Hann raised-cosine window (1 + Cos[2 Pi x])/2 on [-1/2 1/2] (exact for exact input)
HannWindow[x]
HannWindow[x, α]
Plot[HannWindow[x], {x, -1, 1}]
→ -Graphics-Plot3D[HannWindow[x]HannWindow[y], {x, -1, 1}, {y, -1, 1}, PlotRange -> All, Exclusions -> None]
→ -Graphics3D-HannWindow[0.1]
→ 0.9045084971874737