I do not think that mod operator is the source of artifacts, at least in the general case. Recently I also experimented with raymarching of distance fields and all worked pretty fine. Check my screens lower. My trusty guide in this stuff isare tutorials and papers of Iñigo Quílez. If you don`t know about this guy, taste his strawberry. More concretely: iterative shapes, distance functions and raymarching distance fields.

