Attraction Distance
To avoid small unrecognizable inaccuracies in geometry definition, new vertices or edges cannot be created very close to the existing objects. Creation of new geometric objects is controlled by the value we denote by ε and call the attraction distance.
The following rules apply to creation of new vertices and edges.
New vertices cannot be created within 2·ε-neighborhood of the existing vertex.
New edge cannot connect the ends of the existing edge and lie inside its ε-neighborhood.
If the vertex being added lies in the ε-neighborhood of an existing edge, the vertex is projected on the edge. The edge, in its turn, is split by the new vertex into two parts. The same is true when an existing vertex lies in the ε-neighborhood of the edge being added. In such case the vertex position remains unchanged and two new edges, connected at the vertex' position, are added to the model.
The value of ε is proportional to the size of the visible region. So, to avoid undesired vertex or edge merges creating especially small details, you would have to zoom in the model window in advance.