Wavelet-based methods have proven their efficiency for the visualization at different levels of detail, progressive transmission, and compression of large data sets. The required core of all waveletbased methods is a hierarchy of meshes that satisfies subdivisionconnectivity: this hierarchy has to be the result of a subdivision process starting from a base mesh. Examples include quadtree uniform 2D meshes, octree uniform 3D meshes, or 4-to-1 split triangular meshes. In particular, the necessity

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... of subdivision-connectivity prevents the application of wavelet-based methods on irregular triangular meshes. In this paper a "wavelet-like" decomposition is introduced, that works on piecewise constant data sets over irregular triangular surface meshes. The decomposition/reconstruction algorithms are based on an extension of wavelet-theory allowing hierarchical meshes without subdivision-connectivity property. Among others, this approach has the following features: it allows exact reconstruction of the data set, even for nonregular triangulations, it extents previous results on Haar-wavelets over 4-to-1 split triangulations.