1. A method of enabling touch determination based on an output signal from a touch-sensitive apparatus, the touch-sensitive apparatus comprising a rectangular touch surface with first and second spatial dimensions and being configured to propagate signals across the touch surface along a plurality of detection lines, said method comprising: processing the output signal to generate a set of data samples indicative of transmitted signal energy for at least a subset of the plurality of 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 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; wherein the step of processing the data points comprises: 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.