Alignments by means of the tikzmark macro

Question

This is a follow up of my previous question Issues and potentiality of the tikzmark macro: dynamic box adaptation. I thought should have been better to ask two questions although they are related to the same argument because problems are different.

In a recent answer Horizontal alignment within an equation using Tikz, I derived a slightly different version of the macro to align text.

My solution is a bit fragile, in the sense that it strongly depends on the \textdim parameter, used to set the text width option.

Here are the examples. With this code:

\documentclass[11pt]{article}
\usepackage{xparse}
\usepackage{tikz}
\usepackage{amsmath,amssymb}

\newlength\textdim
\setlength{\textdim}{2cm}

%% code by Andrew Stacey 
% https://tex.stackexchange.com/questions/51582/background-coloring-with-overlay-specification-in-algorithm2e-beamer-package#51582

\makeatletter
\tikzset{%
     remember picture with id/.style={%
       remember picture,
       overlay,
       save picture id=#1,
     },
     save picture id/.code={%
       \edef\pgf@temp{#1}%
       \immediate\write\pgfutil@auxout{%
         \noexpand\savepointas{\pgf@temp}{\pgfpictureid}}%
     },
     if picture id/.code args={#1#2#3}{%
       \@ifundefined{save@pt@#1}{%
         \pgfkeysalso{#3}%
       }{
         \pgfkeysalso{#2}%
       }
     }
   }

   \def\savepointas#1#2{%
  \expandafter\gdef\csname save@pt@#1\endcsname{#2}%
}

\def\tmk@labeldef#1,#2\@nil{%
  \def\tmk@label{#1}%
  \def\tmk@def{#2}%
}

\tikzdeclarecoordinatesystem{pic}{%
  \pgfutil@in@,{#1}%
  \ifpgfutil@in@%
    \tmk@labeldef#1\@nil
  \else
    \tmk@labeldef#1,\pgfpointorigin\@nil
  \fi
  \@ifundefined{save@pt@\tmk@label}{%
    \tikz@scan@one@point\pgfutil@firstofone\tmk@def
  }{%
  \pgfsys@getposition{\csname save@pt@\tmk@label\endcsname}\save@orig@pic%
  \pgfsys@getposition{\pgfpictureid}\save@this@pic%
  \pgf@process{\pgfpointorigin\save@this@pic}%
  \pgf@xa=\pgf@x
  \pgf@ya=\pgf@y
  \pgf@process{\pgfpointorigin\save@orig@pic}%
  \advance\pgf@x by -\pgf@xa
  \advance\pgf@y by -\pgf@ya
  }%
}
\makeatother

\NewDocumentCommand{\tikzmarkin}{m O{white} m}{%
      \tikz[remember picture,overlay,baseline]
      \draw[line width=1pt,rectangle,rounded corners,fill=#2,draw=none,outer sep=1pt,inner sep=1pt]
      (pic cs:#1) ++(0.065,-0.32) rectangle (-0.05,0.6);
      \tikz[baseline=(current bounding box.-7)] \node [align=center,text width=\textdim]at(pic cs:#1){\ensuremath{#3}}
      ;}

\newcommand\tikzmarkend[2][]{%
\tikz[remember picture with id=#2] #1;}


\begin{document}
\begin{alignat*}{3}
A &= \tikzmarkin{a}[red!20]{\dfrac{\partial u}{\partial x} + \lambda}\tikzmarkend{a}&&+ \dfrac{\partial f}{\partial y}\dfrac{\partial f}{\partial z}\\
B &= \tikzmarkin{b}[blue!20]{\dfrac{\partial u}{\partial z} + \dfrac{\partial^2 u}{\partial x}} \tikzmarkend{b}&&+ \gamma  \\
CD &=\tikzmarkin{c}[green!20]{\dfrac{\partial f}{\partial x}}\tikzmarkend{c}&& + \dfrac{\partial f}{\partial z} \\
E &= \tikzmarkin{d}[orange!20]{\Gamma(x)}\tikzmarkend{d} &&+ \Xi(y)
\end{alignat*}


\end{document}

it is possible to get:

Let's now do a simple modification:

\setlength{\textdim}{4cm}

After having compiled twice, you get:

which is really unexpected. This behavior could be corrected by setting:

\tikz[baseline=(current bounding box.-3)]...

to achieve:

Is there any particular reasons for which an horizontal change affects the vertical positioning?

