Bleed correction for chapterthumbs in KOMA

Question

I want to send a dissertation document with chapterthumbs to the printing house. The document is written in KOMA-Script and a good solution for bleed correction is found here. Since chapterthumbs are sitting at the edge of the pages in black I want to make sure that there is a bleed correction for that too before sending out the document. Otherwise, due to imprecise cutting process (which is expected), unwanted white margins could appear at the edge of the pages where chapterthumbs are sitting.

Question
How to provide a bleed correction for the chapterthumbs themselves, i.e. extend their black color to the bleed correction area as in the image below?

Output

Consider the MWE below, where chapterthumbs are provided and bleed correction using crop package. What is missing is bleed correction for the chapterthumbs themselves.

MWE Code

\documentclass[paper=a4, twoside=semi]{scrbook} % A4 is 210mm x 297mm
\usepackage[cross,center,width=216mm,height=303mm]{crop} % bleed is 6mm
\usepackage{blindtext}
\usepackage{calc}%
\usepackage{chapterthumb}% package should be installed in the same active folder
\usepackage{etoolbox}%
\usepackage{nameref}%
\usepackage{totcount}
\regtotcounter{chapter}%
%=====================Bibliography=============================
\usepackage{biblatex}
\usepackage{filecontents}
\begin{filecontents}{jobname.bib}
@article{rand_objective_1971,
    title = {Objective criteria for the evaluation of clustering methods},
    volume = {66},
    pages = {846},
    number = {336},
    journaltitle = {Journal of the American Statistical Association},
    author = {Rand, William M.},
    date = {1971}
}

\end{filecontents}
\addbibresource{jobname.bib}
%=========================Chapter thumbs========================
\AtBeginDocument{%
\ifnumequal{\number\totvalue{chapter}}{0}{%
\renewcommand{\chapterthumbwidth}{\paperheight}%
\renewcommand{\chapterthumbskip}{\paperheight}%
}{%
\typeout{There are \number\totvalue{chapter} chapter in this document}%
\renewcommand{\chapterthumbskip}{\paperheight/\number\totvalue{chapter}}%
\renewcommand{\chapterthumbwidth}{\paperheight/\number\totvalue{chapter}}%
}%
}%

\lohead[\putchapterthumb]{\putchapterthumb}
\addtokomafont{chapterthumb}{\bfseries}

\newrobustcmd{\SetChapterThumbHeader}{%
\lohead[\putchapterthumb]{\putchapterthumb}%
}%

\newrobustcmd{\ClearChapterThumbHeader}{%
\lohead[]{}%
}%

\newrobustcmd{\refcommand}[1]{%
\nameref*{#1}%
}%

\newtoggle{UseChapterThumb}%
\toggletrue{UseChapterThumb}%

\renewcommand*{\chapterthumbformat}{\refcommand{chapter::title::\number\value{chapter}}}%

\makeatletter

\newcounter{totalchaptercounter}%

\newrobustcmd{\CreateUniqueChapterLabel}[1]{% 
\refstepcounter{totalchaptercounter}%
\label{chapter::title::\number\value{totalchaptercounter}}%
}%


\let\LaTeXStandardChapter\chapter%

\newcommand{\chapter@noopt}[1]{%
\iftoggle{UseChapterThumb}{\SetChapterThumbHeader}{\ClearChapterThumbHeader}
\LaTeXStandardChapter{#1}%
\CreateUniqueChapterLabel{#1}%
}%

\newcommand{\chapter@opt}[2][]{%
\iftoggle{UseChapterThumb}{\SetChapterThumbHeader}{\ClearChapterThumbHeader}
\LaTeXStandardChapter[#1]{#2}%
\CreateUniqueChapterLabel{#2}% Must appear after chapter title is done
}%


\newcommand{\unstarredchapter}{%
\@ifnextchar[{\chapter@opt}{\chapter@noopt}%
}%

\newcommand{\starredchapter}[1]{%
\ClearChapterThumbHeader% Clear the headers -> no chapterthumb here 
\LaTeXStandardChapter*{#1}%   
}%

\renewcommand{\chapter}{%
\@ifstar{\starredchapter}{\unstarredchapter}%
}%


\let\LaTeXStandardTOC\tableofcontents%
\renewcommand{\tableofcontents}{%
\begingroup%
\renewcommand{\chapter}{\LaTeXStandardChapter}%
\LaTeXStandardTOC%
\endgroup%
}%


\let\LaTeXStandardLOF\listoffigures%
\renewcommand{\listoffigures}{%
\begingroup%
\renewcommand{\chapter}{\LaTeXStandardChapter}
\LaTeXStandardLOF%
\endgroup
}%

\let\LaTeXStandardLOT\listoftables%
\renewcommand{\listoftables}{%
\begingroup%
\renewcommand{\chapter}{\LaTeXStandardChapter}
\LaTeXStandardLOT%
\endgroup
}%


