MergeVectorsDifferentLengths

Diagnostic signature

MergeVectorsDifferentLengths

The Manifold library emits this error when the mergeFromVert and mergeToVert arrays in a MeshGL structure have different lengths, preventing valid vertex merging.

What it means

The MergeVectorsDifferentLengths diagnostic is an error status emitted by the Manifold geometry library when it encounters a mismatch in the lengths of the mergeFromVert and mergeToVert vectors within an input MeshGL or MeshGL64 object. These vectors are utilized to specify which vertices (often duplicated to accommodate sudden changes in vertex properties like normals or UV coordinates across seams) correspond to the same physical location. They guide the engine in losslessly stitching these vertices back together to recover a valid topological manifold. When the lengths of these two vectors differ, Manifold cannot pair every source vertex with a target vertex, resulting in an invalid merge specification.

Why it happens

This error is triggered during the construction of a manifold::Manifold from a raw graphics mesh (e.g., calling Manifold(const MeshGL&) in C++ or its JavaScript equivalent). The internal validation logic strictly checks the sizes of the mergeFromVert and mergeToVert arrays. If a developer or an importer script populates one array but not the other, or pushes mismatched numbers of indices into them, the constraint mergeFromVert.size() == mergeToVert.size() is violated. Consequently, the library aborts the construction of the solid and sets the object's status to MergeVectorsDifferentLengths, typically returning an empty or invalid mesh representation.

Minimal reproduction

#include <manifold.h>
#include <vector>

int main() {
    manifold::MeshGL mesh;
    mesh.numProp = 3;
    mesh.vertProperties = {
        0.0f, 0.0f, 0.0f,
        1.0f, 0.0f, 0.0f,
        0.0f, 1.0f, 0.0f
    };
    mesh.triVerts = {0, 1, 2};
    
    // Cause the error by providing mismatched merge arrays
    mesh.mergeFromVert = {1, 2};
    mesh.mergeToVert = {1};
    
    manifold::Manifold m(mesh);
    if (m.Status() == manifold::Manifold::Error::MergeVectorsDifferentLengths) {
        return 1;
    }
    return 0;
}

How to fix it

Ensure that the mergeFromVert and mergeToVert arrays in the MeshGL structure are of exactly the same length, or omit them entirely if vertex merging is unneeded.

```C++
#include <manifold.h>
#include <vector>

int main() {
    manifold::MeshGL mesh;
    mesh.numProp = 3;
    mesh.vertProperties = {
        0.0f, 0.0f, 0.0f,
        1.0f, 0.0f, 0.0f,
        0.0f, 1.0f, 0.0f
    };
    mesh.triVerts = {0, 1, 2};
    
    // Fix the error by matching lengths or clearing both vectors
    mesh.mergeFromVert = {1, 2};
    mesh.mergeToVert = {0, 0}; // Same length as mergeFromVert
    
    manifold::Manifold m(mesh);
    if (m.Status() != manifold::Manifold::Error::NoError) {
        return 1;
    }
    return 0;
}
```

Step 1

• Locate where the MeshGL or MeshGL64 object is being constructed or parsed before it is passed to the Manifold constructor.

Step 2

• Check the logic responsible for populating the mergeFromVert and mergeToVert vectors. Both vectors must always contain the exact same number of elements.

Step 3

• If you are manually defining topological merges for vertices duplicated along property seams (such as sharp normals or UV boundaries), verify that for every index appended to mergeFromVert, a corresponding target index is appended to mergeToVert.

Step 4

• If no vertex deduplication or merging is required for the input mesh, completely omit or clear both the mergeFromVert and mergeToVert arrays.

Upstream references

manifold/manifold.h Header File

SplitByPlane() doesn't handle empty Manifold objects < Issue #1515

Manifold is merging coplanar faces despite unique vertex properties < Issue #1316