Increase vertical space beetwen underbrace bracket and content

Question

How can I increase the space marked in red?

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[
\underbrace{x+2}_{\text{content}}
\]
\end{document}

Answer 1

Here's a solution that employs a 2-parameter macro called \ShiftDown. The first argument must be a length -- say, 3ex or 7mm -- and the is the text you wish to display below the brace.

\documentclass{article}
\usepackage{amsmath} % for '\underbrace' macro
\newcommand\ShiftDown[2]{\raisebox{-#1}{\upshape\scriptsize #2}}
\begin{document}
\[
\underbrace{x+2}_{\text{content}} 
\underbrace{x+2}_{\ShiftDown{1.5ex}{content}}
\underbrace{x+2}_{\ShiftDown{3.0ex}{content}}
\underbrace{x+2}_{\ShiftDown{4.5ex}{content}}
\]
\end{document}

Answer 2

thankx to Bernard -- https://tex.stackexchange.com/a/459154/197451

\documentclass[a4paper]{article}   
\usepackage{mathtools}

\usepackage{bigstrut}

\begin{document}
   \begin{equation}\label{eq:reprojection_error_final}
        f=\min_{\hat{P}^i, \hat{X}_j}
        \sum_{ij} \Bigl[\underbracket[1.5pt][8pt]{d(\hat{P}^i \hat{X}_j, x^i_j)}_{\text{Projektionsdiff.}} + \underbracket[1.5pt][8pt]{d(L_k^m - L_{k+1}^m ,\hat{X}_k^m - \hat{X}_{k+1}^m)}_{\text{3D-Punkt-Differenz}} + \underbracket[1.5pt][8pt]{d(\hat{X}_0, 0)}_{\text{Ursprungsdiff.}}\Bigr]^2
        \end{equation}


        \begin{equation}\label{eq:reprojection_error_final}
        f=\min_{\hat{P}^i, \hat{X}_j}
        \sum_{ij} \Bigl[\underbracket[1.5pt][8pt]{\bigstrut[b]d(\hat{P}^i \hat{X}_j, x^i_j)}_{\text{Projektionsdiff.}} + \underbracket[1.5pt][8pt]{\bigstrut[b]d(L_k^m - L_{k+1}^m ,\hat{X}_k^m - \hat{X}_{k+1}^m)}_{\text{3D-Punkt-Differenz}} + \underbracket[1.5pt][8pt]{\bigstrut[b]d(\hat{X}_0, 0)}_{\text{\clap{Ursprungsdiff.}}}\Bigr]^2
        \end{equation}

\end{document}