||We treat digital straight line segments in two different cases,
in the 8-connected plane and in the Khalimsky plane. We investigate them
using a new classification, dividing them into a union of horizontal and
Then we study necessary and sufficient conditions for straightness in both
cases, using vertical distances for certain points. We also establish
necessary and sufficient conditions in the 8-connected plane as well as in
the Khalimsky plane by transforming their chain codes. Using this
technique we can transform Khalimsky lines to the 8-connected case.