Optical touch tomography
An optical touch-sensitive device has the capability to determine touch locations of multiple simultaneous touch events. The touch events disturb optical beams propagating across the touch sensitive surface. With multi-touch events, a single beam can be disturbed by more than one touch event. In one aspect, a non-linear transform is applied to measurements of the optical beams in order to linearize the effects of multiple touch events on a single optical beam. In another aspect, the effect of known touch events (i.e., reference touches) is modeled in advance, and then unknown touch events are determined with respect to the reference touches.
1. A method of resolving a multi-touch event occurring at a plurality of touch regions on a touch sensitive surface, the method comprising a computer processor executing the steps of:
receiving a plurality of beam measurement values b n resulting from the multi-touch event, wherein different beam measurement values b n result from different optical beams transmitted across the touch sensitive surface and the multi-touch event disturbs optical beams in a vicinity of the touch region;
applying a previously determined pseudo-inverse matrix of a linear transfer function from binding values p i to the beam measurement values b n , the binding values p i measuring optical beam disturbance at corresponding locations L i of the touch-sensitive surface, wherein applying the pseudo-inverse matrix to the plurality of beam measurements b n generates binding value estimates p′ i estimating optical beam disturbance at the corresponding locations L i of the touch-sensitive surface; and
determining the touch regions for the multi-touch event from the binding value estimates p′ i .
2. The method of claim 1 , wherein:
the linear transfer function is described by b=M p, where:
b is a column vector of the beam measurement values b n ,
p is a column vector of the binding values p i , and
M is the linear transfer function, and each column of the matrix M is the beam measurement values b n resulting from a reference touch at location L i , said reference touch corresponding to a binding value p i =1; and
applying the pseudo-inverse matrix comprises evaluating p′=R b, where:
p′ is a column vector of the binding value estimates p′ i , and
R is the pseudo-inverse matrix.
3. The method of claim 2 , wherein the column vector p includes elements corresponding to at least 1000 different locations L i .
4. The method of claim 1 , wherein the pseudo-inverse matrix R is determined during design of the touch sensitive surface.
5. The method of claim 1 , wherein the pseudo-inverse matrix R is determined by pre-calibration during manufacture of the touch sensitive surface.
6. The method of claim 1 , wherein the pseudo-inverse matrix R is determined during run-time operation of the touch sensitive surface.
7. The method of claim 1 , wherein determining the pseudo-inverse matrix R comprises:
determining the linear transfer function M; and
applying an inversion process to the linear transfer function M to determine the pseudo-inverse matrix R.
8. The method of claim 7 , wherein determining the linear transfer function M comprises:
applying, to the touch sensitive surface, references touches with known binding values at known locations;
measuring beam measurements resulting from the reference touches; and
constructing the linear transfer function M based on the known binding values of the references touches and the resulting beam measurements.
9. The method of claim 7 , wherein elements of the pseudo-inverse matrix R are quantized to one bit per element, and the inversion process comprises counting a number of active optical beams in a given touch region.
10. The method of claim 1 , wherein determining the pseudo-inverse matrix R comprises:
determining the linear transfer function M by simulation; and
applying an inversion process to the linear transfer function M to determine the pseudo-inverse matrix R.
11. The method of claim 1 , wherein the pseudo-inverse matrix R is a sparse matrix.
12. The method of claim 1 , wherein non-zero elements of matrix R are encoded to few bits per entry.
13. The method of claim 1 , wherein determining the touch regions for the multi-touch event comprises applying a threshold to the binding value estimates p′ i , and identifying candidate touch regions based on which binding value estimates p′ i exceed the threshold.
14. The method of claim 12 , wherein the non-zero elements of the pseudo-inverse matrix R are encoded to few bits per entry using a quantization encoder.
15. The method of claim 1 , wherein the column vector p includes elements corresponding to non-uniformly spaced locations L i .
16. The method of claim 1 , wherein the column vector p includes elements corresponding to locations L i that are all at least 2 mm apart from each other.
17. The method of claim 1 , wherein:
applying the pseudo-inverse matrix comprises applying a plurality of pseudo-inverse matrices to the plurality of beam measurements b n to generate binding value estimates P′ i .
18. The method of claim 17 , wherein a first pseudo-inverse matrix of the plurality of pseudo-inverse matrices corresponding to a first set of the locations L i , a second pseudo-inverse matrix of the plurality of pseudo-inverse matrices corresponding to a second set of the locations L i that is offset from the first set by half a pitch in a horizontal direction, a third pseudo-inverse matrix of the plurality of pseudo-inverse matrices corresponding to a third set of the locations L i that is offset from the first set by half a pitch in a vertical direction, and a fourth pseudo-inverse matrix of the plurality of pseudo-inverse matrices corresponding to a fourth set of the locations L i that is offset from the first set by half a pitch in a diagonal direction.
19. The method of claim 1 , further comprising:
interpolating binding value estimates for locations located between the locations L i , wherein determining the touch regions for the multi-touch event is further based on the interpolated binding value estimates.