How to get one component of a tikz/PGF coordinate?

Question

I'm trying to draw a horizontal line across my diagram. The Y coordinate of the line should be halfway between points a and b (a is below b). The left and right endpoints of the line are on the bounding box of the tikzpicture. Here's how I'm doing this now, using the intersection operator:

\coordinate (h0) at ($(a.north)!0.5!(b.south)$);
\draw (h0 -| current bounding box.west) -- (h0 -| current bounding box.east);

This strikes me as rather roundabout. What I'd rather do is get the Y coordinate of (h0) and the X coordinates of the east and west sides of the bounding box, and compose the coordinates myself. I'd like to do this, but it isn't supported syntax:

\coordinate (h0) at ($(a.north)!0.5!(b.south)$);
\draw (current bounding box.west.x,h0.y) -- (current bounding box.east.x,h0.y);

Is there a way to reference individual components of coordinates that I'm missing?

Solution 1

You can get at the components inside a let operation. Look it up in the PGF manual for the works, but from memory:

\draw
  let
    \p1=($(a.north)!0.5!(b.south)$),
    \p2=(current bounding box.west),
    \p3=(current bounding box.east)
  in
    (\x2,\y1) -- (\x3, \y1);

That'll probably need debugging... EDIT: and now has been thanks to the questioner.

Solution 2

Alternatively, use

\pgfextractx{<dimension>}{<point>}
\pgfextracty{<dimension>}{<point>}

These are raw PGF commands, so it may be less convenient to use them.

Solution 3

Another option I just found from this shows that you can get relative positioning from a node by doing this:

\node at (2, 1) (a) {};
\draw (a) -- ++(right:2);

This will draw a line from node a to the point 2 units to the right of point a. Like in the example I linked, this is useful when you don't know the exact coordinates of a node, and need to draw things relative to it.

Comments

[uckelman]

Thanks! Debugging: The comma ending the \p3 line needs to be deleted, a.north needs a closing ')' and b.south needs an opening '('. Once that's done, this works perfectly.

[leemes]

But <point> really needs to be a lowlevel pgfpoint. To make it work with points defined using \coordinate at ..., you have to say \pgfpointanchor{coordinate}{center}, since \coordinate (coordinate) at (1,2); defines a node of shape coordinate (having one ancher called center) and \pgfpointanchor returns the pgf lowlevel point for this anchor.

[tobiasBora]

@leemes Any idea how to make \pgfpointanchor work for pgfpoints (i.e. if the coordinate is in fact a pgfpoint, then just return that point)? Or maybe to make the distinction between pgfpoints and coordinate? I'm asking because in path \tikztotarget can be both a point or a coordinate depending on whether the start is A or A.east.