System and method for differentiating between pointers used to contact touch surface

[0001] The present invention relates generally to touch systems and in particular to a touch system and method for differentiating between pointers used to contact a touch surface.

[0002] Touch systems are well known in the art and typically include a touch screen having a touch surface on which contacts are made using a pointer. Pointer contacts with the touch surface are detected and are used to generate corresponding output pointer position data representing areas of the touch surface where the pointer contacts are made. There are basically two general types of touch systems available and they can be broadly classified as “active” touch systems and “passive” touch systems.

[0003] Active touch systems allow a user to generate pointer position data by contacting the touch surface with a special pointer that usually requires some form of on-board power source, typically batteries. The special pointer emits signals such as infrared light, visible light, ultrasonic frequencies, electromagnetic frequencies, etc. that activate the touch surface.

[0004] Passive touch systems allow a user to generate pointer position data by contacting the touch surface with a passive pointer and do not require the use of special pointers in order to activate the touch surface. A passive pointer can be a finger, a cylinder of some material, or any other suitable object that can be used to contact some predetermined area of interest on the touch surface. Since special active pointers are not necessary in passive touch systems, battery power levels and/or pointer damage, theft, or pointer misplacement are of no concern to users.

[0005] Although passive touch systems provide advantages over active touch systems, many prior art passive touch systems suffer disadvantages in that they are generally not able to tell what type of pointer is used to contact the touch surface i.e. whether a contact is made using a finger, stylus, pen, or the like.

[0006] To deal with this shortcoming, techniques have been considered to provide the ability to differentiate between pointers used to contact a touch surface. For example, U.S. Pat. No. 5,448,263 to Martin, assigned to SMART Technologies, Inc., assignee of the present invention, discloses a passive touch system including an analog resistive touch screen coupled to a computer. Image data generated by the computer is conveyed to a projector, which in turn projects a corresponding image on the touch surface of the touch screen. When a user contacts the touch surface of the touch screen, the touch screen generates pointer position data that is conveyed to the computer. Depending on the mode of operation of the touch system, the computer either records the pointer position data as writing or uses the pointer position data to control execution of an application programs executed by the computer. The computer also updates the image data so that the image presented on the touch surface by the projector reflects the pointer activity.

[0007] To assist the user in making pointer contacts on the touch surface, a tool tray is disposed adjacent the bottom edge of the touch surface. The tool tray includes a number of slots, each slot of which accommodates a different colored pen. When a pen is lifted from its slot, the touch screen generates pen color data that is conveyed to the computer allowing the computer to determine the color of the lifted pen. With the pen color known, when the touch system is in a write mode and the pen is used to contact the touch surface, the computer uses the pointer position data and the pen color data to update the image data so that writing in the color of the selected pen is projected onto the touch surface.

[0008] International PCT Application No. PCT/CA01/00980 filed on Jul. 5, 2001 and published under number WO 02/03316 on Jan. 10, 2002, assigned to SMART Technologies, Inc., assignee of the present invention, discloses a camera-based touch system comprising a touch screen that includes a passive touch surface on which a computer-generated image is presented. A rectangular bezel or frame surrounds the touch surface and supports digital cameras at its corners. The digital cameras have overlapping fields of view that encompass and look along the touch surface. The digital cameras acquire images of the touch surface from different locations and generate image data. The image data acquired by the cameras is processed by digital signal processors to determine if a pointer exists in the captured image data. When it is determined that a pointer exists in the captured image data, the digital signal processors convey pointer characteristic data to a master controller, which in turn processes the pointer characteristic data to determine the location of the pointer relative to the touch surface using triangulation. The pointer location data is conveyed to a computer executing one or more application programs. The computer uses the pointer location data to update the computer-generated image that is presented on the touch surface. Pointer contacts on the touch surface can therefore be recorded as writing or used to control execution of an application program executed by the computer. Similar to the touch system disclosed in U.S. Pat. No. 5,448,263, the camera-based touch system also includes a tool tray accommodating a number of different colored pens that provides pen color data to the computer when a pen is lifted from the tool tray.

