de la Vallee Poussin (Parzen) window function; piecewise cubic on [-1/2 1/2]
ParzenWindow[x]
Plot[ParzenWindow[x], {x, -1, 1}]
→ -Graphics-Plot3D[ParzenWindow[x]ParzenWindow[y], {x, -1, 1}, {y, -1, 1}, PlotRange -> All, Exclusions -> None]
→ -Graphics3D-ParzenWindow[0.1]
→ 0.808