Regularisation matrix for the linear case. Each row corresponds to the regularisation function for one pixel in space and time. For displaying purposes, we assume an observation with dimensions n_t = 3, n_y = 4, n_x = 4 and regularisation weights α = 1, β = 1, and γ = 2.5. The outermost bands (darker blue) correspond to the temporal regularisation terms, whereas the inner light-blue terms originate from the spatial regularisation.