JTS allows specifying styling parameters to control the appearance of generated buffer polygons.
End Cap styles have been available for a while. End caps can be Round (the default), Flat, or Square. For Round caps, the Quadrant Segments parameter controls the degree of approximation to a true circular arc.
JTS 1.9 adds the ability to specify the Join Style. The Join style controls how the offset curves for consecutive line segments are joined together. The Join type can be Round (the default), Mitre, or Bevel. If Mitre is specified, a further Mitre Limit parameter is provided to control how long mitred joins can be before they are beveled off. The maximum length of a mitred corner is equal to the Mitre Limit multiplied by the buffer distance.
The JTS TestBuilder has been enhanced to allow specifying all of these parameters, so it can be used to experiment with the visual effects of the various styles.
44 minutes ago