Abstract:
An interactive display device ( 100 ) has an image layer ( 102 ) for presenting user perceivable visual content and a touching surface ( 101 ) covering the image layer ( 102 ) or formed by the image layer ( 102 ). Further, there are a plurality of invisible light sources ( 110 ) for illuminating alternatingly different sub-regions of the touching surface ( 101 ) and a plurality of touch detection units ( 107 ) for capturing images of illuminated sub-regions from behind the image layer ( 102 ). From the captured images contact areas are detected where a pointing object ( 112 ) contacts the touching surface ( 101 ) and thus causes correspondingly one or more marks in the captured images. The remaining ones of the invisible light sources ( 110 ) are controlled to be operable while adjacent touch detection units and associated invisible light sources are controlled to be non-operable so as to reduce undesired reflections of invisible light from non-contacting objects in the vicinity of the touching surface.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention generally relates to an interactive display. 
       BACKGROUND OF THE INVENTION 
       [0002]    There are interactive displays which comprise e.g. a projector or a Liquid Crystal Display (LCD) panel to form an image on a surface, a camera or other optical detector behind the surface, and a computer vision circuitry for detecting user input. 
         [0003]    There are numerous different technologies for optical multi-touch detection. Many of these are described in the publication “ThinSight: Versatile Multi-touch Sensing for Thin Form-factor Displays” by Steve Hodges, Shahram Izadi, Alex Butler, Alban Rrustemi and Bill Buxton, see http://www.billbuxton.com/UISTthinSight.pdf. Optical touch detection is advantageous for a variety of reasons including ability to detect multiple touch points or areas on the display. However, the optical recognition of touching is exposed to illumination related changes in the environment. Typically, infrared (IR) light is used to illuminate the touch screen and the reflecting or shadowing of the IR light is measured. The operation of such touch screens depends, for example, on the frequency and amplitude distribution of ambient light, direction of ambient light, and reflectance and size of user&#39;s fingers. 
         [0004]    It is an object of the present invention to avoid problems associated with prior art and/or to provide an alternative to existing technology. 
       SUMMARY 
       [0005]    According to a first aspect of the invention there is provided an interactive display device comprising: 
         [0006]    an image layer configured to provide a user with user perceivable visual content; 
         [0007]    a touching surface covering the image layer or formed by the image layer; characterized by: 
         [0008]    a plurality of invisible light sources configured to illuminate alternatingly different sub-regions of the touching surface; 
         [0009]    a plurality of touch detection units configured to capture images of illuminated sub-regions; 
         [0010]    a first processor configured to detect from the captured images contact areas where a pointing object contacts the touching surface and thus causes correspondingly one or more marks in the captured images; and 
         [0011]    a second processor configured to control at least one of the invisible light sources to be operable while remaining ones of the invisible light sources are controlled to be non-operable so as to reduce undesired reflections of invisible light from non-contacting objects in the vicinity of the touching surface. 
         [0012]    Advantageously, undesired reflections from non-contacting objects may be reduced by controlling some invisible light sources to be inoperable. Such reduction of reflections may help the detection of the areas where the pointing object contacts the touching surface. 
         [0013]    A common processor may operate as both the first and second processors. Alternatively, one or both of the first and second processors may comprise two or more different processors. 
         [0014]    The second processor may be a relatively simple control processor with few or no other tasks in addition to controlling into operable or non-operable state the invisible light sources. 
         [0015]    The touch detection units may be configured to operate with the invisible light provided by the associated invisible light source. The invisible light may be infrared light. 
         [0016]    The touch detection units and associated invisible light sources may be arranged into groups so that at least some adjacent touch detection units and associated invisible light sources are distributed in different ones of the groups. 
         [0017]    The touch detection units may be configured to capture the images from behind the image layer 
         [0018]    The touch detection units may be arranged in a grid. 
         [0019]    The touch detection units may comprise camera units. The camera units may be rolling shutter camera units, such as commonly available complementary metal oxide-semiconductor (CMOS) camera units. The rolling shutter camera units may be synchronized in such a way that the rolling shutter operation extends from one rolling shutter camera unit to another. 
         [0020]    The image layer may be configured to pass through light with the invisible frequency range. 
         [0021]    Each of the marks may appear as a shadow or highlight. 
         [0022]    The image capture unit may be configured to obtain the captured images through the image layer. Alternatively, the image capture unit may be formed on the image layer or in front of the image layer such that the images produced by the image layer are visible to the user through the image capture unit. 
