In legal terms, the author is the person who expresses creative ability in an original manner when developing a literary or artistic work: the standard is one of intellectual creation. Where choices are dictated by technical considerations, rules or constraints, then the criterion of intellectual creation is not met. An example is when footballers play in a football match. This could not be protected by copyright because the players play the game in accordance with pre-existing rules.
Joint or co-authorship arises where two or more people have contributed the right level of intellectual creation to a copyright work and their contributions cannot be separated. For example, in a collection of essays authorship in each of the essays will reside with the individual author because they can be readily be separated from each other. Where however two or more authors have collaborated in painting a picture, and it is not possible to point to part of that picture and say that one author rather than another painted that part, then the authors with be joint authors in law.