\let\LaTeXStandardPrintBibliography\printbibliography%


\newrobustcmd{\commonprintbibliographysetup}[1][]{%
\iftoggle{UseChapterThumb}{\SetChapterThumbHeader\refstepcounter{chapter}}{\ClearChapterThumbHeader}
\LaTeXStandardPrintBibliography[#1]
\iftoggle{UseChapterThumb}{%
\immediate\write\@auxout{%
\string\newlabel{chapter::title::\number\value{chapter}}{{\thepage}{\thepage}{Bibliography}{}}%
}%
}{}%
}%



\newrobustcmd{\printbibliography@@noopt}{%
\commonprintbibliographysetup[]%
}%

\newrobustcmd{\printbibliography@@opt}[1][]{%
\commonprintbibliographysetup[#1]%
}

\renewcommand{\printbibliography}{%
\@ifnextchar[{\printbibliography@@opt}{\printbibliography@@noopt}%
}%

\makeatother
%==================================================
\title{Title of the Dissertation}
\author{Author Name} 
\publishers{Faculty Name}

\begin{document}
\maketitle
\pagestyle{plain}
\pagenumbering{roman}
\tableofcontents
\listoffigures
\listoftables
\cleardoublepage

\chapter*{Acknowledgments}
\addcontentsline{toc}{chapter}{Acknowledgments}

\chapter*{Abstract}
\addcontentsline{toc}{chapter}{Abstract}
\cleardoublepage
\lohead[\putchapterthumb]{\putchapterthumb}
\mainmatter
\pagestyle{scrheadings}

\chapter{Introduction}
\begin{figure}
\caption{dummy figure}
\end{figure}

\chapter{Materials \& Methods} 

\chapter{Results} 
\begin{table}
\caption{dummy table}
\end{table}

\chapter{Discussion}
In his reference \parencite{rand_objective_1971}, the author did a nice job.
\backmatter
\cleardoublepage
\printbibliography[heading=bibintoc]
\end{document}

Update1: MWE based on Symbol's solution
I wonder what might cause this inequality in the crosses.

Answer 1

Replacing

\usepackage[cross,center,width=216mm,height=303mm]{crop} % bleed is 6mm

by

\usepackage[mirror,cross,center,width=216mm,height=303mm]{crop} % bleed is 6mm
    \makeatletter
    \newsavebox\WholePageBigBox
    \renewcommand*\CROP@genreflect[1]{%
        \leavevmode
        \savebox\WholePageBigBox{\hb@xt@\paperwidth{\vbox to\paperheight{#1\vss}\hss}}
        \usebox\WholePageBigBox\dimen0\CROP@horigin\kern2\dimen0\reflectbox{\usebox\WholePageBigBox}}
    \makeatother

results and .

Idea

In the following MWE we could see that every page is duplicated and reflected. Hence every thumb index will not stop at the logical edge but seems exceeding it. This trick works no matter how you generate thumb indexes or any other typographic thing requiring bleed correction.

Be careful that since the black box below is not a single box but consists of two, the logical edge may still be uninked. This can be easily fixed by replacing \kern2\dimen0 by \kern2\dimen0\kern-1pt.

\documentclass[oneside]{book}
\usepackage[mirror,cam,a2,center]{crop}
\usepackage{thumb,blindtext}
\makeatletter
\newsavebox\WholePageBigBox
\renewcommand*\CROP@genreflect[1]{
    \leavevmode
    \savebox\WholePageBigBox{\hb@xt@\paperwidth{\vbox to\paperheight{#1\vss}\hss}}
    \usebox\WholePageBigBox\dimen0\CROP@horigin\kern2\dimen0\reflectbox{\usebox\WholePageBigBox}}
\makeatother
\title{MyTitle}
\begin{document}
    \blinddocument
\end{document}

Antisymmetry

For antisymmetry, please add % at appropriate position so that the newline will not produce any space in horizontal mode.

\renewcommand*\CROP@genreflect[1]{
    \leavevmode
    \savebox\WholePageBigBox{\hb@xt@\paperwidth{\vbox to\paperheight{#1\vss}\hss}}%
    \usebox\WholePageBigBox\dimen0\CROP@horigin\kern2\dimen0\reflectbox{\usebox\WholePageBigBox}}

TOC items duplicated issue

This happens simply because every toc-command is executed twice. So you may either

• Delete duplicated items in .toc file by hand; or

• Turn on and off \contentsline on the fly, as follows.

  \makeatletter
  \newsavebox\WholePageBigBox
  \renewcommand*\CROP@genreflect[1]{%
      \leavevmode
      \savebox\WholePageBigBox{\hb@xt@\paperwidth{\vbox to\paperheight{#1\vss}\hss}}%
          \write\@auxout{\noexpand\TurnOn@writefile}%
          \usebox\WholePageBigBox%
          \write\@auxout{\noexpand\TurnOff@writefile}%
          \dimen0\CROP@horigin\kern2\dimen0\reflectbox{\usebox\WholePageBigBox}}
  \let\old@writefile=\@writefile
  \newcommand*\lazy@writefile[2]{}
  \newcommand*\TurnOn@writefile{\renewcommand\@writefile\old@writefile}
  \newcommand*\TurnOff@writefile{\renewcommand\@writefile\lazy@writefile}
  \makeatother