Q:Why is it necessary to distinguish between internal lines and base lines in the system?

A:Internal lines are involved in triangulation, while base lines are not.

"Triangulation" can be simply understood as "the process of generating a mesh".

Some operations may affect the arrangement of the mesh, such as sewing, topstitches, setting the fold angle, etc. The operations that will affect the mesh arrangement theoretically require triangulation and need to be set as internal lines. In practice, not all line information coming from the pattern needs to affect the mesh arrangement, such as fabric lines, arrows indicating pleat direction, measurement-related lines, etc. If all the lines are involved in triangulation it will affect the simulation to a great extent. Therefore, it is currently necessary for the user (by outlining) to manually select and generate the internal lines.