Patent Grant
7492357
The present invention relates generally to interactive input systems and in particular to an apparatus and method for detecting a pointer relative to a touch surface.
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 in order to generate user input. Pointer contacts with the touch surface are detected and are used to generate corresponding output depending on areas of the touch surface where the pointer contacts are made. Common touch systems utilize analog resistive, electromagnetic, capacitive, acoustic or machine vision techniques to identify pointer contacts on the touch surface.
For example, International PCT Application No. PCT/CA01/00980 filed on Jul. 5, 2001 and published under No. 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 generally across the plane of the touch surface. The digital cameras acquire images looking across the touch surface from different locations and generate image data. Image data acquired by the digital 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 in (x,y)-co-ordinates relative to the touch surface using triangulation. The pointer co-ordinate data is conveyed to a computer executing one or more applications programs. The computer uses the pointer co-ordinate 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 drawing or used to control execution of applications programs executed by the computer.
Although the above touch system works extremely well, improvements in vision-based touch systems are continually being sought.
It is therefore an object of the present invention to provide a novel apparatus and method for detecting a pointer relative to a touch surface.
According to one aspect of the present invention there is provided an apparatus for detecting a pointer relative to a touch surface comprising at least two spaced imaging devices having overlapping fields of view encompassing the touch surface. The imaging devices see the touch surface in three-dimensions as a perspective view. The imaging devices acquire images from different locations. A processor receives and processes image data generated by at least one of the imaging devices to determine the location of the pointer relative to the touch surface.
Each imaging device is calibrated to establish the relationship between points (X,Y,Z) in its perspective view and points (x,y) in acquired images. Each imaging device generates pointer co-ordinate data when a pointer is captured in an acquired image. The processor triangulates the pointer co-ordinate data to determine the location of the pointer relative to the touch surface.
In one embodiment, the apparatus includes a pair of imaging devices with each imaging device being positioned adjacent a different corner of the touch surface. Each imaging device is spaced from and spaced in front of the touch surface. Each imaging device is positioned relative to the touch surface so that at a minimum the entire periphery of the touch surface is within its perspective view.
In one embodiment, during calibration, calibration points (X,Y,Z) on the touch surface and image points (x,y) corresponding to the calibration points are measured. Collinearity equations are solved using the measured calibration and image points to determine external and internal orientation parameters of the imaging devices. The collinearity equations are solved using a least-squares method. The calibrations points are at spaced location along the periphery of the touch surface such as at the corners and edge mid-points of the touch surface. In an alternative embodiment, the external orientation parameters of the imaging devices are determined using a vanishing point method. In yet another embodiment, the external and internal orientation parameters of the imaging devices are determined using planar homography. In still yet another embodiment, the external orientation parameters of the imaging devices are determined using a three-point method.
In one embodiment, each imaging device generates a certainty value representing the degree of certainty that the imaging device has positively identified the pointer of the acquired image. The certainty value is used by the processor to determine pointer co-ordinate data to be used to determine the position of the pointer relative to the touch surface. The processor ignores pointer co-ordinate data generated by the imaging device when the certainty value associated therewith is below a threshold level.
According to another aspect of the present invention there is provided a camera-based touch system comprising a generally rectangular passive touch surface on which contacts are made using a pointer. Camera devices are removably mounted adjacent at least two corners of the touch surface. Each of the camera devices has a field of view looking across and back towards the touch surface and is disposed in front of the plane of the touch surface. The fields of view of the camera devices overlap over the touch surface. The camera devices acquire images of the touch surface. A processor receives and processes the image data to determine the location of the pointer relative to the touch surface via triangulation.
According to yet another aspect of the present invention there is provided an apparatus for detecting a pointer relative to a generally rectangular touch surface comprising at least two spaced imaging devices having overlapping fields of view encompassing the touch surface. The imaging devices see the touch surface in three-dimensions as a perspective view with the perspective view at a minimum including the four corners of the touch surface. The imaging devices acquire overlapping images from different locations. A processor receives and processes image data generated by at least one of the imaging devices to determine the location of the pointer relative to the touch surface using triangulation.
