicator may be a human.

[0019] The target substrate may be part of the skin of a human subject.

[0020] The system may comprises an applicator for dispensing a medium for applying the artwork and the or each processor may be configured to, if the comparison indicates conformity between the markers in the image and the candidate locations, transmit a signal to the applicator for permitting the applicator to dispense the medium.

[0021] The or each processor may be configured to, if the comparison indicates conformity between the markers in the image and the candidate locations, determine an amount of the medium in dependence on the graphic and transmit a signal to the applicator for permitting the applicator to dispense that amount of the medium.

[0022] The one or more processors may be configured to: if the comparison indicates conformity between the markers in the image and the candidate locations, transmit an to the applicator an indication of a location relative to the markers at which the graphic is to be applied to the target substrate.

[0023] According to a second aspect there is provided a method for applying an artwork to a substrate, the method comprising: receiving an image; receiving an indication of a set of marker positions; applying markers to the substrate, each marker being applied in a region of a respective one of the indicated marker positions; recording the positions of the markers as applied to the substrate; and applying the image to the substrate in the region of the markers.

[0024] According to a third aspect there is provided a tattoo applicator comprising: an inlet for receiving ink; a needle for applying ink to a substrate; a valve located in a flow path between the inlet and the needle for controlling the flow of ink between the inlet and the needle; a communication interface for receiving data over a data network; and one or more processors coupled to the interface and the valve, the processors being configured to process a message received at the communication interface to authenticate that message, and in response to the message being authenticated and the message being of a predetermined form, cause the valve to open

[0025] According to a fourth aspect there is provided a computer program and / or a data carrier storing a computer program, the program being adapted for causing one or more processors to perform a function set out above.

[0026] DESCRIPTION OF THE DRAWINGS

[0027] The present invention will be described by way of example with reference to the accompanying drawings. In the drawings:

[0028] Figure 1 is a schematic diagram of a system for the provision and authentication of images.

[0029] Figure 2 shows an example of an image including authentication markers.

[0030] Figure 3a shows an example of windows and intended marker locations.

[0031] Figure 3b shows an example of applied markers.

[0032] DETAILED DESCRIPTION The following description is presented to enable any person skilled in the art to make and use the invention and is provided in the context of a particular application. Various modifications to the disclosed embodiments will be apparent to those skilled in the art. The general principles defined herein may be applied to other embodiments and applications without departing from the present invention. Embodiments are described by way of example only.

[0033] In one example of the system to be described below, a user can browse an online marketplace of images and select an image for purchase. Then the user can engage an artist to manually reproduce that image on a specific substrate. Before or as the artist is reproducing the image, the system guides the artist to incorporate markers in the image. The markers may be applied before other components of the image are applied. Alternatively, some or all of the image may be applied before the markers are applied. The system records the disposition of the markers. Subsequently, the markers can be used to authenticate the image, for example as having properly been derived from the original creator of the image.

[0034] The original image could be drawn by a human artist or generated by another entity such as a suitably programmed computer. The image could be acquired through the online marketplace or in other ways, for example in a physical shop or store. The image could be applied manually, which includes by a person physically manipulating a mechanical applicator such as a tattoo applicator, or by a machine such as a robotic arm which moves an applicator under program control.

[0035] Figure 1 shows a distributed system including multiple terminals 1 to 5 connected via a network 6 to a server 7. The terminals could, for example, be cellular phones, tablets, personal computers. The server could be implemented by one or more computer devices which could be co-located or distributed across multiple locations. Each terminal has a processor 8 and a memory 9 storing in non-transient form instructions executable by the processor to cause the processor to implement the functions of the terminal as described below. Each computer device of the server has a processor 10 and a memory 11 storing in non-transient form instructions executable by the processor to cause the processor to implement the functions of the server as described below. The memory 11 of the server also stores a database for holding information on available art, applied markers, artist details and so on. Each terminal may comprise a display 12 for displaying information including images and a camera 13 for capturing images. The terminals and the server may communicate with the network using any suitable data link: for example wired or wireless data links. The network may be, at least in part, a publicly accessibly network such as the internet.

[0036] The operation of the system of figure 1 will now be described.

