Geometry Flip

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The command GEOMETRY FLIP, which is used to reverse the direction of the local Z-axis of a surface, has been enhanced. The following new options have been added:

1. GEOMETRY FLIP set_name
2. GEOMETRY FLIP ALL
3. GEOMETRY FLIP CONSISTENT {set_name [ref_surface_name]} | ALL
4. GEOMETRY FLIP FOCAL {set_name [ref_surface_name] [focal_point]} | ALL

In consistent mode, the surfaces within a set are selectively flipped such that the

normals of topologically connected surfaces within that set are aligned in a consistent manner. The effect of GEOMETRY FLIP CONSISTENT will be to orient the surfaces such that their normals either (i) all point outwards, or (ii) all point inwards with respect to the inside of a shell. A reference surface may be specified optionally in order to select a reference normal that defines a sense of alignment. The reference surface itself is never flipped. If no reference surface is specified, then the first surface in the set is taken as a

default reference surface.





a) A set of inconsistently aligned surfaces, prior to a flip





b) A set of consistently aligned surfaces, after a flip



Figure 6: Example of using the GEOMETRY FLIP CONSISTENT command



In focal mode, the surfaces within a set are selectively flipped such that the normal of each surface is aligned with a direction defined by the surface's 'focal vector'. Compared to consistent mode, the sense of alignment (or otherwise) is determined entirely by geometric information. In general, the result of a focal flip on a set of surfaces is to redefine surfaces, where necessary, such that all the normals of surfaces in the set are either (i) pointing away from a focal point, or (ii) pointing towards a focal point. The focal point may be defined explicitly by the user as a point name, or specified indirectly (the default) as the centroid of the set. A focal vector is then associated with each surface in the set. The focal vector is taken from the set's focal point to the centroid of the surface in question. For each surface, a sense of alignment can then be determined by assessing the direction of the surface's normal against its focal vector. Under a focal flip, a surface is flipped if its sense of alignment is opposite to the sense of alignment of a reference surface. The user may explicitly specify the reference surface, otherwise the reference surface is taken as the first surface in the set.



Under certain circumstances, the alignment and location of the reference surface with respect to the focal point may cause indeterminacy. This is the case where the focal vector is perpendicular to the surface normal, and where the centroid of a surface coincides with the focal point. In these cases, no flip is undertaken and a warning message is issued.



In addition, the number of surfaces that have been flipped is reported. Also, any prism that may be affected by the flip is examined. If the topology of the prism has been compromised then a suitable error message is produced so that an immediate UNDO can be effected.

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