  An object's interior material properties, while invisible to the human eye,
determine motion observed on its surface. We propose an approach that estimates
heterogeneous material properties of an object from a monocular video of its
surface vibrations. Specifically, we show how to estimate Young's modulus and
density throughout a 3D object with known geometry. Knowledge of how these
values change across the object is useful for simulating its motion and
characterizing any defects. Traditional non-destructive testing approaches,
which often require expensive instruments, generally estimate only homogenized
material properties or simply identify the presence of defects. In contrast,
our approach leverages monocular video to (1) identify image-space modes from
an object's sub-pixel motion, and (2) directly infer spatially-varying Young's
modulus and density values from the observed modes. We demonstrate our approach
on both simulated and real videos.