[0009] Although the above touch systems provide for discrimination between different color pens confusion can occur. For example, if a user lifts a pen from its slot in the tool tray and then uses a finger to write on the touch surface with the pen slot vacant, the computer will treat the pointer position data generated by the touch screen in response to the finger contact as if the pointer position data was generated in response to contacts made using the lifted pen.

[0010] Touch systems that treat pointer position data differently depending on the areas of the touch surface where contacts are made also exist. In these touch systems, when a computer desktop image is projected onto the touch surface, areas on the touch surface where inherent computer operating display elements such as tool bars, icons etc. are presented, are treated as non-active areas. Pointer contacts on the touch surface over the non-active areas are treated as mouse events irrespective of the type of pointer used to contact the touch surface. Pointer contacts on the touch surface within active areas are treated as writing. Although this type of touch system allows pointer contacts to be differentiated, differentiation of the pointer contacts is based on the areas of the touch surface where contacts are made and not based on pointer type. As will be appreciated touch systems that provide for better differentiation between pointers used to contact touch surfaces are desired.

[0011] It is therefore an object of the present invention to provide a novel touch system and method for differentiating between pointers used to contact a touch surface.

[0012] According to one aspect of the present invention there is provided a touch system comprising:

[0013] a touch surface to be contacted by a pointer;

[0014] at least one imaging device having a field of view looking generally along said touch surface; and

[0015] at least one processor communicating with said at least one imaging device and analyzing images acquired by said at least one imaging device to: (i) determine the type of pointer used to contact said touch surface; and (ii) determine the location on said touch surface where pointer contact is made.

[0016] According to another aspect of the present invention there is provided a method of updating image data used to present a displayed image on a touch surface in response to pointer contact on said touch surface, said method comprising the steps of:

[0017] displaying an image on said touch surface;

[0018] generating pointer position data in response to pointer contact with said touch surface and acquiring images of pointers used to contact said touch surface;

[0019] analyzing the acquired images to determine the type of pointer used to contact the touch surface; and

[0020] updating image data used to present the displayed image on said touch surface based on the generated pointer position data and the determined type of pointer.

[0021] According to yet another aspect of the present invention there is provided in a touch system including a touch surface, at least one pen tool to contact said touch surface, a processor recording data generated in response to contacts made on said touch surface using said at least pen tool and a camera to acquire an image of said at least one pen tool when said at least one pen tool is generally in contact with said touch surface, a method of updating recorded data comprising the steps of:

[0022] examining the acquired image to determine which end of the at least one pen tool is used to contact the touch surface; and

[0023] modifying the recorded data based on the end of the at least one pen tool that is used to contact the touch surface.

[0024] According to yet another aspect of the present invention there is provided in a touch system including a touch surface on which an image is presented, a processor responsive to contacts made on the touch surface using a pointer and generating image data used to present said image, a method of updating said image data comprising the step of:

[0025] assessing a pointer attribute to determine the type of pointer used to make said contacts; and

[0026] updating the image data based on the type of pointer and the pointer activity on the touch surface.

[0027] According to still yet another aspect of the present invention there is provided a touch system comprising:

[0028] a touch surface on which an image is presented; and

[0029] a processor responsive to pointer contacts on the touch surface and generating image data that is used to present said image, said processor assessing a pointer attribute to determine the type of pointer used to make contacts and updating the image data based on the type of pointer and the pointer activity on the touch surface.

[0030] According to still yet another aspect of the present invention there is provided a camera-based touch system comprising:

[0031] a pair of cameras at spaced locations having overlapping fields of view encompassing an area on which pointer contacts are to be made, said cameras acquiring images and generating corresponding image data; and

[0032] a processor receiving and processing the image data generated by said cameras to determine the location of a pointer relative to the area when the pointer is captured in images acquired by the cameras and analyzing the image data to determine the type of pointer used to contact the area.

[0033] According to still yet another aspect of the present invention there is provided an interactive input system comprising:

[0034] at least one imaging device having an input region within its field of view into which a pointer is moved to generate user input; and

[0035] at least one processor communicating with said at least one imaging device and analyzing each image acquired by said at least one imaging device to determine the type of pointer moved into said input region.

