Help with creating geometries from Python

I think you could use the FMEFace class.

Here's how you can create a simple FMEFace in the PythonCreator:

import fmeobjects

class FeatureCreator(object):
    def __init__(self):
        pass
        
    def input(self,feature):
        newFeature = fmeobjects.FMEFeature()
        points = [(0,0,0),(1,0,0),(1,1,0)]
        mode = 0
        newFeature.setGeometry(fmeobjects.FMEFace(points,mode))
        self.pyoutput(newFeature)
        
    def close(self):
        pass

The points variable holds the coordinate tuples of your TIN so you just need to loop through your triangles creating FMEFaces and then you could create a MultiSurface or Mesh by passing in the newly created FMEFaces.

Also, have a look at FMETriangleFan & FMETriangleStrip, I think you can create them in a similar way and they might be useful in your scenario.

FME and Python is a really powerful combo, that's for sure.

Based on @gerhardatsafe, here's the code adapted to the specific use case with aggregate / MultiSurface generation:

from fmeobjects import *

MY_TRIANGLES = [((0,0,0),(1,0,0),(2,1,1)), ((0,0,0),(2,1,1),(2,2,1))]

class CreateTriangles(object):

    def __init__(self):
        pass
        
    def input(self, feature):
        triangles = MY_TRIANGLES
        
        aggregate = FMEAggregate()  # or FMEMultiSurface()
        for triangle in triangles:
            # Add first vertex as last to close polygon
            vertices = list(triangle) + [triangle[0],]
            mode = 0
            aggregate.appendPart(FMEFace(vertices, mode))
                
        feature.setGeometry(aggregate)
        self.pyoutput(feature)
        
    def close(self):
        pass

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