# Hey, Emacs!  This is a -*- mode: org -*- file!

#+TITLE:     Support for the banner of the newsletter "La Lettre de GUTenberg"
#+AUTHOR:    Association GUTenberg
#+EMAIL:     secretariat@gutenberg-asso.fr
#+DESCRIPTION: ...
#+KEYWORDS:  ...
#+LANGUAGE:  en
#+OPTIONS:   num:t toc:nil title:nil

#+LATEX_COMPILER: lualatex

#+LaTeX_CLASS: letgut
#+LaTeX_HEADER: \setmainfont{KpRoman}
#+LaTeX_HEADER: \setsansfont{KpSans}
#+LaTeX_HEADER: \setmonofont[Scale = MatchLowercase]{RobotoMono}
#+LaTeX_HEADER: \letgutsetup{editorial=false}
#+LaTeX_HEADER: \input{listings-conf}
# #+LaTeX_HEADER: \input{codehigh-conf}
# #+LaTeX_HEADER: \letgutsetup{editorial=false,final}

#+PROPERTY: header-args :padline no :tangle ../../../tex/lualatex/letgut/letgut-banner.sty :exports both :noweb yes :eval always
#+export_file_name: ../../../doc/lualatex/letgut/letgut-banner-code

* Readme file
:PROPERTIES:
:CUSTOM_ID: Readmefile-dlkgxqt0fck0
:END:

#+begin_src markdown :tangle no :exports none
  letgut-banner - Support for the banner of the newsletter
                  "La Lettre de GUTenberg"
  =============================================================

  About
  -------
  This package automatically draws the banner of the newsletter "La Lettre de
  GUTenberg" with the current main font.

  Release
  -------
  2024-10-07 v0.10

  Development
  -----------
  Follow development, submit issues, and suggest improvements at
  https://framagit.org/gutenberg/classe-lettre-gut.
#+end_src

#+NAME: preamble
#+begin_src
-------:| ---------------------------------------------------
 letgut:| Package for the banner of the newsletter
        | "La Lettre de GUTenberg"
 Author:| Association GUTenberg
 E-mail:| secretariat@gutenberg-asso.fr
License:| Released under the LaTeX Project Public License
        |  v1.3c or later
    See:| http://www.latex-project.org/lppl.txt

Copyright (C) 1994-2024 by Association GUTenberg
                  <secretariat@gutenberg-asso.fr>

This work may be distributed and/or modified under the
conditions of the LaTeX Project Public License (LPPL), either
version 1.3c of this license or (at your option) any later
version.  The latest version of this license is in the file:

http://www.latex-project.org/lppl.txt

This work is "maintained" (as per LPPL maintenance status)
by Association GUTenberg.
#+end_src

* Implementation
:PROPERTIES:
:CUSTOM_ID: Implementation-ckogxqt0fck0
:END:

** Identification
:PROPERTIES:
:CUSTOM_ID: ImplementationIdentification-gwqgxqt0fck0
:END:

#+begin_src
  This is file `letgut-banner.sty,
  generated with the Emacs Org-babel utility.

  The original source file is letgut-banner.org
  %% <<preamble>>
#+end_src

These document classes can only be used with \LaTeXe, so we make
sure that an appropriate message is displayed when another \TeX{}
format is used.

#+begin_src latex
\NeedsTeXFormat{LaTeX2e}[2020-10-01]
#+end_src

Now, we can announce the package name and its version:

#+begin_src latex
\ProvidesExplPackage{letgut-banner}
{2024-10-07}
{0.10}
{
  Package for the banner of the newsletter "La Lettre de GUTenberg"
}
#+end_src

** Packages loaded
:PROPERTIES:
:CUSTOM_ID: ImplementationPackagesloaded-etvgxqt0fck0
:END:

Till the Lua-based color will be  added to the core \package{l3color} approach,
we have to rely on \package{xcolor}

#+begin_src latex
\RequirePackage{xcolor}
#+end_src

We want the material added to the first page to be not selectable in order to let the
reader who wants to copy the interesting text be able to select it (see below).