The present invention provides advantages in that since the imaging devices see the touch surface in three-dimensions as a perspective view, the imaging devices see the entire touch surface as well as its surrounding area. As a result, during image processing it is not necessary to process the entire images captured by the imaging devices but rather only pixels corresponding to information within the boundaries of the touch surface. Noise and other aberrations occurring in areas outside of the touch surface can be disregarded. In addition, the three-dimensional perspective of the imaging devices allows the apparatus to be automatically calibrated and calibrated on a continuing basis without the need for user intervention. Furthermore, the three-dimensional perspective of the imaging devices allows (x,y,z) co-ordinates to be assigned to each pointer appearing within the fields of view of the imaging devices. Thus, the apparatus is able to disambiguate between multiple pointers contacting the touch surface.
The present invention also provides advantages in that since the imaging devices are portable, they can be used to turn basically any surface into a touch surface. The imaging devices simply need to extend forwardly of the surface a sufficient distance so that their fields of view looking back and across the touch surface see the corners of the surface and are not obstructed by any bezel or framing surrounding the surface. The use of portable imaging devices that see the touch surface in three-dimensions as a perspective view also supports arbitrarily large or curved touch surfaces.
Embodiments of the present invention will now be described more fully with reference to the accompanying drawings in which:
a to 8d show the number of pixel rows in a captured image that must be processed for different spacings between an imaging assembly and the plane of the touch surface;
a to 12c show different pointer contacts on the touch surface of the apparatus of
Turning now to
Turning now to
Housing 72 also accommodates a circuit board on which the digital camera 74 and image processing circuitry 80 are mounted as shown in
The general operation of the touch system 50 will firstly be described. When a pointer is brought into the fields of view of the digital cameras 74 and contacts the touch surface 54, each of the digital cameras 74 acquires an image including the pointer. The DSP 82 of each imaging assembly 58 moves the image data from the digital camera 74 to the SRAM 84 and then compares the image data with a mathematical model describing the boundary of the touch surface 54 as seen by the digital camera 74. This enables a pixel subset of the captured image including only relevant pixels to be processed. The DSP 82 of each imaging assembly 58 in turn processes the pixel subset to determine the (x,y) position of the pointer within the captured image. The imaging assemblies 58 in turn convey this pointer co-ordinate data to the computer 60, which uses the pointer co-ordinate data to calculate the (X,Y,Z) location of the pointer contact on the touch surface 54 using triangulation. The pointer location data is then either recorded by the computer 60 as writing or drawing or interpreted as a mouse or other event.
In order to enable pointer contacts relative to the touch surface 54 to be calculated accurately using triangulation, the touch system 50 needs to be calibrated so that either or both imaging assemblies 58 know how a particular point in a captured image relates to a physical three-dimensional location on the touch surface 54 (the “scene”). During calibration, a transformation that establishes the relationship between any point in the three-dimensional scene that each imaging assembly 58 observes and any point in a captured two-dimensional image is established. This step is necessary, since when the touch system 50 is set up, a priori information concerning the locations and orientations of the digital cameras 74 with respect to a touch surface 54 are not known.
The relationship between a point in a three-dimensional scene and its two-dimensional position in a captured image is summarized according to the collinearity equations as follows:
where:
(x,y) are the co-ordinates of a point in a captured image corresponding to a point (X,Y,Z) in the three-dimensional scene;
(x0, y0) are the co-ordinates of the principal point of the digital camera 74, the location at which the optical axis of the digital camera meets the focal plane of the digital camera with the optical axis being approximately normal to the focal plane;
(Δx,Δy) represent distortion terms introduced due to the imperfect nature of the lens 88;
f is the digital camera focal length; and
(R1,R2,R3) are terms depending on point (X,Y,Z), the spatial location of the optical center of the digital camera (X0,Y0,Z0) and the orientation angles (ω,φ,κ) of the digital camera optical axis with respect to the three-dimensional co-ordinate system of the touch surface 54.
