When is a line a line

Conformance testing of graphics systems is a very complex and exhausting task. Years of practice with the GKS testing tools have shown a need for the automatic testing of visual output. Indeed, with regard to graphics systems which are more precisely specified than GKS like the Computer Graphics Interface (CGI), conformance testing is not manageable to all unless a major part can be automated. This paper discusses different strategies for the automatic testing of pictorial effect. It concentrates on the definition of lines and describes a strategy to answer the question put in the title by the testing system. Finally, automatic testing of simple graphical operations such as segment highlighting and visibility is discussed