[0037] First, artists create images, also known as graphics, either electronically on an artist’s terminal 1 or using physical media. Images made using physical media can be scanned or photographed using a camera, which may be integrated into the artist’s terminal. An artist then uploads a created image to the server 7. The server 7 stores the image, together with details of the artist such as their name, location and biography. The artist may designate a price for the image which is also stored by the server. The server may store information about the image, such as its style, size and subject matter. Many images can be collected in this way at the server.

[0038] An end user can connect to the server using an end user’s terminal 2. On the end user’s terminal the end user can browse images stored on the server, together with the details stored for each image. On finding a suitable image the end user may decide to buy the image, or a licence to it, for application to a suitable substrate by a reproducer. As will be discussed below, the substrate could be a substrate to which the end user has access. The reproducer could be a person or a machine. The reproducer may be the same person as the end user. The reproducer may also be termed an applicator. A corresponding financial transaction is undertaken between the end user and an operator of the server. In that transaction the end user pays the operator of the server for access to the image. Upon receipt of payment, the operator may vend the image in a suitable format, such as a digital file for download by the end user or by the reproducer / applicator. Then the operator of the server may pay the artist who created the image. Alternatively, the transaction might not involve any payment at all.

[0039] Once the end user has bought or licensed the image, they can engage a reproducer to reproduce the image on the intended substrate. If the substrate is mobile, for example if the substrate is the end user’s skin and the intention is for the image to be reproduced as a tattoo on the end user’s skin, then the end user may visit the reproducer to have the selected image reproduced. If the substrate is immobile, for example if the substrate is a wall and the intention is for the image to be reproduced as a mural on the wall, then the reproducer may visit the location of the wall. The server may facilitate the introduction of the end user to a reproducer who is appropriately situated and has the appropriate skills to reproduce the image on the desired substrate. The stored information related to the image (e.g. style) may be used to produce a candidate list of reproducers for selection by the end user. The reproducer may, for example, be a tattoo artist or a mural artist. The reproducer may have particular specialisms, such as styles of tattooing. The end user may themself be a reproducer of the image. The reproducer may be or may use a machine to perform the reproduction.

[0040] When the reproducer and the server are together, the reproducer accesses the server to obtain the image that is to be reproduced. If the reproducer is a human artist then they can use a reproducer’s terminal 3 to view the image. At this stage, the server begins the process of enabling the image to be authenticated. Alternatively, this process may be done at another time.

[0041] Figure 2 shows at 20 an image that may have been bought by an end user. To allow the image to be authenticated, the server determines a set of locations for fiducial markers that can be incorporated into the image. Figure 2 shows at 21 an example of the same image incorporating fiducial markers 22.

[0042] To determine the locations for the fiducial markers, the server selects a set of locations that can be determined to be unique in comparison to other location sets that the server is already aware of. Those other location sets may be location sets associated with images that have already been reproduced and whose details are stored by the server. To do this, the server may use one or more of the following criteria.

[0043] 1 . The server may select a desired number of fiducial markers to be incorporated into the image. In one example, the number of fiducial markers may be determined in dependence on another attribute stored in the database in association with the image. For example, the greater the price of the image, the fewer markers might be selected. 2. The server may select the locations such that the set of locations is different from any of its previously stored sets, optionally even taking into account that the reproducer might introduce some error when positioning the markers. For example, a window of a predetermined shape and size may be defined. The server may then determine a set of locations that are at the centre of such windows, the windows being such that wherever a marker is located in the respective window, the resulting set of markers will be distinct from previously stored sets. The windows may, for example, be round or square in shape. The windows may, for example, have a greatest dimension of 3mm, 5mm or 10mm. The greatest dimension could be higher or lower than these values, suitably being selected depending on the technique being used to apply the image and its expected precision.