The above collinearity equations represent a pinhole model. Thus, each digital camera 74 is idealized as a very small aperture at the location of the digital camera's optical center (focal point), which is taken to be the position of the digital camera in three-dimensional space. The three-dimensional nature of the digital cameras' view is important in that it allows the digital cameras to see over the bezels 56, if the touch surface 54 is assumed to be planar allows the plane of the touch surface to be determined, allows a determination to be made at any point as to whether a pointer is in contact with the touch surface 54 or hovering above the touch surface 54, and allows the position of the pointer relative to the touch surface 54 to be determined.
The above collinearity equations express that a point in a three-dimensional scene with co-ordinates (X,Y,Z) projects into a two-dimensional image at point (x,y). In order to establish the transformation using the collinearity equations, the external orientation parameters (X0,Y0,Z0) and ω,φ,κ and the internal orientation parameters f,x0,y0 and Δx,Δy of the digital cameras 74 need to be determined.
The distortion of each lens 88 can be represented by terms relating specifically to both radial distortion and decentering distortion. Due to the relativity low quality of each lens 88 and captured image data, in the present embodiment, only the first-order radial distortion term is recorded. As a result the lens distortion terms can be expressed as:
Δx=(x−x0)K1((x−x0)2+(y−y0)2)
Δy=(y−y0)K1((x1−x0)2+(y−y0)2)
Thus, lens distortion can be summarized through parameter K1.
As will be appreciated, ten (10) parameters for each digital camera 74 need to be determined from the collinearity equations to calibrate each digital camera, namely:
X0,Y0,Z0,ω,φ,κ,f,x0,y0,K1
In the present embodiment, a self-calibration technique is used to calibrate the touch system 50. Self-calibration is based on the fact that if the three-dimensional positions of reference points on an object are known and the two-dimensional positions of the same points can be measured in one or more images of the object, these data completely specify the location of the imaging assembly capturing the image, the angular orientation of the imaging assembly and parameters relating to the lens of the imaging assembly.
The positions (X0,Y0,Z0)of the digital cameras 74 in three-dimensional space may be measured in absolute units (e.g., centimeters) or in relative terms by assuming a unit of length corresponding to a reference length such as for example the shorter dimension of the touch surface 54. Each digital camera's angular orientation is represented by the three angles ω, φ, κ allowing a rotation matrix R for each digital camera 74 to be defined. The rotation matrix R describes the transformation between the co-ordinate system of the three-dimensional scene and that of the digital camera. Calculating the focal length f, principal point (x0,y0), and lens distortion coefficient(s) for each digital camera 74 is not necessary if precise values for these digital camera parameters are known.
During self-calibration, it is assumed that the touch surface 54 corresponds to the X-Y plane, and that the Z axis is pointing outward from the touch surface 54 at an angle generally normal to the plane of the touch surface. If image positions (x,y) corresponding to a number of scene points (X,Y,Z) are measured from an image, and the positions of the scene points (X,Y,Z) are known (e.g., in centimeters), then the collinearity equations may be set up for each point and solved using a least-squares technique to enable the external and internal orientation parameters to be determined. The least-squares method is used due to the non-linear nature of the collinearity equation model.
In the present embodiment, eight (8) calibration points around the periphery of the touch surface 54 are chosen since by doing so yields sixteen (16) equations and ten (10) unknowns, which is sufficient for a good least-squares solution. In particular, the four corners and the mid-points along each side edge of the touch surface 54 are selected as the calibration points since the (X,Y) positions at these calibration points are easy to measure, provide reproducible calibration points, are easily located by users and at each of these (X,Y) positions Z=0 cm. The corresponding image points are defined by either measuring the positions of a pointer at the calibration points captured in an image, or by measuring the positions of markers at the calibration points in a captured image.
