In my reference in mesh processing I came across the following equation
Where $\bf{I}$ is the first fundametal form of some surface patch $\bf{x}$, I don't know the exact meaning of the notation $\bf{x}_u$ orthogonal (I don't know how to do the symbol in the formula). I've tried to expand the expression, but I don't end up with anything.
My assumption was to write the rejection of $\bf{x}_u$ respect to $\bf{x}_v$, but it doesn't seem to lead me to anything... Any suggestion?