         [0023]    According to a second aspect of the invention there is provided a method in an interactive display device, comprising:
       providing a user with user perceivable visual content on an image layer, wherein there is a touching surface covering the image layer or formed by the image layer;
 
characterized by:
   illuminating alternatingly different sub-regions of the touching surface with a plurality of invisible light sources;   capturing images of illuminated sub-regions with touch detection units, wherein each of the invisible light sources is associated with one or more touch detection units;   detecting from the captured images contact areas where a pointing object contacts the touching surface and thus causes correspondingly one or more marks in the captured images; and   controlling at least one of the invisible light sources to be operable while controlling remaining ones of the invisible light sources to be non-operable so as to reduce undesired reflections of invisible light from non-contacting objects in the vicinity of the touching surface.       
 
         [0029]    The method may further comprise taking rolling shutter images of the sub-regions and advancing capturing of images from one sub-region to another such that the rolling shutter operation extends from one sub-region and associated touch detection unit to adjacent sub-region and associated touch detection unit. 
         [0030]    According to a third aspect of the invention there is provided a computer program configured to cause when executed by a computer a method according to the second aspect of the invention. 
         [0031]    According to a fourth aspect of the invention there is provided a computer readable memory medium embodied with a computer program which when executed by a computer causes a computer to perform a method according to the second aspect of the invention. 
         [0032]    According to a fifth aspect of the invention there is provided a computer program product comprising a non-transitory computer readable medium having computer executable program code stored thereon, which when executed by at least one processor causes an apparatus at least to perform a method according to the second aspect of the invention. 
         [0033]    Various embodiments of the present invention have been illustrated only with reference to certain aspects of the invention. It should be appreciated that corresponding embodiments may apply to other aspects and embodiments as well to produce further non-limiting examples. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]    The invention will be described, by way of example only, with reference to the accompanying drawings, in which: 
           [0035]      FIG. 1  shows a block diagram of a system according to an embodiment of the invention; 
           [0036]      FIG. 2  shows a simplified block diagram of the structure of a control unit shown in  FIG. 1  according to an embodiment of the invention; 
           [0037]      FIG. 3   a  shows an example of distribution of a touching surface into a plurality of sub-regions; 
           [0038]      FIG. 3   b  shows a schematic flow chart according to an embodiment of the invention for illustrating a first phase in a process of determining a pointing object from an Infrared (IR) image captured by an IR image capture unit from behind of the pointing object under clear daylight or strong and relatively coherent ambient IR light; and 
           [0039]      FIG. 4  shows a system to illustrate further details regarding possible circuitries suited e.g. for implementing the sub-regions shown in  FIG. 3   a.    
       
    
    
     DETAILED DESCRIPTION 
       [0040]    In the following description, like numbers denote like elements. 
         [0041]    In the following examples of various embodiments of the invention an LCD screen is used as an example of an image layer. The LCD screen may comprise a film that forms a visible image and optionally also other elements such as background illumination, infrared (IR) illumination, incoming IR intensity detection across the screen, and/or one or more speakers for outputting sound. It is appreciated that the invention may likewise be applied with any other types of image layers as well, or screens for simpler expression. Generally, an image layer is any means for producing a visible image for a user, such as an electric display, a screen displaying a projected image or a substance carrying printed, dyed image, or woven image. However, the image layer should be sufficiently transparent or translucent for IR light that is used for detecting a pointing object through the image layer as will be explained in more detail in the following. 
         [0042]      FIG. 1  shows a block diagram of a system  100  according to an embodiment of the invention.  FIG. 1  also shows a user  113  for facilitating the description of particular order of different elements. The system  100  is suited for use as an interactive user interface device e.g. as a built in dynamic menu in a restaurant, as a display screen at a ticket office, or generally wherever an interactive display and optical pointing recognition is desired. 
         [0043]    The system  100  comprises as an outmost element or as facing to the user  113  a touching surface  101  such as a toughened glass plate, then an LCD film  102  as a display screen and a diffusion layer  103  behind the screen. Preferably, the touching surface, the screen  102  and the diffusion layer  103  are all in a compact stack such that the distance from the touching surface to the diffusion layer is as low as possible for machine view performance reasons that will be described in more detail in this description. 