[0044] To illustrate the process in more detail, suppose the server is storing the following marker positions for previously reproduced images, the positions representing distances in mm on orthogonal X and Y axes from a reference location:

[0045] Image 1 : [3.4, 1.2] , [6.0, 1.3] , [11.1 ,7.6]

[0046] Image 2: [0.2,10.9] , [5.7, 2.9] , [5.5, 4.0]

[0047] Image 3: [9.0, 13.4] , [11.8, 15.8] , [5.0, 4.0] , [14.3, 7.1]

[0048] To determine the locations for a new set of markers, the server first determines how many markers are to be in the new set, for instance three. Suppose the marker is to be a square 1 mm x 1 mm. Suppose the window size to accommodate imprecision in drawing the marker is a 5mm x 5mm square. Suppose the server is pre-configured to assume that markers will be considered distinct if they are more than 1 mm away from each other. These criteria can suitably be decided when implementing the present system, depending on the technique being used to apply the image, its expected precision and the degree of accuracy desired in authenticating images. Then the server determines the selected number of locations such that when a single marker is placed at any location in each of the windows, a pattern will result whose markers are distinct from the set of markers in each of the already-stored sets. In doing this, the server may consider rotation of the determined set of positions, so that it is distinct from the already-stored sets in any orientation. It may also consider sub-sets of markers in already-stored sets of markers, so that the newly determined set as a whole will be distinct from any subset of markers in an already-stored set. The server determines the positions relative to the graphic so that the markers will be distinguishable from the graphic once applied. Thus, each marker may be spaced or offset from non-blank parts of the graphic that is to be applied, or may be in a different colour from adjoining parts of the graphic. The markers or parts of them could be reversed out of the graphic. If desired, one or a set of the markers may be directionally specific, for instance in the shape of a line, arrow or L-shape. This can allow an entity that is visually assessing the markers to establish a specific orientation and avoid the need for multiple possible rotational configurations of the pattern to be considered.

[0049] Once the server has determined a suitable set of marker locations, it presents the set of marker locations to the reproducer, e.g. via the reproducer’s terminal 3. The markers may be represented as an overlayed layer in the digital image. The markers may be accentuated by being, for example, a different colour from a majority of the image, or the lines of the image, or having a greater local density than other areas of the image. The reproducer then attempts to apply markers to the substrate in those locations. In practice, due to factors such as unevenness of the substrate and the imprecision associated with manual drawing, the markers might not be placed precisely as indicated to the reproducer. However, if the reproducer is capable of placing the markers within the predetermined windows then the pattern of markers will still be unique due to the manner in which the marker locations were selected, as described above. Such placement will be discussed in relation to figures 3a and 3b below.

[0050] Once the reproducer has laid down the markers, it may be advantageous to record the position of the markers as laid down. This can help to identify the markers later, and can afford the server greater flexibility in defining marker positions for subsequent artworks. The reproducer may use their terminal 3, or another terminal 4 such as a mobile phone, or a dedicated camera, to photograph the markers as laid down on the substrate. Then the reproducer can transmit that photo to the server. The server can then verify that the markers as laid down are within the predetermined windows and / or are distinguishably unique among the art for which it stores marker data. If the markers as laid down are not distinguishably unique, e.g. due to errors in their positioning by the reproducer, then the server may instruct the reproducer to apply one or more additional markers to generate a distinguishably unique pattern. The server stores the locations of the markers as laid down in association with the end user’s purchase of the image. If an image of the markers is not transmitted to the server then the server can store the marker positions as instructed to the reproducer in association with the end user’s purchase of the image.

[0051] The markers may be applied and photographed for identification prior to the image being reproduced. Alternatively, the markers may be laid down contemporaneously with other features of the image being reproduced. It may be that the completed image partially or fully overlaps the markers.

[0052] The reproducer can then apply the image itself to the substrate. The server may instruct the reproducer on the location where the image is to be applied relative to the markers. To logically associate the image with the markers, the image may be positioned so that for one of the markers there is a non-blank part of the image located on a straight line across the substrate running between that marker and another one of the markers. With the image between two of the markers in this way, it can be plain that the markers are associated with that image. To more emphatically associate the markers with the image, it may be the case that for each of the markers there is at least one other marker located such that the straight line across the substrate between the marker in question and that other marker intersects a non-blank part of the image. A non-blank part of the image may be a part applied by the reproducer. Put another way, for at least one of the markers there may exist a straight line (23 in figure 2) across the substrate which extends from that marker to another one of the markers and also passes through a non-blank part of the image. This may be true for multiple of the markers, or even for each of the markers there may exist a straight line across the substrate which extends from that marker to another one of the markers and also passes through a non-blank part of the image

[0053] Once the image is complete the reproducer may capture another photograph of the completed artwork, including the markers, and transmit that to the server as a record of successful completion. The photograph of the completed artwork may be associated with a stored file of the image on the server. The photograph may be accessible to users browsing the image marketplace as an example of the image applied to a substrate.

