Question: I'm trying to use the `tikz-feynman` package to produce a fairly simple diagram, I produced the following plot:

```
\documentclass[tikz]{standalone}                                                                                                                                                                     
\usepackage[compat=1.1.0]{tikz-feynman}                                                                                                                                                              

\begin{document}                                                                                                                                                                                     
\feynmandiagram [horizontal=a to b] {                                                                                                                                                                
  i1 [particle=\(q\)] -- [fermion] a -- [fermion] i2[particle=\(\bar{q}\)],                                                                                                                          
  a -- [gluon, edge label=\(g\)] b,                                                                                                                                                                  
  f1 [particle=\(g\)] -- [gluon] b -- [dashed, red] f2 [red, particle=\(G\)]],                                                                                                                       
};  
\end{document} 

```

and would like to add a gluon radiation off the line that connects the `i1` and `a` vertexes. My understanding is that I would have to redo the diagram placing the vertexes manually. Is that so?

Answer: 
Once the diagram has been placed with [Ti*k*Z-Feynman](http://jpellis.me/projects/tikz-feynman) ([CTAN](http://ctan.org/pkg/tikz-feynman)), you can access the vertices as you would with other coordinates in Ti*k*Z.

In your case, you probably don't want to redraw the whole diagram as you want to make it salient as it is the same diagram except with initial state radiation extra. In the solution below, I add an extra vertex `(r)` which I then connect with a gluon line to 80% of the distance from `(i1)` to `(a)`.

```
\RequirePackage{luatex85}
\documentclass[tikz, border=10pt]{standalone}
\usepackage[compat=1.1.0]{tikz-feynman}

\begin{document}
\begin{tikzpicture}
  \begin{feynman}
    \diagram [horizontal=a to b] {
      i1 [particle=\(q\)]
        -- [fermion] a
        -- [fermion] i2[particle=\(\bar{q}\)],
      a -- [gluon, edge label=\(g\)] b,
      f1 [particle=\(g\)]
        -- [gluon] b
        -- [dashed, red] f2 [red, particle=\(G\)]],
    };

    \vertex [above left=of f1] (r);
    \draw [gluon] ($(i1)!0.8!(a)$) -- (r);
  \end{feynman}
\end{tikzpicture}
\end{document}

```

[![output](https://i.stack.imgur.com/uasBB.png)](https://i.stack.imgur.com/uasBB.png)
This is one case where manual placement can come to the rescue, if you want to do anything beyond the simple case of the question (with an s-channel 2-to-2 diagram and a single emission in the initial state). This gives you complete freedom on the look and feel of the diagram.

```
\documentclass[tikz, border=10pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{shapes,arrows,positioning,automata,backgrounds,calc,er,patterns}
\usepackage[compat=1.1.0]{tikz-feynman}

\begin{document}
    \begin{tikzpicture}
        \begin{feynman}

            \vertex [large, blob] (blob) at (0,0) {};
            \vertex (b)  at (-1.1,0.55);
            \vertex (c) at (-0.4,1.);
            \vertex (g1)  at (-2,1) {};
            \vertex (g2) at (-2,-1)  {};
            \vertex (t2) at (2,1)  {};
            \vertex (t1) at (2,-1)  {};
            \vertex (b2)  at (0.6,1.5) {};
            \vertex (b1) at (0.65,0.9)  {};

            \diagram* {
                (g1) -- [gluon] (b) -- [gluon] (blob) -- [gluon] (g2),
                (c) -- [gluon] (b),
                (b1) -- [fermion] (c) -- [fermion] (b2),
                (t1) -- [fermion] (blob) -- [fermion] (t2),
            };
        \end{feynman}
    \end{tikzpicture}
\end{document}

```

[![feynman diagram](https://i.stack.imgur.com/Y6J3c.png)](https://i.stack.imgur.com/Y6J3c.png)