[0036] The present invention provides advantages in that the type of pointer used to contact the touch surface is determined automatically. This enables the touch system to differentiate between contacts made on the touch surface using a finger and pen tool accurately and quickly. As a result, a user is provided with the ability to write, draw or annotate with a pen tool and then immediately use a finger to generate mouse events and manipulate the pen tool input without having to put the pen tool down. This provides the user with a seamless ability to write into and control application programs without purposefully having to change the mode of operation of the touch system prior to initiating touch events with different pointers.

[0037] Furthermore, since the type of pointer used to contact the touch surface is determined automatically, the need for tool tray electronics that generate pointer color data when a pointer is lifted from the tool tray is reduced or eliminated. This of course reduces costs.

[0038] In addition, since pointer contacts and pointer type are determined from images, multiple simultaneous contacts on the touch surface using different pointers can be determined.

[0039] The present invention also provides advantages in that since the touch system is able to differentiate between pointer tip and pointer backend contacts with the touch surface, different functions can be assigned to opposite ends of a pointer. As a result, a single pointer can be used to perform both a writing function and an erase function without purposefully having to change the mode of operation of the touch system. Using a pointer backend to initiate an erase function is of course highly user intuitive. Providing identifying glyphs on or shaping the tips of different pointers uniquely also allows different attributes, such as colors assigned to the different pointers, to be determined automatically.

[0050] The present invention relates to a touch system and method that differentiates between pointers used to contact a touch surface so that pointer position data generated in response to a pointer contact with the touch surface can be processed in accordance with the type of pointer used to contact the touch surface. The touch system includes a touch surface to be contacted by a pointer and at least one imaging device having a field of view looking generally along the touch surface. At least one processor communicates with the at least one imaging device and analyzes images acquired by the at least one imaging device to determine the type of pointer used to contact the touch surface and the location on the touch surface where pointer contact is made. The determined type of pointer and the location on the touch surface where the pointer contact is made are used by a computer to control execution of an application program executed by the computer. Preferred embodiments of the present invention will now be described with reference to FIGS. 1 to 10.

[0051] Turning now to FIGS. 1 and 2, a camera-based touch system in accordance with the present invention is shown and is generally identified by reference numeral 50. Camera-based touch system 50 is similar to that disclosed in U.S. patent application Ser. No. 10/354,168 for an invention entitled “Illuminated Bezel and Touch System Incorporating Same” filed on Jan. 30, 2003 assigned to SMART Technologies Inc., assignee of the present invention, the content of which is incorporated herein by reference. As can be seen, touch system 50 includes a touch screen 52 coupled to a digital signal processor (DSP) based master controller 54. Master controller 54 is also coupled to a computer 56. Computer 56 executes one or more application programs and provides computer-generated image output that is presented on the touch screen 52. The touch screen 52, master controller 54 and computer 56 form a closed-loop so that pointer contacts with the touch screen 52 can be recorded as writing or drawing or used to control execution of an application program executed by the computer 56.

[0052] FIG. 2 better illustrates the touch screen 52. Touch screen 52 in the present embodiment includes a high-resolution display device such as a plasma display 58, the front surface of which defines a touch surface 60. The touch surface 60 is bordered by an illuminated bezel or frame 62 coupled to the display device. Illuminated bezel 62 includes elongate side frame assembly 64 that are coupled to the sides of the plasma display 58. Each side frame assembly 64 accommodates a generally continuous illumination source (not shown). The ends of the side frame assembly 64 are joined by corner pieces 68 that house DSP-based CMOS digital cameras 70 (see FIG. 3). Each digital camera is mounted within its respective corner piece 68 so that its field of view encompasses and looks across the entire touch surface 60.

[0053] A tool tray 76 is positioned adjacent the bottom edge of the touch surface 60 and accommodates a plurality of different color pointers in the form of pen tools 76a used to contact the touch surface 60 as shown in FIGS. 6a and 6b. Each pen tool has a generally conical tip 76b defining an opening angle ø. The opening angle is an invariant property of the pen tool irrespective of the pen tool orientation when captured in an image. In the present embodiment, the tool tray 76 is passive and simply provides a convenient location to place the pen tools. If desired however, the tool tray 76 may include active functionality.

