In the article called Offset (exterior) we have seen how we can create fictitious bars (external lines) to simulate the connecting elements (shackles, turnbuckles, … ) and absorb the bulk of the tubes or other structural elements. To do this we get new points that are located outside of the membrane, that is to say, the volume occupied has become larger.

However there are many cases in which once obtained the shape of the membrane, we must create clearance but without increasing the area occupied:

    1. The membrane goes into a rigid, fixed frame as they can be walls that form a courtyard, an existing structure of bars, etc.
    2. We have used the coordinates of the structure (for example, tripods) as final coordinates to find the form, and now we have to do the smaller membrane.
    3. Auxiliaries: shackles, turnbuckles, etc are larger than we thought and do not fit in the margin we assumed.

To solve these issues we have the option to Offset (interior). The internal offset uses a negative value in the input box:

Offset (interior) negative

In contrast to the case of the Offset (outer), we create a new point at a distance equal to the absolute value of the separation (30 cm in the example) away from the vertex or apex. But this time it is towards the interior of the membrane following the same direction that we had taken in the case of an offset outside.

If didn’t do anything else we would have a cable towards the inside of the membrane:

Cable to inside

But obviously this is not useful. What the program will make therefore is to recalculate the form of the membrane, using interior point found as new vertex, and using the previous vertex as end of the cable outside.

Cable outward

To see exactly what happened, in the following image you can see the initial form of the membrane (in red) and the final form with the outside cable (in blue) within the previous volume.

Final form within the previous volume