[0054] At a later time, the end user or a third party can verify the artwork as applied to the substrate, for example to check that it derives from the original artist. To do this the party in question can use a terminal , e.g. a mobile phone, to capture an image of the artwork as applied to the substrate, including the markers, and transmit that image to the server. The server can then process the image to identify the positions of the markers, and compare those marker positions to the marker positions stored in its database. The comparison may take into account distortions, e.g. trapezoidal distortion due to variance between the camera image plane and a plane of the substrate and rotation. If there is a match then the server can report to the terminal 5 details of the artwork and / or transaction that were matched, for example the artist, the purchase date and the purchaser. In this way it can be verified that the artwork as applied to the substrate has derived properly from the artist.

[0055] The markers may be of any suitable shape and size. For example, they could be dots, crosses, full or open squares, full or open circles, full or open ovals or lines. Any suitable number of markers may be used. Conveniently the number of markers may be in the range from three to 15. The number of markers may be in the range four to 10. The markers may be of any suitable size, e.g. a size that permits them to be identified reliably. For example, the markers may be 1 mm square or 1 mm diameter; or may have a greatest dimension that is not greater than 5mm and / or a smallest dimension that is not less than 1 mm. The markers may be deposited in the same medium as the image, for example ink or paint.

[0056] The designation of marker positions and windows within which each marker should be situated will be discussed with reference to exemplary figures 3a and 3b. Figure 3a shows an image area 300 and windows 301 indicated as circular areas. Within each window 301 there is an intended marker position 302 indicated as a solid cross. The centre of the intended marker position 302 is central to a respective window 301 .

[0057] Figure 3b illustrates an equivalent image area 301 as in figure 3a. Marker 303 is shown as ideally applied, at the intended marker position 302 (the centre of the cross) within the window 301 . Marker 304 is shown as offset from the intended marker position, but still within the defined window. Such a marker can be considered to have been successfully applied according to being within the predetermined window generated by the server. Marker 305 is shown as located outside a respective window 301 , such a marker can be considered to have been unsuccessfully applied. Marker 306 is shown having a different rotational orientation compared to markers 303, 304 and 305. The marker 306 can be assessed in terms of orientation relative to other markers. As discussed above, the orientation of markers to be applied may be specified by the server in order to generate a unique set of markers.

[0058] To assist the reproducer in accurately positioning the markers, the reproducer’s terminal may provide a template, for example in the form of an adhesive mask that can be applied to the substrate to guide application of the markers, or it may project the intended marker positions on to the substrate. The reproducer’s terminal may be configured to cause a thermal printer to print a transfer image for tattooing.

[0059] The server is provided with software for detecting the markers in images provided to it. This may be any suitable image processing software, for example software trained by machine learning to detect the markers. Such software can conveniently be trained by assembling a set of images of artworks analogous to the ones that are to be reproduced, artificially applying markers to those images in known locations to form a set of training images, and training the machine learning algorithm to detect the markers in the known locations in the training images.

[0060] Another application of the system described above is in surgery. Tattoos can be used to mark sites on or in the human body at which surgery is to be performed. Tattoos are used particularly for applying markers for radiation therapy. This can reduce the chance of mistakes by a surgical procedure being performed on the wrong patient or in the wrong location. Using the system as described above, in a pre-surgical step a healthcare professional (HCP) identifies a site for surgery. The HCP communicates with the server to register a target image for the surgical site, analogously to the end user as described above obtaining an artwork. The server determines a distinguishably unique set of markers, as described above, and instructs the HCP to apply the markers at the surgical site together with the target, analogously to the reproducer applying markers and an image. The markers may be recorded in the server’s database together with details of the patient to whom the markers were applied and / or the intended surgical procedure. At a later time, an HCP who is to perform surgery can identify the target and validate the markers by sending an image of them to the server. The server can return the information stored about that set of markers. In that way, the HCP who is to perform the surgery can check that the correct patient is at hand and / or that the correct procedure is to be undertaken.