[0054] One of the digital cameras within a corner piece 68 is shown in FIG. 3. As can be seen, each digital camera 70 includes a two-dimensional CMOS image sensor and associated lens assembly 80, a first-in-first-out (FIFO) buffer 82 coupled to the image sensor and lens assembly 80 by a data bus, and a digital signal processor (DSP) 84 coupled to the FIFO 82 by a data bus and to the image sensor and lens assembly 80 by a control bus (see FIG. 4). A boot EPROM 86 and a power supply subsystem 88 are also included. In the present embodiment, the CMOS camera image sensor is configured for a 20×648 pixel subarray that can be operated to capture image frames at rates in excess of 200 frames per second since arbitrary pixel rows can be selected. Also, since the pixel rows can be arbitrarily selected, the pixel subarray can be exposed for a greater duration for a given digital camera frame rate allowing for good operation in dark rooms as well as in lit rooms.

[0055] The DSP 84 provides control information to the image sensor and lens assembly 80 via the control bus. The control information allows the DSP 84 to control parameters of the image sensor and lens assembly 80 such as exposure, gain, array configuration, reset and initialization. The DSP 84 also provides clock signals to the image sensor and lens assembly 80 to control the frame rate of the image sensor and lens assembly 80.

[0056] An infrared pass filter 89 is provided on the digital camera image sensor and lens assembly 80 to blind the digital camera 70 to frequencies of light other than the light broadcasted by the illuminated bezel 62.

[0057] Master controller 54 is best illustrated in FIG. 4 and includes a DSP 90, a boot EPROM 92, a serial line driver 94 and a power supply subsystem 95. The DSP 90 communicates with the DSPs 84 of the digital cameras 70 over a data bus via a serial port 96 and communicates with the computer 56 over a data bus via a serial port 98 and the serial line driver 94.

[0058] The master controller 54 and each digital camera 70 follow a communication protocol that enables bi-directional communications via a common serial cable similar to a universal serial bus (USB). The transmission bandwidth is divided into thirty-two (32) 16-bit channels. Of the thirty-two channels, six (6) channels are assigned to each of the DSPs 84 in the digital cameras 70 in to the DSP 90 and the master controller 54 and the remaining two (2) channels are unused. The master controller 54 monitors the twenty-four (24) channels assigned to the DSPs 84 while the DSPs 84 monitor the six (6) channels assigned to the DSP 90 of the master controller 54. Communications between the master controller 54 and the digital camera 70 are performed as background processes in response to interrupts.

[0059] The operation of the touch system 50 will now be described. Each digital camera 70 acquires images looking along the touch surface 60 within the field of view of its image sensor and lens assembly 80 at a desired frame rate and processes each acquired image to determine if a pointer is in the acquired image. During image capture, the illuminated bezel provides adequate backlighting for the digital cameras 70. If a pointer is in the acquired image, the image is further processed to determine characteristics of the pointer contacting or hovering above the touch surface 60 such as pointer type. Pointer information packets (PIPs) including pointer characteristics, status and/or diagnostic information are then generated by the digital camera 70 and the PIPs are queued for transmission to the master controller 54. The digital cameras 70 also receive and respond to command PIPs generated by the master controller 54.

[0060] The master controller 54 polls the digital cameras 70 for PIPs. If the PIPs include pointer characteristic information, the master controller 54 triangulates pointer characteristics in the PIPs to determine the position of the pointer relative to the touch surface 60 in Cartesian rectangular coordinates and the type of pointer used to contact the touch surface 60. The master controller 54 in turn transmits calculated pointer position data, pointer type data, status and/or diagnostic information to the computer 56. In this manner, the pointer position data transmitted to the computer 56 can be recorded as writing or drawing or can be used to control execution of an applications program executed by the computer 56 depending on the type of pointer used to contact the touch surface 60. The computer 56 also updates the computer-generated image output conveyed to the plasma display 58 so that the information presented on the touch surface 60 reflects the pointer activity.

