A discussion about violin reduction: geometric analysis of contour lines and channel of minima

Some early violins have been reduced during their history to fit imposed morphological standards, while more recent ones have been built directly to these standards. We can observe differences between reduced and unreduced instruments, particularly in their contour lines and channel of minima. In a recent preliminary work, we computed and highlighted those two features for two instruments using triangular 3D meshes acquired by photogrammetry, whose fidelity has been assessed and validated with sub-millimetre accuracy. We propose here an extension to a corpus of 38 violins, violas and cellos, and introduce improved procedures, leading to a stronger discussion of the geometric analysis. We first recall the material we are working with. We then discuss how to derive the best reference plane for the violin alignment, which is crucial for the computation of contour lines and channel of minima. Finally, we show how to compute efficiently both characteristics and we illustrate our results with a few examples.

Validation of a photogrammetric approach for the study of ancient bowed instruments

Some ancient violins have been reduced throughout their history. We propose an objective photogrammetric approach to differentiate between a reduced and an unreduced instrument, where a three-dimensional mesh is studied geometrically by examining 2D slices. First, we validate the accuracy of the photogrammetric mesh by the way of a comparison with reference images obtained with medical imaging. Then, we show how contour lines and channels of minima can be automatically extracted from the photogrammetric meshes, allowing to successfully highlight differences between instruments.