[0061] Algorithms for identifying suitable marker locations to be advised to the reproducer will now be described.

[0062] The server has access to a database of previously registered sets of markers. Each set of markers may be stored as a set of coordinates for each marker, for example X and Y coordinates. If the system is to be used with substrates that are substantially non-planar then a projection of the markers on to a plane may be considered, or the server may make use of data defining the three-dimensional shape of the substrate, e.g. as provided to it by a LIDAR scan of the substrate made by the end user or the reproducer. The server stores data defining zones of potential inaccuracy in the application and / or identification of a marker. In the case of application, it is possible that the reproducer might apply a marker in a position somewhat offset from an intended position. In the case of identification, it may be that due to camera resolution, camera lens distortion or non-planarity of the substrate there is a difference between the position at which the server identifies a marker in an image and the actual position of the marker, either absolutely or relative to nearby markers. These zones of potential inaccuracy may conveniently be considered to be square or round. They may be ascribed the same or different widths. They may be considered separately, or a larger zone of potential inaccuracy may be employed to account for both sources of inaccuracy. In one example, to determine suitable locations at which to instruct the reproducer to place markers, the server identifies a set of locations such that with a respective marker placed anywhere in a window of a predefined shape and size about each of those locations, those placed markers will form a pattern that is distinct from any of the previously stored patterns, or any subset of the previously stored patterns. The latter condition allows later-applied patterns with fewer markers than earlier- applied patterns to nevertheless be distinguished from the prior patterns. In addition to choosing the positions of the markers, the server may instruct that markers of different sizes or shapes should be applied. The distinguishable uniqueness of a pattern of markers may take into account the shape and / or orientation of the markers. This may allow further variations in pattern to be achieved.

[0063] It may be desirable to use a specific medium to apply the markers and / or the image to the substrate. For example, it may be desirable to use an ink of the type described in our co-pending GB patent application No. 2400946.6 entitled “A Particle”. In some circumstances it may be advantageous to securely associate the dispensing of the medium with the application of a specific image. This may be achieved as follows. The reproducer is provided with a supply of the medium, e.g. in a reservoir 14 such as a bottle. The reproducer attaches the reservoir to a dispensing device 15, e.g. a tattooing machine or a paint spray gun. The reservoir may be physically configured, e.g. with a neck of a specific shape, so that it can only mate with certain dispensing devices. The dispensing device is configured only to dispense medium when suitably authorised. The dispensing device communicates with the server. When the end user gains access to an image for application by a reproducer, the end user communicates to the server the identity of the reproducer who is to be engaged. The dispensing device is logically associated at the server with that reproducer. When the reproducer uses their terminal to contact the server to initiate the process of applying the image, as described further above, the server communicates with the dispensing device to authorise it to dispense an amount of the medium. The amount of the medium to be dispensed may be determined by the image. For example, an image for a tattoo having a size of 10 x 10 cm and only outlines, without shading, may have an associated ink volume of 5 cm3. In this way, the use of the medium can be reliably constrained to images acquired in the manner described above. A volume of a specific medium for applying only the markers may be dispensed. For example, a tattoo ink having a particular colour.

[0064] In the examples given above, it is described that the graphic is applied to the substrate by a human operator. The human operator could use a drawing tool such as a needle, pencil, pen, brush or spray. Alternatively, the reproducer could be an automated device such as a printer or a robotic drawing tool. If the reproducer is an automated device, it may be provided with the marker positions by the server over a suitable communications link. It may attempt to apply the markers in the designated positions, or as close as it can given any factors such as unevenness of the substrate, and then automatically capture an image of the markers and return it to the server for verification analogously to the process described above. It may be provided with the image by the server, and it may then automatically apply the image to the substrate, in association with the markers. The robotic device may, for example be an automated tattoo applicator.

