Hi,
The null segments are used in the Sequence operators to create an empty space instead of a brick. There's one sequence operator for each side of the corner and we need to alternate between a brick and a gap, but offset on each side. Imagine in the column below that 1 = brick and 0 = gap. The null segments are used in this case to create the gaps.
1 | 0
0 | 1
1 | 0
0 | 1
1 | 0
0 | 1
The Null wired to the Compose operator performs a slightly different function. When a compose operator is wired to a corner input the number of attached nodes determines how the segments are placed in relation to the corner vertex. An
even number will always result in an asymmetric composition whereas an
odd number will be centred. Therefore the null segment in this case is used to create an odd number of inputs and centre the bricks on the corner. You can read more about this in the "
How to Fine Tune Corners" chapter of our
Next Steps with RailClone guide.
The corner problem shown in your screen shot is caused by a known issue when using sequence operators on the first vertex in a closed spline that causes a mismatch in the counters. This will be addressed in a future release, but in the meantime it is possible to get around it by giving the first vertex a different type, for example a Bezier-Corner for the first vertex and a Corner for all the others. You can then use a Conditional operator to detect that corner and use an adjusted version of the existing sequence operator. In this case I just needed to swap the order.
![](http://i.imgur.com/C1cWkJx.png)
I've attached a revised version of the wall to illustrate how this works. I hope that helps clear things up a bit.
Many thanks!
Paul