A method of enabling touch determination based on an output signal from a touch-sensitive apparatus, the touch-sensitive apparatus comprising a panel configured to conduct light signals from a plurality of peripheral incoupling points to a plurality of peripheral outcoupling points, thereby defining actual detection lines that extend across a rectangular surface portion of the panel between pairs of incoupling and outcoupling points, at least one signal generator coupled to the incoupling points to generate the light signals, and at least one light signal detector coupled to the outcoupling points to generate the output signal, the 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 an actual detection line and is defined by a signal value and two dimension values ( φ, s; β, α) that define the location of the actual detection line on the surface portion, wherein the actual detection lines are arranged according to a first geometry, processing the set of data samples to generate a set of matched samples, wherein the matched samples are indicative of estimated signal values for fictitious detection lines at predetermined locations in the two-dimensional sample space that match a second geometry for tomographic reconstruction, wherein the second geometry is different to the first geometry and wherein the second geometry is a standard geometry in the field of tomographic reconstruction, and wherein each estimated signal value is generated by interpolation of the signal values of neighboring data samples in the two-dimensional sample space, processing (44, 46) the set of matched samples by tomographic reconstruction to generate data indicative of an attenuation field within at least part of the surface portion, wherein the attenuation field is a distribution of attenuation values corresponding to positions on the at least part of the surface portion, and where each attenuation value is indicative of the attenuation of the light signals at the corresponding position, and, processing the data indicative of the attenuation field for touch determination. processing the output signal to generate a set of data samples in a two-dimensional sample space, wherein each data sample is representative of an actual detection line and is defined by a signal value and two dimension values ( φ, s; β, α) that define the location of the actual detection line on the surface portion, wherein the actual detection lines are arranged according to a first geometry, processing the set of data samples to generate a set of matched samples, wherein the matched samples are indicative of estimated signal values for fictitious detection lines at predetermined locations in the two-dimensional sample space that match a second geometry for tomographic reconstruction, wherein the second geometry is different to the first geometry and wherein the second geometry is a standard geometry in the field of tomographic reconstruction, and wherein each estimated signal value is generated by interpolation of the signal values of neighboring data samples in the two-dimensional sample space, processing (44, 46) the set of matched samples by tomographic reconstruction to generate data indicative of an attenuation field within at least part of the surface portion, wherein the attenuation field is a distribution of attenuation values corresponding to positions on the at least part of the surface portion, and where each attenuation value is indicative of the attenuation of the light signals at the corresponding position, and, processing the data indicative of the attenuation field for touch determination.