Conditions for fancy chapter title

Question

I use a book class and here is a part of my fancy title page for the mainmatter chapters.

\newlength\chapterwidth
\settowidth\chapterwidth{\huge\chaptertitlename}
\titleformat{\chapter}[display]
{\normalfont\filcenter}
{\tikz[remember picture,overlay]{
\node[fill=nicedarkblue,font=\sffamily\fontsize{96}{72}\bf\selectfont\color{white},anchor=north east, minimum width=3cm, minimum height=3.4cm] 
at ([xshift=-1cm,yshift=-1cm]current page.north east) 
(numb) {\thechapter};
\node[rotate=90,anchor=south,inner sep=4pt,font=\Huge\sffamily] at (numb.west) {Chapter};
}
}{20pt}{\scshape\Huge\color{nicedarkblue}#1}[\vskip10pt\Large***]

I do not master conditions in latex, but I would like to have four different styles:

• chapter in frontmatter
• chapter in mainmatter
• appendix in mainmatter
• chapter in backmatter

How to branch on these conditions ?

Answer 1

Generally speaking, rather than including "conditionals" in the title format to work out where in the document the title is used, you either (a) include macros in the format itself which will expand appropriately (e.g. \chaptertitlename which will be redefined once \appendix is given) or (b) redefine with a new \titleformat at the appropriate point in the document.

Since you didn't give a complete compilable MWE I'm guessing somewhat about your settings. But here's a short example. This is basically your code, but (a) instead of "hardcoding" the word "Chapter" in the bit that prints the colored box, I've used \chaptertitlename and (b) instead of hard-coding a color in the two places you used it, I put a macro which can therefore easily be redefined.

\documentclass{book}
\usepackage[explicit]{titlesec}
\usepackage{lipsum}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\newcommand{\chaptercolor}{blue}
\usepackage{tikz}

\titleformat{\chapter}
[display]
{\normalfont\filcenter}
{\tikz[remember picture,overlay]
  {\node[fill=\chaptercolor,%<--- Not hardcoded color
       font=\sffamily\fontsize{96}{72}\bf\selectfont\color{white},anchor=north east, 
       minimum width=3cm, 
       minimum height=3.4cm] 
       at ([xshift=-1cm,yshift=-1cm]current page.north east) 
         (numb) {\thechapter};
     \node[rotate=90,
           anchor=south,
           inner sep=4pt,
           font=\Huge\sffamily]
       at (numb.west) {\chaptertitlename};%<-- Not hardcoded "CHAPTER"
    }}
  {20pt}
  {\Huge\bfseries\color{\chaptercolor}#1}%< Not hardcoded color
  [\vskip10pt\Large\bfseries***]
\begin{document}

\frontmatter

\chapter{Preface}

\lipsum

\mainmatter\renewcommand{\chaptercolor}{red}

\chapter{Chapter}

\lipsum

\appendix\renewcommand{\chaptercolor}{green!40!black!60}

\chapter{Appendix Something}

\end{document}