Turning now to
With the touch system 50 calibrated, the three-dimensional position of a pointer above or in contact with the touch surface 54 can be calculated via triangulation. During triangulation it is assumed that all of the camera orientation parameters are known and the pointer position (X,Y,Z) is to be determined given corresponding (x,y) measurements from the image captured either by one or both digital cameras 74.
Triangulation is then performed using the collinearity equations referred earlier either using the pointer co-ordinate data from both imaging assemblies 58 (step 128) or using the pointer co-ordinate data from one imaging assembly 58 (step 130). Since the collinearity equations relate image position (x,y) to spatial position (X,Y,Z), two (x,y) positions, one from each digital camera 74, are necessary to compute a unique (X,Y,Z) spatial position for the pointer. This yields four equations and three unknowns. The collinearity equations are rearranged to produce a linear least-squares problem, making triangulation an efficient procedure. Since the results of the triangulation in this case yield an (X,Y,Z) spatial position for the pointer, multiple pointers appearing within the fields of view of the imaging assemblies 58 can be tracked separately thereby to provide pointer disambiguation.
When performing triangulation using pointer co-ordinate data from a single imaging assembly 58, it is assumed Z=0 (cm). In this case, one of the unknowns in the collinearity equations is eliminated. In other words, spatial position (X,Y) is determined from image position (x,y). Using images from a single imaging assembly 58 provides advantages in that the touch system 50 can still determine pointer contacts with the touch surface 54 even in instances where one of the imaging assemblies 58 is unable to see the pointer.
Once the triangulation results are available, the triangulation results can be refined using a non-linear least-squares technique if desired.
The use of imaging assemblies 58 that see the entire touch surface 54 in three-dimensions as a perspective view as well as its surrounding area provides advantages. For example, during image processing, pointers crossing the boundaries of the touch surface 54 can be recognized prior to contact on the touch surface. This information can be used by the DSPs 82 to limit image processing to pixels within the relevant pixel subset adjacent the boundary crossover points. The touch system 50 also provides shadow/object discrimination. Generally, as a pointer is brought towards the touch surface 54, one of the imaging assemblies 58 will see the pointer before the other. The imaging assembly seeing the pointer first can provide pointer information to the other imaging assembly identifying the region of its relevant pixel subset that should be examined to locate the pointer. This helps to increase the probability of locating the pointer accurately and quickly. Planar homography is used to relate the two digital camera focal planes to one another, allowing the pointer information to be effectively exchanged between the imaging assemblies 58.
Another approach is to make use of a different relationship between the views of the imaging assemblies, through an entity known as the fundamental matrix, or the closely-related essential matrix. Here, if the location of a point is known in one digital camera view, the fundamental matrix translates this point into a line in the other digital camera image. Thus, it is only necessary to search an image along this line (known as an epipolar line) to locate the corresponding point. This approach has an advantage in that it severely limits the search region in the second digital camera view and helps to eliminate false positives.
In the above-described embodiment, the imaging assemblies 58 are shown as being disposed in front of the plane of the touch surface 54 by a distance equal to approximately 2.5 cm. As mentioned, this distance ensures that the bezel 56 does not obstruct the imaging assemblies' views of the entire touch surface 54. The distances over which the imaging assemblies 58 are disposed in front of the plane of the touch surface 54 can of course vary depending on the environment although, the distances have an impact on the size of the relevant pixel subset corresponding to the boundary of the touch surface that must be processed. The imaging assemblies 58 are positioned so that, at a minimum, the four corners of the touch surface 54 are within their fields of view.
Although a self-calibration technique has been described, other techniques can be used to calibrate the touch system 50. For example, the vanishing points for the touch surface 54 can be determined as described in the publication entitled “Geometric Computation for Machine Vison”, Oxford University Press, Oxford 1993, authored by Kanatani. Alternatively planar homography as described in the publication entitled “Multiple View Geometry in Computer Vision”, Cambridge University Press, Cambridge 2001, authored by Hartley et al. or the three-point method as described in the publication entitled “Minimum Solutions for Orientations in Calibration and Orientation of Cameras in Computer Vision”, Springer-Verlag, New York 2001, authored by Wrobel can be used to calibrate the touch system.