Answer 1

The reason for the shift is that you are setting the baseline of the tikzpicture by using a point on the boundary of the bounding box that is specified by an angle. This is in the key baseline=(current bounding box.-7). This refers to the point on the boundary given by the angle -7 degrees. As the aspect ratio of the picture changes, the height of this changes. Here's an example:

Notice in particular how the boundary point at angle 20 shifts from the top to the side as the aspect ratio changes.

Given that you are putting text in a node inside the tikzpicture, probably the best coordinate to use to get the baseline correct is the base anchor of this node. Thus:

\tikz[baseline=(h.base)] \node (h) [align=center,text width=\textdim]at(pic cs:#1){\ensuremath{#3}};

(maybe choose a better node name, though it isn't all that important). This results in:

which is what I guess you want.

Here's the full code for that:

\documentclass[11pt]{article}
%\url{http://tex.stackexchange.com/q/57538/86}
\thispagestyle{empty}
\usepackage{xparse}
\usepackage{tikz}
\usetikzlibrary{calc}
\usepackage{amsmath,amssymb}

\newlength\textdim
\setlength{\textdim}{2cm}
\setlength{\textdim}{4cm}

%% code by Andrew Stacey 
% http://tex.stackexchange.com/questions/51582/background-coloring-with-overlay-specification-in-algorithm2e-beamer-package#51582

\makeatletter
\tikzset{%
     remember picture with id/.style={%
       remember picture,
       overlay,
       save picture id=#1,
     },
     save picture id/.code={%
       \edef\pgf@temp{#1}%
       \immediate\write\pgfutil@auxout{%
         \noexpand\savepointas{\pgf@temp}{\pgfpictureid}}%
     },
     if picture id/.code args={#1#2#3}{%
       \@ifundefined{save@pt@#1}{%
         \pgfkeysalso{#3}%
       }{
         \pgfkeysalso{#2}%
       }
     }
   }

   \def\savepointas#1#2{%
  \expandafter\gdef\csname save@pt@#1\endcsname{#2}%
}

\def\tmk@labeldef#1,#2\@nil{%
  \def\tmk@label{#1}%
  \def\tmk@def{#2}%
}

\tikzdeclarecoordinatesystem{pic}{%
  \pgfutil@in@,{#1}%
  \ifpgfutil@in@%
    \tmk@labeldef#1\@nil
  \else
    \tmk@labeldef#1,(0pt,0pt)\@nil
  \fi
  \@ifundefined{save@pt@\tmk@label}{%
    \tikz@scan@one@point\pgfutil@firstofone\tmk@def
  }{%
  \pgfsys@getposition{\csname save@pt@\tmk@label\endcsname}\save@orig@pic%
  \pgfsys@getposition{\pgfpictureid}\save@this@pic%
  \pgf@process{\pgfpointorigin\save@this@pic}%
  \pgf@xa=\pgf@x
  \pgf@ya=\pgf@y
  \pgf@process{\pgfpointorigin\save@orig@pic}%
  \advance\pgf@x by -\pgf@xa
  \advance\pgf@y by -\pgf@ya
  }%
}
\makeatother

\NewDocumentCommand{\tikzmarkin}{m O{white} m}{%
      \tikz[remember picture,overlay,baseline]
      \draw[line width=1pt,rectangle,rounded corners,fill=#2,draw=none,outer sep=1pt,inner sep=1pt]
      (pic cs:#1) ++(0.065,-0.32) rectangle (-0.05,0.6);
      \tikz[baseline=(h.base)] \node (h) [align=center,text width=\textdim]at(pic cs:#1){\ensuremath{#3}}
      ;}

\newcommand\tikzmarkend[2][]{%
\tikz[remember picture with id=#2] #1;}


\begin{document}
\begin{alignat*}{3}
A &= \tikzmarkin{a}[red!20]{\dfrac{\partial u}{\partial x} + \lambda}\tikzmarkend{a}&&+ \dfrac{\partial f}{\partial y}\dfrac{\partial f}{\partial z}\\
B &= \tikzmarkin{b}[blue!20]{\dfrac{\partial u}{\partial z} + \dfrac{\partial^2 u}{\partial x}} \tikzmarkend{b}&&+ \gamma  \\
CD &=\tikzmarkin{c}[green!20]{\dfrac{\partial f}{\partial x}}\tikzmarkend{c}&& + \dfrac{\partial f}{\partial z} \\
E &= \tikzmarkin{d}[orange!20]{\Gamma(x)}\tikzmarkend{d} &&+ \Xi(y)
\end{alignat*}
\end{document}

(Note also the correction to the coordinate system as per my comment to the question.)