  We generalize the `filtering spectrum' [1] to probe scales along different
directions by spatial coarse-graining. This multi-dimensional filtering
spectrum quantifies the spectral content of flows that are not necessarily
homogeneous. From multi-dimensional spectral information, we propose a simple
metric for shape anisotropy at various scales. The method is applied to
simulations of 2D and 3D Rayleigh-Taylor (RT) turbulence, which is
inhomogeneous and anisotropic. We show that 3D RT has clear shape anisotropy at
large scales with approximately $4:3$ vertical to horizontal aspect ratio, but
tends toward isotropy at small scales as expected [2,3,4]. In sharp contrast,
we find that RT in 2D simulations, which are still the main modeling framework
for many applications, is isotropic at large scales and its shape anisotropy
increases at smaller scales where structures tend to be horizontally elongated.
While this may be surprising, it is consistent with recent results in [5];
large-scale isotropy in 2D RT is due to the generation of a large-scale
overturning circulation via an upscale cascade, while small scale anisotropy is
due to the stable stratification resultant from such overturning and the
inefficient mixing in 2D.
