When I change the equation color, there is a blank line

Question

I want to change the all the equation color, so I redefined the equation environment code, but there is one blank line after the equation, my codes as following:

case 1:

\documentclass[]{article}
\usepackage{lipsum}
\usepackage{xcolor}
\usepackage{amsmath}

\makeatletter

\renewenvironment{equation}{%
\incr@eqnum
\mathdisplay@push\color{red}
\st@rredfalse \global\@eqnswtrue
\mathdisplay{equation}%
}{%
\endmathdisplay{equation}%
\mathdisplay@pop
\ignorespacesafterend
}
\makeatother

\begin{document}
\lipsum[2]
\begin{equation}\label{eq1}
    {x_{n + 1}} = F({x_n},\lambda ),{x_n} \in [a,b]. 
\end{equation}

\lipsum[2]

\end{document}

Codes without \color{red} as following:

case 2:

\documentclass[]{article}
\usepackage{lipsum}
\usepackage{xcolor}
\usepackage{amsmath}

\makeatletter

\renewenvironment{equation}{%
\incr@eqnum
\mathdisplay@push
\st@rredfalse \global\@eqnswtrue
\mathdisplay{equation}%
}{%
\endmathdisplay{equation}%
\mathdisplay@pop
\ignorespacesafterend
}
\makeatother

\begin{document}
\lipsum[2]
\begin{equation}\label{eq1}
    {x_{n + 1}} = F({x_n},\lambda ),{x_n} \in [a,b]. 
\end{equation}

\lipsum[2]

\end{document}

As the following image shows, when \color{red} is added there is extra space under the red equation. I hope someone can tell me why.

Answer 1

If you simplify your example it is easier to see where the space is coming from (and the fact that it is a line of a paragraph, not a space)

\documentclass[]{article}

\usepackage{xcolor}
\usepackage{amsmath}

\showoutput
\showboxdepth4
\begin{document}


1
\begin{equation}\label{eq1}
   xx
\end{equation}


2


\makeatletter

\renewenvironment{equation}{%
\incr@eqnum
\mathdisplay@push\color{red}%%%%
\st@rredfalse \global\@eqnswtrue
\mathdisplay{equation}%
}{%
\endmathdisplay{equation}%
\mathdisplay@pop
\ignorespacesafterend
}
\makeatother

3
\begin{equation}\label{eq2}
   xx
\end{equation}


4



\end{document}

produces

Looking after the log you see that the first equation and the following paragraph "2" is

...\hbox(7.5+2.5)x178.21527, shifted 166.78473
....\hbox(4.30554+0.0)x11.43054, display []
....\kern154.00691
....\hbox(7.5+2.5)x12.77782, display []
...\penalty 0
...\glue(\belowdisplayshortskip) 6.0 plus 3.0 minus 3.0
...\glue(\parskip) 0.0 plus 1.0
...\glue(\baselineskip) 3.05556
...\hbox(6.44444+0.0)x345.0, glue set 324.99998fil
....\hbox(0.0+0.0)x15.0
....\OT1/cmr/m/n/10 2

so you just have the normal space before a paragraph plus \belowdisplayshortskip space.

Compare with the the second equation and the paragraph "4"

...\hbox(7.5+2.5)x178.21527, shifted 166.78473
....\hbox(4.30554+0.0)x11.43054, display []
....\kern154.00691
....\hbox(7.5+2.5)x12.77782, display []
...\penalty 0
...\glue(\belowdisplayshortskip) 6.0 plus 3.0 minus 3.0
...\glue(\baselineskip) 9.5
...\hbox(0.0+0.0)x345.0, glue set 345.0fil
....\pdfcolorstack 0 pop
....\penalty 10000
....\glue(\parfillskip) 0.0 plus 1.0fil
....\glue(\rightskip) 0.0
...\glue(\parskip) 0.0 plus 1.0
...\glue(\baselineskip) 5.55556
...\hbox(6.44444+0.0)x345.0, glue set 324.99998fil
....\hbox(0.0+0.0)x15.0
....\OT1/cmr/m/n/10 4

Here you see that the paragraph that started "3" and was interrupted by the math display has a final (white) line that just contains the colour restore.

The colour restore is not just a switch of state like restoring a font at the end of a group it is a real node in the list (much like an empty box, technically a whatsit node) and as there is another node in the list display math returns first to horizontal mode, and then the blank line ends the paragraph so you get a final all-white line of paragraph.

Note this is not vertical space and would not for example be dropped at a page break.

If you insert the colour nodes in safer places then

\documentclass[]{article}

\usepackage{xcolor}
\usepackage{amsmath}

\showoutput
\showboxdepth4
\begin{document}


1
\begin{equation}\label{eq1}
   xx
\end{equation}


2


\makeatletter

\renewenvironment{equation}{%
\incr@eqnum
\mathdisplay@push
\st@rredfalse \global\@eqnswtrue
\let\zzz\tagform@
\def\tagform@##1{\textcolor{red}{\zzz{##1}}}%%%
\mathdisplay{equation}%
\begingroup\color{red}%%%%%
}{%
\endgroup\endmathdisplay{equation}%
\mathdisplay@pop
\ignorespacesafterend
}
\makeatother

3
\begin{equation}\label{eq2}
   xx
\end{equation}


4



\end{document}

Answer 2

Seems that an \egroup or something similar during \endmathdisplay causes the problem.

To confirm test the code (that coloring using \textcolor without the unwanted space):

\documentclass[]{article}
\usepackage{lipsum}
\usepackage{xcolor}
\usepackage{amsmath}

\makeatletter

\renewenvironment{equation}{%
\incr@eqnum
\mathdisplay@push
%\textcolor{blue}\bgroup
\st@rredfalse \global\@eqnswtrue
\mathdisplay{equation}%
\textcolor{blue}\bgroup
}{%
\egroup
\endmathdisplay{equation}%
%\egroup
\mathdisplay@pop
\ignorespacesafterend
}
\makeatother

\begin{document}
\lipsum[2]
\begin{equation}\label{eq1}
    {x_{n + 1}} = F({x_n},\lambda ),{x_n} \in [a,b]. 
\end{equation}

\lipsum[2]

\end{document}

If you use the same code starting end ending in the commented start and end you will see the problem coming back.

So, there (in \endmathdisplay) is hidden the reason of your problem.