This paper provides an efficient method to find all feasible offsets for a given separation in a very large-scale integration (VLSI) channel-routing problem in one layer. The previous literature considers this task only for problems with no single-sided nets. When single-sided nets are included, the worst-case solution time increases from $\Theta ( n )$ to $\Omega ( n^2 )$, where n is the number of nets. But if the number of columns c is $O( n )$, the problem can be solved in time $O( n^{1.5} \lg n )$, which improves upon a “naive” $O( cn )$ approach. As a corollary of this result, the same time bound suffices to find the optimal offset (the one that minimizes separation). Better running times result when there are no two-sided nets or all single-sided nets are on one side of the channel. This paper also gives improvements upon the naive approach for $c \ne O( n )$, including an algorithm with running time independent of c. An interesting algorithmic aspect of the paper is a connection to discrete convolution.