[0061] The master controller 54 also receives commands from the computer 56 and responds accordingly as well as generates and conveys commands to the digital camera 70. Specifics concerning the triangulation of pointer characteristics in PIPs are described in PCT Application No. WO 02/03316, assigned to SMART Technologies, Inc., assignee of the present invention, the content of which is incorporated herein by reference. Accordingly, specifics will not be described herein.

[0062] To enable automatic pointer discrimination, each DSP 84 is programmed to analyze the acquired images to determine whether a pen tool, or a finger is used to contact the touch surface 60. Each DSP 84 reports the determined pointer type to the master controller 54, which in turn reports the pointer type and pointer position to the computer 56. In this manner, since each DSP 84 can differentiate between the types of pointers used to contact the touch surface 60, the function assigned to each pointer can be determined from the images acquired by the digital cameras 70. This allows the computer 56 to process the pointer position and pointer type data and update the image data in the appropriate manner.

[0063] In order to determine the type of pointer used to contact the touch surface 60 from the images acquired by the digital cameras 70, each DSP 84 uses a curve of growth method to differentiate between different pointers. During this method, a horizontal intensity profile (HIP) is formed by calculating a sum along each row of pixels in the images (FIG. 5a) thereby to produce a one-dimensional profile having a number of points equal to the row dimension of the image (see FIG. 5b). A curve of growth is then generated from the HIP by forming the cumulative sum from the HIP (FIG. 5c). In other words, starting at the left most point of the HIP, a successive integration of the HIP from the left most point of the HIP to the right most point of the HIP is performed. The resulting integrated area corresponds to a specific point on the curve of growth. For a pen tool, the curve of growth has a distinct non-linear shape, while for a finger tip, the curve of growth is very close to linear. This is due to the fact that the tip of the pen tool is generally conical whereas a fingers tip is generally cylindrical. To distinguish between non-linear and linear curves of growth, a least-squares line is fitted to the curve of growth, the linear trend from the curve of growth is subtracted and the sum of squares of residuals (otherwise known as 2vχ) is formed. The sum of squares of residuals is defined as:

[0064] $X_v^2=\frac{1}{N-2}\sum _{i=1}^N{\left[\mathrm{COG}-\mathrm{CALC}\right]}^2$
The factor of N−2 is the number of degrees of freedom for a linear fit to N points. This number reduces the problem of pointer-finger recognition to just examining the value of the sum of squares of residuals. If the sum of squares of residuals is close to 1, then a finger tip is present while if it is much larger than a pen tool is present.

[0065] In the present embodiment, when the DSPs 84 identify the pointer that is used to contact the touch surface as a finger, the computer 56 treats the pointer contact as a mouse event. In this manner, a user is able to use a finger to manipulate (i.e. move, resize etc.) pen tool writing or control execution of an applications program with a pen tool in hand without having to change the mode of operation of the touch screen 52. When the DSPs 84 identify the pointer that is used to contact the touch surface as a pen tool, the computer 56 treats the pointer position data as writing.

[0066] If desired, different pen tools can be provided with uniquely shaped conical tips and assigned different attributes, each conical tip having a unique opening angle as shown in FIGS. 6a and 6b. In this case, if a pen tool is used to contact the touch surface 60, the DSPs 84 further analyze the images to recognize the unique shape of the pen tool tip.

[0067] To differentiate between different pen tools, the curves of growth are examined. Since the tip of each pen tool has its own unique shape, the curve of growth generated for each pen tool is different. This allows different pen tools used to contact the touch surface 60 to be determined. For example, if each pen tool is assigned a different color, the unique curve of growth generated for each pen tool can be used to assign the appropriate color to the pen tool so that writing made on the touch surface 60 using each pen tool appears in the appropriate color.

[0068] Although the pen tools are described as having conical tips with different opening angles, the various pen tools can be formed with different shaped tips of virtually any configuration that result in unique curves of growth being generated to provide for pointer discrimination.

[0069] Although the operation of the touch system is described with reference to single contacts made on the touch surface 60, as will be appreciated by those skilled in the art, since camera images are used to determine pointer contacts, multiple pointer contacts on the touch surface 60 can be determined since each pointer will appear in the images captured by each camera. The type of each pointer can be determined in the same manner described above and used during processing of the resulting pointer location data. In this manner a user may contact the touch surface simultaneously with different color pen tools or with a pen tool an a finger.

