There are various problems with your code. First, multirooted structures are not formally trees, so you can't expect tree drawing packages to handle them directly. See the following question for more discussion on techniques for this:
Furthermore, the syntax for a gb4e example requires that the example be introduced by the \ex macro (within the {exe} environment.)
To handle the multidominance, I've used Ignasi's suggestion to create a node relative to the -| path between node A and E. You could also use the TikZ positioning library, to manually place the node. I've also removed the tikz-qtree package, since it's not needed if you're using forest. Since your 'tree' grows upwards, I've added baseline to one of the terminal nodes so that it will align properly with the example number, and set the growth function globally for the tree. Also, you had a particular style for nodes in your example, but you never applied that style, and I've done that too.
You should also load gb4e after most other packages, since it can cause problems otherwise.)
\documentclass{article}
\usepackage{forest}
\usepackage{gb4e}
\begin{document}
\begin{exe}
\ex
\begin{forest}
qtree edges/.style={for tree={parent anchor=north, child anchor=south}},
for tree={grow=north},qtree edges
[A, name=A,
[C, name=C
[E,baseline,name=E ]
[D, ]
]
[B ]
]
\node (F) at (A-|E) {F};
\draw (F.north) -- (C.south);
\end{forest}
\end{exe}
\end{document}
Here are the definitions for forked edge and forked edges from forest-lib-edges.sty.
\forestset{
declare dimen={fork sep}{0.5em},
forked edge/.style={
edge={rotate/.pgfmath=grow()},
edge path'={(!u.parent anchor) -- ++(\forestoption{fork sep},0) |- (.child anchor)},
},
forked edges/.style={
for tree={parent anchor=children},
for descendants={child anchor=parent,forked edge}
},
}
So we see that forked edges not only applies forked edge to all descendants, but adjusts the parent anchor for the tree and the child anchor for the descendants as well.
I don't think that forked edge is correct here. It is true that you don't want forked edges because then you'll have
for tree={
for tree={
...
which can cause problems.
However, if you use \forked edge without setting the parent/child anchors, then Forest defaults to using the (<node>) itself and TikZ then selects a border anchor in its usual way. However, the initial part of the forked path is drawn relative to the starting point. The starting point is also (<node>) but this time TikZ is measuring relative to rather than tracing a path from. So rather than using the node's border anchor, it uses the node's default anchor, which is .center by default.
So although the path starts from the appropriate border anchor, its initial segment takes it towards a location measured relative to the .center anchor and not measured relative to the border anchor.
Here's a visualisation:
The path starts from (a.east), which is right, but the point ++(.5em,0) from (a) is measured from (a.center). This is shown in red. The first part of the path, therefore, is the part shown in blue which is not .5em, of course. The remainder of the path is then in green.
We can see the same thing in the default growth direction where the edge takes rotate=-90.
I don't understand your second question. Why should l sep affect the branching point? I think you are asking why you don't get something like the following.
This can be done by adjusting fork sep.
\begin{forest}
forked edges,
for tree={
font=\footnotesize,
draw,
semithick,
rounded corners,
align = center,
inner sep = 3mm,
edge = {semithick, -stealth},
grow = east,
l sep = 12mm,
fork sep=6mm,
}
[Multimedia\\ RSs
[Multimedia\\ Content]
[Multimedia\\ Content]
]
\end{forest}
But note also, that l sep is a minimum distance between the closest border point of the parent and the closest border point of each child. That distance may end up being larger than the specified dimension. Forest just stops it being any smaller. (Unless it is altered later, of course, either for the whole tree or more locally.)
So you can't guarantee the midpoint in this way. To do that, you would need to alter fork sep later, as the tree is packed because you'd need, I think, to change it for each level before the sub-trees of the nodes on that level are packed. (Or you'd have to repack afterwards.) I'm not certain of this, however - it might be sufficient to do it after the entire tree is packed, at least in many cases. (Of course, you'd want to do it before the x and y values are computed.)
\documentclass[border=10pt,multi,tikz]{standalone}
\usepackage[edges]{forest}
\begin{document}
\begin{tikzpicture}
\node [draw] (a) at (0,0) {a};
\node [draw] (b) at (1,.25) {b};
\node [draw] (c) at (1,-.25) {c};
\foreach \i in {b,c}
{
\draw [rotate=0] (a) -- ++(.5em,0) |- (\i);
\draw [rotate=0, green] (a) ++(.5em,0) |- (\i);
\draw [rotate=0, blue] (a) -- ++(.5em,0);
}
\draw [red] (a.center) -- ++(.5em,0);
\end{tikzpicture}
\begin{tikzpicture}
\node [draw] (a) at (0,0) {a};
\node [draw] (b) at (-.25,-1) {b};
\node [draw] (c) at (.25,-1) {c};
\foreach \i in {b,c}
{
\draw [rotate=-90] (a) -- ++(.5em,0) |- (\i);
\draw [rotate=-90, green] (a) ++(.5em,0) |- (\i);
\draw [rotate=-90, blue] (a) -- ++(.5em,0);
}
\draw [rotate=-90, red] (a.center) -- ++(.5em,0);
\end{tikzpicture}
\begin{forest}
forked edges,
for tree={
font=\footnotesize,
draw,
semithick,
rounded corners,
align = center,
inner sep = 3mm,
edge = {semithick, -stealth},
grow = east,
l sep = 12mm,
fork sep=6mm,
}
[Multimedia\\ RSs
[Multimedia\\ Content]
[Multimedia\\ Content]
]
\end{forest}
\end{document}
Best Answer
Set the
text heightandtext depthexplicitly such that all nodes have the same height and baseline no matter what the letter looks like.