A method of enabling touch determination based on an output signal from a touch-sensitive apparatus, the touch-sensitive apparatus comprising a light transmissive panel having a touch surface and an arrangement of light emitters and light detectors distributed along the periphery of the touch surface, each pair of an emitter and a detector defines a detection line that corresponds to the propagation path for an emitted signal from the emitter to the sensor, the touch-sensitive apparatus being configured to propagate light signals inside the light transmissive panel by internal reflections across the touch surface, said method comprising: processing the output signal to generate data samples in a two-dimensional sample space, wherein each data sample is representative of a detection line and is defined by a signal value and two dimension values that define the location of the detection line on the surface portion, wherein the the data samples are indicative of transmitted signal energy on parallel detection lines at a number of different angles across the touch surface; processing the data samples to generate interpolated Fourier coefficients at grid points in a regular grid in a Fourier domain; operating a two-dimensional inverse Fourier transform on the interpolated Fourier coefficients so as to generate an interaction pattern indicative of attenuation of the light signals on the touch surface resulting from touch interaction; and processing the interaction pattern for touch determination, characterized wherein the step of processing the data samples comprises: sequentially generating the interpolated Fourier coefficients for individual groups of grid points, wherein each individual group comprises grid points that have equal distance to an origin in the regular grid, and wherein the grid points in each individual group are mapped onto each other by one or more lines of symmetry in the regular grid. processing the output signal to generate data samples in a two-dimensional sample space, wherein each data sample is representative of a detection line and is defined by a signal value and two dimension values that define the location of the detection line on the surface portion, wherein the the data samples are indicative of transmitted signal energy on parallel detection lines at a number of different angles across the touch surface; processing the data samples to generate interpolated Fourier coefficients at grid points in a regular grid in a Fourier domain; operating a two-dimensional inverse Fourier transform on the interpolated Fourier coefficients so as to generate an interaction pattern indicative of attenuation of the light signals on the touch surface resulting from touch interaction; and processing the interaction pattern for touch determination, characterized wherein the step of processing the data samples comprises: sequentially generating the interpolated Fourier coefficients for individual groups of grid points, wherein each individual group comprises grid points that have equal distance to an origin in the regular grid, and wherein the grid points in each individual group are mapped onto each other by one or more lines of symmetry in the regular grid.