[0070] FIGS. 7a and 7b show alternative pen tools 78. In this embodiment, each pen tool 78 includes a cylindrical body 78a having a generally conical tip 78b at one end and a square backend 78c. A glyph or distinguishing mark 78d is provided on the pen tool 78 adjacent its tip 78b. The glyph 78d on each pen tool is different and represents a unique attribute, such as color, assigned to the pen tool. For example, in the present embodiment, the red pen tool has a single V-shaped groove on its tip 78b turned along the axis of the pen tool. The blue pen has two V-grooves on its tip 78b turned along the axis of the pen tool. The green pen has three V-shaped grooves on its tip 78b turned along the axis of the pen tool. The black pen tool is devoid of a glyph on its tip 78b. If the touch screen 52 includes additional different colored pen tools, different shaped glyphs are provided on the tips of these additional pen tools to allow the touch system 50 to determine their colors.

[0071] In this embodiment, rather than relying on curves of growth to differentiate between the pointer used to contact the touch surface, each DSP 84 uses an artificial neural network (ANN) to determine the type of pointer used to contact the touch surface 60 from the images. As is well known ANN's are ideally suited to solving problems of an ambiguous nature such as multi-parameter target detection when the parameters are not known. There are no specific algorithms, procedures, or rules that are pre-programmed in an ANN, rather only a training method. The training method allows the ANN to establish its own set of parameters and rules to solve the pattern recognition problem.

[0072] In the present embodiment, the ANN is specifically trained to analyze the images acquired by the digital cameras 70 and recognize whether a pointer tip, a pointer backend, or a finger is used to contact the touch surface 60. During the analysis of each camera image, the ANN generates decision output that identifies the pointer used to contact the touch surface 60 with some degree of probability. For each set of images acquired by the digital cameras 70, the ANN decision output with the highest degree of probability is used by the computer 56.

[0073] When the ANN decision output identifies the pointer that is used to contact the touch surface 60 as a finger, the computer 56 treats the pointer contact data as a mouse event. In this manner, a user is able to use a finger to manipulate (i.e. move, resize etc.) pen tool writing input or control execution of application programs with a pen tool in hand without having to change the mode of operation of the touch screen. When the ANN decision output identifies the pointer that is used to contact the touch surface 60 as a pointer backend 78c, the computer 56 treats the pointer as an erase tool. The pointer backend makes a nice intuitive erase tool because the pen tool itself works like a pencil, its shape is distinct and it avoids the need for pointers to be switched in order to perform an erase function.

[0074] When the ANN decision output identifies the pointer that is used to contact the touch surface 60 as a pointer tip 78b, the computer 56 treats the pointer position data as writing or drawing. In order to determine the color to be assigned to the writing or drawing, the ANN further analyses the images acquired by the digital cameras 70 to recognize the glyph 78d on the pointer tip 78b, since the glyph on the pointer tip represents the color assigned to the pen tool.

[0075] During image analyses, a model fitting approach is used wherein each gray-level value at each pixel (i.e., (x,y) location) of a difference image is described by a model of several parameters. These model parameters include:

[0076] the mean gray level within the pointer, and that in the background;

[0077] the (x, y) location of the pointer tip 78b;

[0078] the orientation angles of the leading and trailing edges of the pointer tip; and

[0079] the resolution of the digital cameras 70 used to acquire the images.

[0080] The mathematical model functions are combinations of Gaussian error functions as disclosed by Blaszka, T. and Deriche, R. in “Recovering and Characterizing Image Features Using an Efficient Model Based Approach”, 1994, Rapport de recherche No. 2422, INRIA. Images are compared pixel-by-pixel, in order to fit the model to the image Statistically, this comparison is performed using the reduced chi-squared statistic

[0081] ${\chi }_v^2=\frac{1}{N-M}\sum _{i,j}{\left[\begin{array}{cc}{I}_{i,j}^{\mathrm{obs}}& -I_{i,j}^{\mathrm{calc}}\end{array}\right]}^2$
Here, v=N−M is the number of degrees of freedom, i.e., the number of data points minus the number of parameters being fitted. To find the best-fit set of parameters, χ_v² is minimized with respect to these parameters.

