The un-normalize operation compensates for the normalization points operation in step 0.
Consider the (un-normalized) fundamental matrix $F$, where $x^T F x' = 0$ for all correspondences $x$ and $x'$. And consider a normalized version of the fundamental matrix $F'$, where $y^T F' y' = 0$ for all normalized points $y = S x$ and $y' = S x'$. The un-normalized matrix $F$ and the normalized matrix $F$ are related by the following expression: $F = S^T F' S$.
Normalizing the points helps to improve the numerical stability of this procedure.
How do you un-normalize F?
The un-normalize operation compensates for the normalization points operation in step 0.
Consider the (un-normalized) fundamental matrix $F$, where $x^T F x' = 0$ for all correspondences $x$ and $x'$. And consider a normalized version of the fundamental matrix $F'$, where $y^T F' y' = 0$ for all normalized points $y = S x$ and $y' = S x'$. The un-normalized matrix $F$ and the normalized matrix $F$ are related by the following expression: $F = S^T F' S$.
Normalizing the points helps to improve the numerical stability of this procedure.