Surface.CoordinateSystemAtParameter ignores surface normals

[ThomasMahon]

Dynamo version

1.3.3

Operating system

Win 10

What did you do?

used Surface.CoordinateSystemAtParameter on surfaces extracted from a Revit roof element

What did you expect to see?

The coordinate systems orientated relative to the surface normals.

What did you see instead?

Inconsistent results; it treats all surfaces with an 'up' direction determined from the global coordinate system, meaning the cs's are effectively located on the back face of the surface.

Z axis of coordinate system should be orientated like this (i.e. use the surface normal):

Instead, it does this (in the example, the underside 'downward' facing surfaces have cs's effectively placed on their backface):

[AndyDu1985]

With node, Element.Geometry, the direction are expected, the direction for down faces are point to down side. But with Topology.Faces or Geometry.Explode, the direction for down faces are point to up. The information is lost. I think this is related to geometry algorithm of Dynamo Core. Do you have any ideas? @QilongTang @mjkkirschner

[ThomasMahon]

Hi @AndyDu1985 it cant be caused by geometry explode for the following reasons:

1. The surfaces are derived from Element.Faces (not unless explode is embedded in this method?)
2. Using Surface.NormalAtParameter yields the correct result - using the same surfaces with Surface.CoordinateSystemAtParameter does not, and that's the problem.

Images in the OP explain the rest of the story.

[AndyDu1985]

@ThomasMahon , thank you for your explaination. So it is only related to Surface.CoordinateSystemAtParameter.

[ThomasMahon]

Based on the empirical evidence, yes!