         [0044]    The purpose of the diffusion layer  103  is to spread the light coming from a background light source (described in the following), so that an image displayed on the screen appears even from many directions. This spreading can be achieved with a diffusion film or with a holographic rear-projection film. By placing the diffusion layer  103  behind the screen  102 , significant advantages may be achieved in comparison to placing the diffusion layer  103  in front of the screen  102  or to omitting the diffusion layer  103 . Namely, the diffusion layer  103  reducing reflections from the typically glossy backside of the screen  102 , which reflections may interfere with the recognizing of pointing objects. It is also advantageous to locate the diffusion layer  103  behind the screen  102  when seen by a user  113 , because otherwise it provides an additional reflecting surface between the screen  102  and the viewer thus impairing the image quality or contrast especially. 
         [0045]    In order to support the aforementioned optical elements, there is provided a casing  104 . The casing  104  comprises a back wall  105  attached to side walls  106 . The side walls are attached from one end to the back wall  105  and from their opposite ends to the touching surface  101 , screen  102  and diffusion layer  103 . A background light source  108  may be located in the casing  104  for background illumination of the screen  102 . The background light source  108  may comprise, for example, one or more of: LED light, light conductor, fluorescent light, and luminescence light. 
         [0046]    In an alternative embodiment, the diffusion layer  103  is omitted. This may particularly be the case when an evenly illuminating background light is provided by a luminescence light that in itself provides an even field of illumination on the LCD film  102 . 
         [0047]    The side walls  106  may be coated from the inner side with some reflective material in order to deliver maximum amount of light to the screen and finally to users of the system. By means of the reflective side walls one may avoid or at least reduce shadows that may be caused to the background light in the IR image captured by a camera behind the screen  102 . The reflective side walls may also help in delivering the background light to the screen in a way that the light can be spread to the users without allowing users to see details inside the system through the screen and at the same time improving the viewing angle of the system. In addition to the reflective side walls (or reflective casing) the diffusion layer  103  may help to achieve this effect. More particularly, the side walls may be for example mirror walls, in which case a particularly good consistency of an image on the image layer may be achieved. 
         [0048]    Inside the casing, there is an image capture unit  107  that is in this particular case an IR camera configured to see through the diffusion layer  103 , screen  102  and the touching surface  101  any objects placed near or at the touching surface  101 . Further in  FIG. 1 , there is drawn as a dotted area an IR light reception space  107 ′ for the IR camera. The camera is configured to detect signals that are outside the visible light wavelengths. There may be for example a filter in front of the lens of the camera providing this effect. The camera may be based on for example CCD (charge-coupled device) or CMOS (complementary metaloxidesemiconductor) technology. Moreover, the image capture unit  107  may comprise more than one camera e.g. for better resolution, for smaller distance between the screen and the cameras, or for covering larger LCD screens  102 . 
         [0049]    The distance between the background light source  108  and the diffusion layer  103  may generally depend on the space that the camera setup requires. The distance may be shortened for more compact sized system e.g. by moving the camera forward and/or turning the camera around (to point away from the screen) and using a mirror or prism in front of the camera to capture the activities on the screen. 
         [0050]    Alternatively, the image capture unit  107  may be formed of a planar sensing structure that has a number of sensors spread over a plane to recognize intensity of incoming light through each pixel or pixel block of the screen  102 . Such a structure may also double as a visible light and/or IR illumination behind the screen  102  for the purpose of background illuminating the LCD screen and/or illuminating objects in front of the LCD screen under weak ambient IR illumination. In case of a planar sensing structure operating as an image capture unit  107 , there is no IR light reception space  107 ′ as that drawn in  FIG. 1  but instead there is merely a small parallel space between the screen and the image capture unit. Further, the image capture unit may be integrated directly into the image plane, for example the screen. 
         [0051]    The camera  107  and the IR LEDs  110  may not be seen from the outside of the system if their shadow is negligent e.g. due to the highly diffused illumination coming from the background lights and as the reflective inner side of the system provides evens further illumination of the display surface. Further, the diffusion layer  103  may prevent the users from seeing any details from inside the system. 
         [0052]    A camera as the IR image capture unit typically provides a more economical solution than a plurality of emitter/detector pairs that are typically used in a planar illumination and sensing element. First, the area covered by one camera typically requires a relatively high number of emitter/detector pairs thus incurring a relatively high number of components and wires. Second, production of the system may be simpler when a camera is used. On the other hand, by using emitter/detector pairs in a planar image capture unit, the size of the system may be reduced and there is no need for accurately positioning and directing the camera in a desired angle with regard to the screen  102 . 
         [0053]    The system  100  may further comprise an IR reflection inhibitor  109  configured to reduce reflection of IR light from the back wall and again from any elements in front of the camera  107 . The IR reflection inhibitor  109  may comprise, for instance, a black plate such as a sooted aluminum plate placed around the background light source  108  or behind the background light source  108 . Alternatively, the IR reflection inhibitor  109  may comprise additionally or alternatively a selective screen between the background light source  108  and the IR light reception space  107 ′. 
