Invariant-based registration of surface patches

3D shape models are often put together from several partial reconstructions. There are good algorithms available now to perform the necessary, precise registration automatically, but only after the partial reconstructions have been brought into approximate positions. This paper proposes a technique to do precisely that. Automatic `crude registration´ is illustrated for Euclidean and affine transformations between parts. The technique is based on the extraction and invariant characterisation of bitangent curve pairs. These can be found and matched efficiently. Registration of these curves yields the crude initialisation that more precise registration algorithms like ICP or mutual information maximisation can start from