If you are using PVector objects you can just calculate the distance. If not you could write a function such as:

1. float lineLength(PVector lineStart, PVector lineEnd) {
2.   return lineStart.dist(lineEnd);  
3. }
4. 
   
5. float lineLength(float x1, float y1, float z1, float x2, float y2, float z2) {
6.  return lineLength(new PVector(x1, y1, z1), new PVector(x2, y2, z2));
7. }
   
8. 
   
9. float x1, y1, z1, x2, y2, z2;
   
10. float threshold = 10;
    
11. 
    
12. if (lineLength(x1, y1, z1, x2, y2, z2) < threshold) {
    
13.   // lets keep it simple and pick the first point
    
14.   point(x1, y1, z1);
    
15. } else {
    
16.   line(x1, y1, z1, x2, y2, z2);
    
17. }

I'm not sure at what point you would want to switch to points. You might have to experiment with that.