If you wish to generate a Dual manifold when truncating the primal, you must check the option ‘Truncate for 'Generate Dual' ’. This ensures that the Dual sits flat on the truncation plane.
When ‘Truncate for 'Generate Dual' ’ is checked the trifan sectors on the Primal truncation plane are flagged and their perimiter edges bisected. The bisection vertex is also flagged so that it isn’t included when generating Primal centroids. Now whenever the Dual is rebuilt, Geodesica knows to move the Dual vertex that would normally lie at the centroid of the trifan sector to its perimiter bisection point. This preserves duality in the envelope proper and means the dual exports correctly.
Fig 1. Bisected perimiter edge with projection lines from the origin.
Fig 2. Dual vertices that would normally lie on the floor of the Primal trifan sectors are now moved to the perimiter bisection points.
Fig 3. Dual envelope using a Primal offset.
The dual floor will move up or down along the projection lines unless the Dual option ‘Keep truncation Y’ is checked in the Envelopes window. The example below shows a negative Dual offset
NOTE: Whilst the truncation plane bisectors are shown in the above diagrams, they are invisible when using the program. This is because they are for internal use only and not intended for modelling operations.