         [0054]    Additionally the system  100  comprises invisible light sources  110  such as IR light sources for facilitating input detection on IR wavelengths. The invisible light sources may comprise for example IR LEDs placed outside the angle of view of the image capture unit. In case that a camera is used as the image capture unit, the invisible light sources  110  may be located outside a cone formed by the view area of the camera. On the other hand, if a planar image capture unit behind or integrated with the LCD screen is used, the LCD screen itself may be configured to provide the IR illumination across its view area or the IR illumination may be configured to be produced such that it illuminates objects at the LCD screen without first passing through the LCD screen. 
         [0055]    The system  100  may further comprise an audio speaker  114  for providing audible signals to the user  113 . The system may be configured to e.g. provide a tapping sound to indicate determined tapping on the touching surface  101  for enhancing user experience of an operable system and to teach users that there is no need for applying substantial force against the touching surface when the recognition of the pointing object  112  is optically performed. 
         [0056]    While  FIG. 1  only shows a single camera, some embodiments of this invention are easier to understand when assuming that there is a matrix of cameras  107  and invisible light sources  110 . For instance, there may be 8 columns×4 rows of cameras  107  and invisible light sources each covering a different sub-region of the touching surface  101 . The different sub-regions may have overlap or the different sub-regions may be non-overlapped. 
         [0057]      FIG. 2  shows a simplified block diagram of the structure of the control unit  111 . The control unit  111  may be based on, for example, a general purpose computer supplied with suitable software and/or on a particularly adapted computing device. While it is possible to implement the control unit  111  by purely hardware based a device, typically it is more economic and faster to produce by making use of software. 
         [0058]    In  FIG. 2 , the control unit  111  is drawn to comprise a memory  201  that comprises a work memory  202 , a non-volatile memory  203  that is configured to store software  204 , presentation information  205  describing content to be presented by the system  100  and/or how pointing at different areas on the screen should be treated, and settings  206  needed e.g. for manual or automatic calibration of the system  100 . The software  204  may comprise any one or more of the following items: operating system, device drivers, display presentation application, hypertext markup language parser, image processing software, and drivers for different external equipment that may be connected to the system such as printers, further displays, further interactive systems  100 , audio systems, and external IR illumination equipment (not shown). 
         [0059]    The control unit  111  further comprises a processor  207  configured to control the operation of the control unit  111  according to the software  204  by executing computer executable program code contained by the software in the work memory  202 . Alternatively, the control unit may be configured to execute the software in place in the non-volatile memory in which case the work memory may not be necessary. The control unit further comprises an input/output unit (I/O) 208 for exchanging signals with other elements of the system  100  and optionally also with external equipment. The I/O  208  may comprise e.g. any one or more of a universal serial bus port, a local area network port, an ISA bus, a PCI express port, an IR port, a Bluetooth element, and a parallel port. Alternatively to being configured capable of communicating with external equipment, the system  100  may be provided with a transferable memory reception unit  209  such as a cd-rom or dvd-rom drive, memory card reader or memory stick reader which enables replacing part of the non-volatile memory e.g. for updating information to be displayed on the LCD screen  102 . 
         [0060]    In order to control the operation of various components of the system and to obtain the captured image, there are connections between the control unit or particularly its input/output unit  208  and other components of the system  100 , while not shown in sake of clarity of the drawing. The control unit has generally the task of receiving a signal from the camera  107 , detecting if and where the touching surface  101  is pointed at and typically also outputting the determination in a standard way e.g. emulating a computer drawing tablet, mouse or other known pointing device. 
         [0061]    Generally, the control unit operation may comprise following acts:
       controlling the screen to show desired images to the user  113 ;   controlling the IR lights  110  to produce IR light on demand for showing a pointing object  112  such as a user&#39;s  113  finger when brought close to the screen;   obtaining signals corresponding to received IR light from the image capture unit  107 ;   detecting from the received signals the pointing object at the touching surface  101 ;   performing a predefined action based on the detected input, e.g. changing the image displayed on the screen  102  or following a hyperlink associated with the area at which the pointing object is detected; and/or   detecting the amount of ambient IR light controlling the IR lights  110  accordingly.       
 
         [0068]    It is appreciated that while the control unit may consist of one separate unit, the control unit  111  may alternatively be integrated with any other element or comprise two or more discreet elements each for one or more of the aforementioned acts. 