[0082] The minimization may be computed in any number of ways, but the most effective methods are Levenberg-Marquardt (LM), and Variable metric (VM). Both methods need the derivatives of χ_v² with respect to each of the model parameters at each point on the image. As the model functions are nonlinear, these derivatives are best computed numerically, using either centered differences:

[0083] $\frac{\partial f}{\partial x}=\frac{f\left(x+h\right)-f\left(x-h\right)}{2h}$
or forward differences:

[0084] $\frac{\partial f}{\partial x}=\frac{f\left(x+h\right)-f\left(x\right)}{h}$
Here, h10⁻⁴ is a small number added to each parameter. So, at each point we compute the gradient of χ_v²:

[0085] $\nabla {\chi }_v^2=\left[\begin{array}{ccccccc}\frac{\partial {\chi }_v^2}{\partial A}& \frac{\partial {\chi }_v^2}{\partial B}& \frac{\partial {\chi }_v^2}{\partial x_0}& \frac{\partial {\chi }_v^2}{\partial y_0}& \frac{\partial {\chi }_v^2}{\partial {\theta }_1}& \frac{\partial {\chi }_v^2}{\partial {\theta }_2}& \frac{\partial {\chi }_v^2}{\partial \sigma }\end{array}\right]$

[0086] In the LM method, this gradient forms one row of the Jacobian matrix J. Assume for the moment that there is some initial set of parameters p which will allow J to be computed for the whole image. To improve this initial estimate, the LM method requires the solution of the matrix equation:
[J^TJ+λI]Δp=−J^Tr
for the correction vector Δp, i.e.,
p_new=p_old+Δp
Here, r is the residual vector, the difference between the observed and computed image brightness at each pixel. Effectively, this residual vector drives the solution process. The LM parameter λ is used to govern the convergence of this iterative procedure; if the magnitudes of the residuals are increasing, λ is increased, otherwise it decreases as the best-fit solution for p is approached. The LM method is a fast and robust procedure. However, it requires the storage and manipulation of some rather large matrices.

[0087] The VM method solves this problem in a convenient way; it constructs directly a matrix similar to:
[J^TJ+λI]⁻¹
known as the inverse Hessian. The Hessian itself rigorously is the matrix of all second derivatives of χ_v² with respect to all of the model parameters. In other words, the calculation
p_new=p_old+H⁻¹Δp
is done by constructing H⁻¹ directly from ∇χ_v². The only minor drawback to this approach is that one must now use a line search to get Δp (and hence H⁻¹) at each iteration, which means numerous evaluations of χ_v² (but not its gradient). The VM method takes several iterations to converge, but each iteration is much faster than a single LM iteration.

[0088] The VM and LM iterative non-linear least-squares techniques adjust all of the model parameters simultaneously and have been found to:

[0089] locate the pointer tip or finger with a precision of approximately 0.1 pixel;

[0090] segment reliably the pointer tip or finger from the background; and

[0091] recognize reliably the type of pointer through the model resolution parameter.

[0092] These iterative non-linear least-squares techniques have also been found to yield sharper images when a pointer tip is used to contact the touch system as compared to when a finger is used to contact the touch surface, in terms of the model resolution parameter. This difference in image sharpness is due to the fact that the model describes the pointer as a corner, i.e., two straight lines connected at a common vertex and separated by a given angle. As will be appreciated, this model description fits the shape of a conical pointer tip much better than it does a finger. To compensate for this difference in image sharpness, the iterative non-linear least-squares technique adjusts the model resolution parameter accordingly. In other words, the iterative non-linear least-squares technique smoothes the corner in order to better approximate a finger shape. The end result is that the model resolution parameter tends statistically to be larger for fingers than for pen tools. The following is a typical result for the mean model resolution parameter:
σ=1.62±0.31 (pen)
σ=2.18±0.24 (finger)
where the units of measurement are pixels. These results were found by analyzing ten images at random from two video sequences (which typically contain 250 to 500 frames). Similar results were found for four other video sequences. FIGS. 8 and 9 show typical fittings generated by a MATLAB simulation in response to touch surface contacts made using a pen tool and a finger, respectively. Note that a larger value of χ_v² is more an indication that the model assumptions have been broken in some way (i.e., it is not possible to model every light variation seen in an image).

