 The Birchfield–Tomasi dissimilarity can then be defined as

In practice the measure can be computed with only a small and constant overhead with respect to the calculation of the simple intensity difference, because it is not necessary to reconstruct the interpolant function.Given that the interpolant is linear within each unit interval centred around a pixel, its minimum is located in one of its extremities.Therefore, formula_9 can be written as
where
denoting with formula_12 and formula_13 the values of the interpolated intensities at the rightmost and leftmost extremities of a one-pixel interval centred around formula_2
The other function formula_16 can be similarly rewritten, completing the expression for formula_17.