         [0069]      FIG. 3   a  shows an example of distribution of a touching surface  101  into a plurality of sub-regions. The sub-regions are drawn to form an 8×4 matrix and denoted with reference signs  1 - 1  to  4 - 8 . The sub-regions may correspond to different compartments formed into an interactive display device (e.g. with separating walls) or the sub-regions may simply represent different parts of the touching surface as seen by different image capture units  107 . In some other embodiments, some of the borders between different sub-regions have compartmentalizing walls while some other sub-regions are not compartmentalized. In the absence of walls that separate different sub-regions, it is understood by the inventor that the invisible light sources  110  configured to illuminate a given sub-region may also to some extent illuminate neighboring sub-regions. This may be particularly the case when the user&#39;s body is near the touching surface and the user wears light clothing that reflects a substantial portion of the invisible light transmitted by the invisible light sources  110 . In result, neighboring sub-regions may severely interfere touch detection at each other. 
         [0070]      FIG. 4  shows some further details regarding possible circuitries suited for e.g. implementing the sub-regions shown in  FIG. 3   a . However, a process description is first presented with reference to  FIG. 3   b.    
         [0071]      FIG. 3   b  shows a schematic flow chart according to an embodiment of the invention for illustrating a process of determining a pointing object from an IR image captured by an IR image capture unit  107  or more precisely by a plurality of image capture units  107 . Let us consider that the image capture units  107  are rolling shutter cameras such as CMOS cameras. In other words, the image is formed on a row-by-row basis. This operation has some adverse effects especially for taking photographs of moving objects, but here new advantages may be obtained. Let us consider that the image capture units  107  are operated in a sequence so that once an image capture unit  107  in the top row completes taking one image of one sub-region, the image capture unit right beneath will commence taking the next image of another sub-region. For instance, each image capture unit may be configured to operate row by row and then advancing next column. In the following, there is described an example process in which two different image capture units  107  are being used simultaneously for illustrating parallel operation with spatial neighbor interference reduction. 
         [0072]    In step  301 , the process for detecting pointing objects  112  starts and the invisible light source and camera units are activated for first (normally non-adjacent) sub-regions, say for sub-regions  1 - 1  and  3 - 5 . These sub-regions are illuminated and image captured for subsequent processing preferably when all the sub-regions have been processed. 
         [0073]    Next in step  302 , the following sub-regions are selected and processed as the first sub-regions in step  301  along the processing order of the sub-regions. It is understood more (or less) than two sub-regions may be processed simultaneously. Moreover, the sub-regions need not be processed exactly simultaneously; the exact timing of parallel processing of different sub-regions may be partly random or there may be a given time offset such as half the image capturing period, for instance (e.g. 1/60 sec/2= 1/120 s). Using  FIG. 3  as a reference, the sub-regions may be thus scanned through each vertical column and then to next column and then jumping back to first column after the last column. Of course, in some other embodiments the scanning may proceed along rows rather than columns. In a preferred embodiment, if rolling shutter cameras are used, the scan order is yet such that the operation may be advanced from one sub-region and its camera to the following accounting for the direction of the rolling shutter so that smooth advancing may be achieved. Therefore, there should be no holes or other discontinuities in images formed by combining plural adjacent sub-images unlike with current systems where rolling shutter images may suffer from temporal disruptions. 
         [0074]    It is then checked  303  if all the sub-regions have been captured already. If not, the process resumes to step  302 , otherwise the process advances to step  304 . 
         [0075]    In step  304 , an image is formed for the entire touching surface  101  based on all the different sub-regions. This step  304  may be carried out after processing of all the sub-regions as drawn in  FIG. 3   b . Alternatively, the step  303  may be carried out when a predetermined number of sub-regions have been processed or when a predetermined threshold is met. 
         [0076]    By processing in temporally parallel manner two or more sub-regions, higher number of cameras may be accounted for and/or other enhancements may be applied such as taking different images of each sub-region with varied invisible illumination intensity. However, for avoiding of reflections from the user, typically best results are achieved by illuminating and imaging the sub-regions one by one. 
         [0077]    The pointing object(s)  112  may be detected using any of the known techniques. For instance, the methods disclosed in the co-pending patent applications PCT/FI2010/050082 and PCT/FI2008/050475 may be employed. 
