1. A method of enabling touch determination based on an output signal from a touch-sensitive apparatus, the touch-sensitive apparatus including a rectangular touch surface with first and second spatial dimensions, the rectangular touch surface configured to propagate signals across the rectangular 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 grid defined by first and second frequency dimension parameters that represent frequencies in the first and second spatial dimensions, respectively; and processing the data points by inverse Fourier transformation to generate an interaction pattern in the first and second spatial dimensions of the rectangular touch surface; wherein the processing the data points includes operating a first one-dimensional inverse Fourier transform on the data points with respect to the second frequency dimension 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, the selected subset defined by the data points within an extent of the rectangular touch surface in the second spatial dimension.