Getting wrong Triangles when trying to tesselate

[CrimNinja]

I have been trying to use libtess2 to do some tesselation. And in some cases I am getting the wrong triangles. I have included one set of points in which I am getting the wrong output.

std::vector Points{1.3515, -0.8156, 1.3672, -0.8156, 1.3329, -0.5057, 1.3174, -0.5057}; TESStesselator pTess = tessNewTess(nullptr); tessAddContour(pTess, 2, Points.data(), 2 sizeof(float), 4); tessTesselate(pTess, TESS_WINDING_NONZERO, TESS_POLYGONS, 3, 2, 0);

I have accounted for the order of vertices using tessGetVertexElements and matched the right indexes from tessGetElements. With or without it, either way, the resulting triangles seem weird.

is the triangle I am getting from this.

Am I missing something from the way I call the functions ? Or is there something else that I need to do to get the right triangles?

[memononen]

Can you show the code you use to read back the triangles?

[CrimNinja]

    const int *pElements = tessGetElements(pTess);
    const int *pOrder = tessGetVertexIndices(pTess);
    const int NumElements = tessGetElementCount(pTess);
    const int NumVertices = tessGetVertexCount(pTess);
    const float *Vertices = tessGetVertices(pTess);

    std::vector<int> IndexMap;
    IndexMap.resize(NumVertices);

    int j = 0;
    for(int i = 0; i < NumVertices; ++i)
    {
        if(pOrder[i] != -1)
            IndexMap[pOrder[i]] = j++;
    }
    rTriangles.reserve(NumElements);
    for(int i = 0; i < NumElements; i++)
    {
        rTriangles.emplace_back(IndexMap[pElements[i * 3 + 0]], IndexMap[pElements[i * 3 + 1]],
                                IndexMap[pElements[i * 3 + 2]]);
    }

This is what I am doing to read the indexes after tesselation.

[memononen]

Which vertex data do you use? That IndexMap does not make sense to me.

[CrimNinja]

I am adding points using Points.data(), and in other parts of the code, the points I have added to the vector are referenced by the index 0, 1, 2,.. But libtess2 seemed to rearrange or add vertices in some cases, so I use an IndexMap to ignore the extra vertex that was added and to refer back to the original order I tried adding the vertices.

[CrimNinja]

Here is an image of the values that I get from the tesselation, the elements from tessGetElements seem to provide 0, 1, 2, 1, 0, 3 in this case. Which seems strange.

[memononen]

pOrder[pElements[i]] should to the original vertex index, or if it is TESS_UNDEF then it was a vertex that was added because there was an intersection.

How does the triangulation look like if you use the tessGetVertices() to plot the points, not your own mapping?

[CrimNinja]

Omg, that works! Thanks!