         [0078]      FIG. 4  shows a system  410  to illustrate further details regarding possible circuitries for implementing the sub-regions shown in  FIG. 3   a .  FIG. 4  also makes clear how the circuitry may be greatly simplified. This simplifying takes advantage of the rolling shutter nature of the image capture units  107 —when the image capture units  107  produce image pixels for different parts of the image on slightly different moments of time. 
         [0079]    In the system  410 , some of the image capture units  107  are laid onto one or more circuit boards into an array. In sake of brevity, the image capture units  107  are now referred to as cameras  107 . In the array, one or more cameras  107  are connected as one common camera unit  420 . 
         [0080]    The common camera unit  420  comprises a field-programmable gate array (FPGA)  422  that is communicatively connected with each of the cameras  107 . The FPGA  422  is configured to synchronise the cameras  107  so that the image formed by adjacent cameras  107  is formed continuously taking into account the rolling shutter. That is, one camera  107  first exposes a first image and the FPGA  422  scans out that first image. Another camera  107  exposes a second image so that the FPGA  422  scans out the second image continuously after the first image, i.e. so that there is no significant delay after scanning out the first image and before scanning out the second image. If the common camera unit  420  comprises a matrix of cameras  107 , then the FPGA  422  controls the cameras  107  of the common camera unit  420  to be scanned out one by one in the direction of the rolling shutter operation (in  FIG. 4 , either horizontally or vertically). The cameras  107  are thus scanned out row by row, column by column, or in some other order. The timing of the scan-out operations may be such that the FPGA  422  receives image data from different cameras  107  as a substantially continuous data stream. 
         [0081]    The system  410  further comprises one or more common camera unit data buses  430  through which the FPGA  422  passes the data stream for subsequent processing. The common camera unit data buses  430  are data channels that are configured capable of transporting image data from all of the FPGAs  422  connected thereto. 
         [0082]    An interfacing unit  440  is connected to each of the common camera unit data buses and configured to pass all the image data and necessary metadata. The metadata comprises e.g. an identification of the camera  107  from which the image data in question comes from. The metadata is typically provided by the FPGA  422 . 
         [0083]    The interfacing unit  440  may comprise a coordinating processor  442  (such as an FPGA circuitry, central processing unit, digital signal processor or the like), a data input port  444  and possibly a control output port  446  for controlling the FPGAs  422 . 
         [0084]    The interfacing unit further comprises a data output port  448  which may comprise a relative small buffer memory  4482  e.g. to allow retransmissions should data be corrupted over a connection  450  between the system  410  and an auxiliary device that receives the image data (such as a computer, not shown). 
         [0085]    When data is scanned out from a camera  107 , the associated FPGA  422  receives and forwards that data, potentially together with identification of the camera  107 , and the interfacing unit  440  further passes on the image data it receives from the different FPGAs  422 . All of this may take place without need to buffer and re-buffer the scanned-out image data. 
         [0086]    The common camera units may altogether lack memory buffers, whereas in normal implementation, each of the FPGAs  422  would be associated with a memory buffer that is large enough to store at least one entire image frame. Thus, the system  410  can be simplified with the advantages that the manufacturing and maintenance become cheaper and faster and debugging of possible problems is also improved in comparison to an alternative implementation in which the memory buffers are provided. Such an alternative implementation may be more advantageous e.g. when fast data buses are not desired for any reason 
         [0087]    It is appreciated that by logically dividing the touching surface  101  into a plurality of sub-regions and by processing temporally separately the different sub-regions, harmful reflections e.g. from user&#39;s clothing may be greatly reduced. In particular, diagonal reflections from the user may be reduced so that the shadows or light marks reflected by the pointing objects have better contrast to surrounding areas that are deemed not to be contacted. 
         [0088]    It should be appreciated that in this document, words comprise, include and contain are each used as open-ended expressions with no intended exclusivity. Moreover, term light here is interchangeable with radiation. While infrared light has in occasions been used, this terming is merely for convenience of explanation the term light is not intended to imply suitability for perception by means of a human eye. 
         [0089]    The foregoing description has provided by way of non-limiting examples of particular implementations and embodiments of the invention a full and informative description of the best mode presently contemplated by the inventors for carrying out the invention. It is however clear to a person skilled in the art that the invention is not restricted to details of the embodiments presented above, but that it can be implemented in other embodiments using equivalent means without deviating from the characteristics of the invention. 
         [0090]    Furthermore, some of the features of the above-disclosed embodiments of this invention may be used to advantage without the corresponding use of other features. As such, the foregoing description shall be considered as merely illustrative of the principles of the present invention, and not in limitation thereof. Hence, the scope of the invention is only restricted by the appended patent claims.