[0093] Although the present technique to differentiate between pointers used to contact a touch system is described as being incorporated in a touch system that uses camera images to generate pointer position data, those of skill in the art will appreciate that the technique may be used in other interactive environments. For example, turning now to FIG. 10, a touch system similar to that disclosed in U.S. Pat. No. 5,448,263 to Martin, assigned to the assignee of the present invention, is shown. As can be seen, touch system 150 includes an analog resistive touch screen 152 having a touch surface 160 on which computer-generated images are projected. A pointer tray 164 is provided along the bottom edge of touch surface 160 and holds pointers in the form of pen tools that are used to contact the touch surface 160. The pointer tray 164, similar to the previous embodiment, simply provides a convenient location to store pen tools. A camera 170 that is aimed generally along the plane of the touch surface 160 is provided adjacent one side of the touch surface and captures images.

[0094] When a pointer contact is made on the touch surface 160, the touch screen 152 generates pointer position data output representing the location on the touch surface where the pointer contact is made. The pointer position data that is output by the touch screen 152 is conveyed to a computer 156. The computer 156 executes one or more application programs and provides computer-generated image output that is displayed on the touch surface 160 via a projector 158. The touch screen 152, computer 156 and projector 158 form a closed-loop so that pointer contacts with the touch surface 160 can be recorded as writing or drawing or used to control execution of application programs executed by the computer 156. The computer-generated image output by the computer 156 is also updated to reflect the pointer activity.

[0095] When the computer 156 receives pointer position data from the touch screen 152, the computer also receives pointer type data from the camera 170. The camera 170 processes captured images in the same manner described above to determine the type of pointer used to contact the touch surface 160 that resulted in the pointer position data being generated. In this manner, the computer can process the pointer position and pointer type data in the appropriate manner. If desired, the captured images can be conveyed to the computer 156 for processing to determine pointer type.

[0096] Although touch systems which make use of image data to determine the type of pointer used to contact the touch surface have been described above, automatic pointer differentiation in touch systems employing active pointers can also be achieved. For example, in one such touch system an electromagnetic digitizer is placed behind an analog resistive touch sensitive screen. When a finger is used to contact the touch sensitive screen, only the touch sensitive screen generates pointer position data. However, when an active pointer is used to contact the touch sensitive screen, both the touch sensitive screen and the electromagnetic digitizer generate pointer position data. This difference in pointer position data output is used by the computer to allow contacts made on the touch surface of the touch sensitive screen using an active pen and a finger to be automatically differentiated. The signature of the active pen can be selected to specify its color allowing the computer to determine the color of the active pen used to contact the touch surface.

[0097] Alternatively, active radio frequency (RF) pens can be used with an analog resistive touch sensitive screen having a controller capable of recording the RFID tags broadcast by such pens when they are used to contact the touch surface. Since the controller of the touch sensitive screen receives pointer position data and an RFID tag when an RF pen is used to contact the touch surface but only receiving pointer position data when a finger is used to contact the touch surface, the computer is able to differentiate automatically between finger and RF pen contacts on the touch surface. The RFID tag can also be selected to specify the color of the RF pen. Active pens can also be used with a camera-based touch system.

[0098] As will be appreciated by those of skill in the art, the present invention is suitable for use in basically any environment where it is desired to distinguish between pointers used to contact a touch surface so that different functions can be invoked automatically without purposefully having to change the mode of operation. Such environments include for example, electronic whiteboards, touch liquid crystal display (LCD) panels, personal digital assistants (PDAs) and portable touch systems.

[0099] Although preferred embodiments of the present invention have been described, those of skill in the art will appreciate that variations and modifications may be made without departing from the spirit and scope thereof as defined by the appended claims.