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 rectangular touch surface with first and second spatial dimensions 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 along a plurality of detection lines, said method comprising: processing the output signal to generate a set of 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 data samples are indicative of transmitted signal energy for at least a subset of the plurality of parallel detection lines; processing the set of data samples for transformation into Fourier coefficients arranged as data points on a regular grid defined by first and second dimension parameters that represent frequencies in the first and second spatial dimensions, respectively; and processing (106, 108) the data points by inverse Fourier transformation so as to generate an interaction pattern in the first and second spatial dimensions of the touch surface indicative of attenuation of the light signals on the touch surface resulting from touch interaction; and processing the interaction pattern for touch determination, characterized by the step of processing the data points comprising: operating a first one-dimensional inverse Fourier transform on the data points with respect to the second frequency dimension, so as to generate first values transformed into the second spatial dimension for the data points; and operating a second one-dimensional inverse Fourier transform on a selected subset of the first values with respect to the first frequency dimension to generate second values that represent the interaction pattern, wherein the selected subset is defined by the data points that fall within an extent of the touch surface in the second spatial dimension. processing the output signal to generate a set of 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 data samples are indicative of transmitted signal energy for at least a subset of the plurality of parallel detection lines; processing the set of data samples for transformation into Fourier coefficients arranged as data points on a regular grid defined by first and second dimension parameters that represent frequencies in the first and second spatial dimensions, respectively; and processing (106, 108) the data points by inverse Fourier transformation so as to generate an interaction pattern in the first and second spatial dimensions of the touch surface indicative of attenuation of the light signals on the touch surface resulting from touch interaction; and processing the interaction pattern for touch determination, characterized by the step of processing the data points comprising: operating a first one-dimensional inverse Fourier transform on the data points with respect to the second frequency dimension, so as to generate first values transformed into the second spatial dimension for the data points; and operating a second one-dimensional inverse Fourier transform on a selected subset of the first values with respect to the first frequency dimension to generate second values that represent the interaction pattern, wherein the selected subset is defined by the data points that fall within an extent of the touch surface in the second spatial dimension. operating a first one-dimensional inverse Fourier transform on the data points with respect to the second frequency dimension, so as to generate first values transformed into the second spatial dimension for the data points; and operating a second one-dimensional inverse Fourier transform on a selected subset of the first values with respect to the first frequency dimension to generate second values that represent the interaction pattern, wherein the selected subset is defined by the data points that fall within an extent of the touch surface in the second spatial dimension.