Draping polygonal features on a TIN?

Hi @evaniko, the Triangulator transformer might help you.

Yes I was wondering how can 2D polygons clip 3D triagle parts perfectly in your first solution, and I tested it and got some problems with the faces in the Sketchup format (*.skp). But now it works great with the amelioration that you added in the update!! thanks you so much for your help Takashi.

1. Maybe these additional transformers work to generate a better result.

1. PlanarityFilter: Set "Yes" to the Expose Surface Normal parameter to expose the normal vector attributes - _surfaceNormalX, _surfaceNormalY, and _surfaceNormalZ.
2. 
   AttributeRounder: Round the three attributes by an appropriate decimal places, to absorb the slight computational error.
3. 
   Dissolver: Set the three attributes to the Group By parameter to dissolve coplanar polygons.

[Addition] Because there is unavoidable computational error, it may not be guaranteed that the resulting non-triangle polygons have planarity strictly.

2. If the number of possible attribute values (i.e. colors) is not so many, I would create an external CSV table to define mapping between each attribute value and corresponding "fme_color" value, and merge the color value to the features with the Joiner. e.g.

Key Attribute, fme_color
1, "0,0,0"
2, "1,0,0"
3, "0,1,0"
4, "0,0,1"
5, "1,1,1"

Then, copy the "fme_color" value to "fme_fill_color" with the AttributeCopier.

The AttributeValueMapper may also be useful.

Or, if the color may be random, the FeatureColorSetter can be used simply (Color Schema: Random, Randomize Based On: Attributes).

1. Although the help doc on the Dissolver says "This transformer accepts two-dimensional polygonal features", in my observation, it also works in fact for 3D polygons except vertical wall-like shapes. See also the attachment: drape-polygon-by-tin-surface-2.fmw (FME 2016.1)
2. I guess that you are using FME Data Inspector in 3D mode to check the result. It seems to mix colors of coplanar features, but I think the color values are set correctly. Check the values for each feature with the Feature Information pane, or view the resulting 3D polygons only.

1. First thanks for sharing your test workflow, then yes you are right Sir! the Dissolver works; it did not work for me before because I converted my polygons into faces with FaceReplacer, in order to be able to colour front side and back side differently (the Dissolver supports only polygons).

The problem now is that the FaceReplacer rejects many polygons in the last FME version 2016.1.1 (I don't know why??), whereas it doesn't eliminate them in the 2016.0.1 version.. .

2. No, I used Sketchup to open the results; the colors are set correctly (I know it), but the fact of faces superposition which causes the color problem (Maybe there's a solution for that?)

1. I found the source of the problem : In the FME 2016.1 version, the FaceReplacer rejects some polygons because they are still aggregated (fme_geometry = "fme_aggregate" and fme_rejection_code = "INVALID_GEOMETRY_TYPE"), whereas the situation is not the same in the 2016.0 version; So to resolve that, we have to change the Mode parameter in the Deaggregator from "Flatten One Level" to "Flatten All Levels".

As a conclusion, I think your proposed method is not always perfect for merging coplanar faces; the FaceReplacer keep rejecting my polygons because of the non-coplanarity and if we rise the precision of attributes (the number of decimal places in attributes), the FaceReplacer rejects fewer polygons but in the other hand merge less number of triagles!

For my case, I discover an alternative option : the Sketchup writer has a parameter to merge coplanar faces, but in other cases, doing the process usins transformers would be the only solution to find! so it's still more practical.

2. Yes! I thought of that solution too and I believe that it is the only :D