During the vanishing point method, advantage of the fact that the touch surface 54 is generally rectangular in shape is taken during computation of the external orientation parameters. In this case, by finding the peripheral edges of the touch surface 54 in a digital camera image, the vanishing points corresponding to the two sets of parallel peripheral side edges of the touch surface may be used to define the external orientation parameters completely. In the touch system, the vanishing points are finite, i.e., they lie within the bounds of a digital camera image and serve to define the rotation matrix R. If a unit of length is assumed, the digital camera positions can then be determined, completing the external orientation parameter determination. The vanishing points can also be used to define the digital camera's focal length and principal point as described in the publication entitled “On Estimating Rotations”, T.U. Munchen, 1999, authored by Foerstner. As will be appreciated, the two vanishing points define a three-dimensional co-ordinate system for the touch system, from which everything else follows. When using this approach it is preferred that the determined external orientation parameters be refined using a least-squares method.
The planar homography calibration approach relates points on the plane of the touch surface 54 to points on the image plane of the digital camera. By measuring the positions of a number of image points corresponding to certain scene points, it is possible to define the nine components of the homography matrix. Once this is done, the homography matrix can be decomposed into the rotation matrix R and a vector representing the digital camera's position in three-dimensional space. Using this calibration method requires some assumptions about the digital camera's internal orientation to be made. The need to make these assumptions can however be avoided by rewriting the homography matrix as another matrix entity known as the image of the absolute conic as described in the previously mentioned Hartley et al. publication. This matrix entity provides direct estimates of the focal length and principal point of the digital camera through its singular value decomposition.
The three-point calibration method makes use of basic trigonometry and the fact that three points define a plane. If the locations and distances between three points in a three-dimensional scene are known and form a triangle, then the image positions of these points may be used to define angles between the points. This information is sufficient to solve for the distances of the three points from the digital camera's optical center, thus giving the digital camera's position in space. Subsequent processing of the image points then provides an estimate of the rotation matrix R. As this method gives an orientation from a minimal data set, it can be used to initialize a least-squares method for refining the orientation, and hence to provide the initial guess input at step 104 during calibration.
Although the above-described calibration techniques make use of three angles to define the orientation of each digital camera 74 in space through a rotation matrix R, alternatives are available. For example rather than defining the rotation matrix R, the orientation of each digital camera in space can be determined based on an entity known as a “quaternion”. A quaternion is a four-element vector with certain properties as described in the previously mentioned Foerstner publication. Quaternion elements take on only values between −1 and 1, with one of the elements being constrained to be 1. This avoids problems associated with abrupt changes in value and assists greatly in the convergence using a least-squares approach. As will be appreciated, when measuring angles, some angle changes create difficulty such as for example when an angle changes from 359 degrees to 360 degrees, which is the same as 0 degrees.
Imaging assemblies that see the touch surface in three-dimensions as a perspective view can also be used in conjunction with large scale touch systems such as those described in U.S. patent application Ser. No. (not available) filed on Jan. 2, 2004 to Hill et al., assigned to SMART Technologies Inc., assignee of the present invention, the content of which is incorporated herein by reference. For example,
For example, as shown in
b and 13c show other positions of pointers on the touch surface 354 and the various triangulation results that can be derived from images captured by the digital cameras.
By pairing the digital cameras, curved and non-planar touch surfaces can be supported since the various pairs of digital cameras need only be responsible for viewing a portion of the touch surface.
As will be appreciated, since the imaging assemblies are able to self-calibrate, the imaging assemblies can be affixed basically to any surface to convert that surface to a touch surface.
Although the touch system 50 is described as including a computer communicating with the DSPs of the imaging assemblies and processing the pointer co-ordinate data using triangulation to determine the position of the pointer relative to the touch surface, other processing architectures can of course be used. For example, the DSP of one of the imaging assemblies may serve as the processor responsible for triangulating the pointer co-ordinate data.
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.
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