Unsharp masking
= Unsharp masking
{title2=$g+\lambda(g-G_t*g)$}
Subtract a smoothed <image signal> and add a scaled residual: $u=g+\lambda(g-G_t*g)$. The Fourier multiplier $1+\lambda(1-e^{-t|\xi|^2})$ is bounded by $1+\lambda$. This enhances apparent contrast but also amplifies noise; it is not a stable exact inverse of heat <image smoothing>.