[0065] The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present invention may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

[0066] The phrase "configured to" or “arranged to” followed by a term defining a condition or function is used herein to indicate that the subject of the phrase is in a state in which it has that condition, or is able to perform that function, without that object being modified or further configured.

Claims

CLAIMS1 . A system for managing the application of artwork to a substrate, the system comprising one or more processors configured to: receive an indication of a graphic; generate a set of candidate locations for markers visually differentiable from the graphic; transmit the candidate locations to a graphic applicator; receive from the applicator an image of a target substrate region having visually differentiable markers applied thereto; and compare the locations of the markers in the image with the candidate locations.

2. A system as claimed in claim 1 , wherein the one or more processors are configured to: if the comparison indicates conformity between the markers in the image and the candidate locations, transmit an instruction to the applicator to apply the graphic to the target substrate; and otherwise, transmit an instruction to the applicator to apply a further marker to the substrate.

3. A system as claimed in claim 1 or 2, wherein the one or more processors are configured to perform image processing on the received image of the target substrate region to identify the locations of markers in that image.

4. A system as claimed in any preceding claim, wherein the one or more processors are configured to store the set of locations of the markers in the image in association with data indicative of a transaction by which the graphic is authorised to be applied to the target substrate.

5. A system as claimed in claim 3 as dependent on claim 4, wherein the one or more processors are configured to, subsequent to the said storing step: receive from a user terminal a second image of the target substrate having the graphic and the markers thereon; process the second image to identify the locations of markers in that image;compare those identified marker locations with sets of stored marker locations to identify a match between the identified marker locations and a set of stored marker locations; and in response to identifying such a match, transmit associated data indicative of a transaction to the user terminal.

6. A system as claimed in any preceding claim, wherein: the system stores sets of marker locations for previously applied graphics, and the step of generating a set of candidate locations for markers comprises: determining a target quantity (n) of candidate locations; and determining n candidate locations subject to the constraint that any set of n markers located such that each such marker is offset from a respective one of the candidate locations by not more than a predetermined deviation is distinct from all of the stored sets of marker locations.

7. A system as claimed in claim 6, wherein the predetermined deviation is a two- dimensional deviation.

8. A system as claimed in any preceding claim, wherein the marker is circular or square.

9. A system as claimed in any preceding claim, wherein the greatest dimension of each marker is less than 5mm.

10. A system as claimed in any preceding claim, wherein the graphic applicator is a human.

11. A system as claimed in any preceding claim, wherein the target substrate is part of the skin of a human subject.

12. A system as claimed in claim 2 or any of claims 3 to 11 as dependent on claim 2, wherein the system comprises an applicator for dispensing a medium for applying the artwork and the or each processor is configured to, if the comparison indicatesconformity between the markers in the image and the candidate locations, transmit a signal to the applicator for permitting the applicator to dispense the medium.

13. A system as claimed in claim 12, wherein the or each processor is configured to, if the comparison indicates conformity between the markers in the image and the candidate locations, determine an amount of the medium in dependence on the graphic and transmit a signal to the applicator for permitting the applicator to dispense that amount of the medium.

14. A system as claimed in claim 2 or any of claims 3 to 13 as dependent on claim 2, wherein the one or more processors are configured to: if the comparison indicates conformity between the markers in the image and the candidate locations, transmit an to the applicator an indication of a location relative to the markers at which the graphic is to be applied to the target substrate.

15. A method for applying an artwork to a substrate, the method comprising: receiving an image; receiving an indication of a set of marker positions; applying markers to the substrate, each marker being applied in a region of a respective one of the indicated marker positions; recording the positions of the markers as applied to the substrate; and applying the image to the substrate in the region of the markers.

16. A tattoo applicator comprising: an inlet for receiving ink; a needle for applying ink to a substrate; a valve located in a flow path between the inlet and the needle for controlling the flow of ink between the inlet and the needle; a communication interface for receiving data over a data network; and one or more processors coupled to the interface and the valve, the processors being configured to process a message received at the communication interface to authenticate that message, and in response to the message being authenticated and the message being of a predetermined form, cause the valve to open.

Images