#+begin_src latex
\RequirePackage{accsupp}
#+end_src

#+begin_src latex
  \cs_generate_variant:Nn \color_fill:nn { nV }
#+end_src

** Declarations
:PROPERTIES:
:CUSTOM_ID: ImplementationDeclarations-u0ygxqt0fck0
:END:

*** Dimensions
:PROPERTIES:
:CUSTOM_ID: ImplementationDeclarationsDimensions-9t0hxqt0fck0
:END:

   #+begin_src latex
     \dim_new:N \g__letgutbanner_area_height_dim
     \dim_new:N \g__letgutbanner_La_height_dim
     \dim_new:N \g__letgutbanner_La_depth_dim
     \dim_new:N \g__letgutbanner_La_yoffset_dim
     \dim_new:N \g__letgutbanner_banner_vertical_contents_width_dim
     \dim_new:N \g__letgutbanner_largest_LETR_box_height_dim
     \dim_new:N \g__letgutbanner_between_letters_yoffset_dim
     \dim_new:N \g__letgutbanner_between_letters_vertical_space_dim
     \dim_new:N \g__letgutbanner_banner_padding_dim
     \dim_if_exist:NF \g__letgutbanner_first_page_shrink_dim {
       \dim_new:N \g__letgutbanner_first_page_shrink_dim
     }
     \dim_new:N \g__letgutbanner_textwidth_dim
   #+end_src

- Dimensions of the page. If the present package is used with the
  \class*{letgut}, they are borrowed from it. Otherwise, they are set to default
  values. For this, we first generate a variant of ~\dim_const:Nn~ (see
  https://github.com/latex3/latex3/issues/947#issuecomment-860950569).
   #+begin_src latex
     \cs_generate_variant:Nn \dim_const:Nn { NV }
     \dim_if_exist:NTF \c__letgut_textheight_dim {
       \dim_const:NV \c__letgutbanner_textheight_dim
       \c__letgut_textheight_dim
     }{
       \dim_const:Nn \c__letgutbanner_textheight_dim
       { 25cm }
     }
     \dim_if_exist:NTF \c__letgut_textwidth_dim {
       \dim_gset_eq:NN \g__letgutbanner_textwidth_dim \c__letgut_textwidth_dim
     }{
       \dim_gset:Nn \g__letgutbanner_textwidth_dim { 14cm }
     }
     \dim_if_exist:NTF \c__letgut_lmargin_dim {
       \dim_const:NV \c__letgutbanner_lmargin_dim
         \c__letgut_lmargin_dim
     }{
       \dim_const:Nn \c__letgutbanner_lmargin_dim
       { 5cm }
     }
     \dim_if_exist:NTF \c__letgut_tmargin_dim {
       \dim_const:NV \c__letgutbanner_tmargin_dim
         \c__letgut_tmargin_dim
     }{
       \dim_const:Nn \c__letgutbanner_tmargin_dim
       { 2.92cm }
       % \dim_const:Nn \c__letgutbanner_tmargin_dim
       % { 3cm }
     }
     \dim_if_exist:NTF \c__letgut_head_dim {
       \dim_const:NV \c__letgutbanner_head_dim
         \c__letgut_head_dim
     }{
       \dim_const:Nn \c__letgutbanner_head_dim
       { 0.6cm }
     }
     \dim_if_exist:NTF \c__letgut_headsep_dim {
       \dim_const:NV \c__letgutbanner_headsep_dim
         \c__letgut_headsep_dim
     }{
       \dim_const:Nn \c__letgutbanner_headsep_dim
       { 0.82cm }
       % \dim_const:Nn \c__letgutbanner_headsep_dim
       % { 0.9cm }
     }
     \dim_if_exist:NTF \c__letgut_marginparwidth_dim {
       \dim_const:NV \c__letgutbanner_marginparwidth_dim
       \c__letgut_marginparwidth_dim
     }{
       \dim_const:Nn \c__letgutbanner_marginparwidth_dim
       { 3.74cm }
     }
   #+end_src
- Dimensions of the banner. If the present package is used with the
  \class*{letgut}, they are borrowed from it. Otherwise, they are set to
  default values.
  - Thickness of the "L".
      #+begin_src latex
        \dim_if_exist:NTF \c__letgut_banner_thickness_dim {
          \dim_const:NV \c__letgutbanner_banner_thickness_dim
          \c__letgut_banner_thickness_dim
        }{
          \dim_const:Nn \c__letgutbanner_banner_thickness_dim
          { 4cm }
        }
        #+end_src
  - Height of the "L" (that of the text area plus 3cm).
      #+begin_src latex
        \dim_if_exist:NTF \c__letgut_banner_height_dim {
          \dim_const:NV \c__letgutbanner_banner_height_dim
            \c__letgut_banner_height_dim
        }{
          \dim_const:Nn \c__letgutbanner_banner_height_dim  {
            \c__letgutbanner_textheight_dim
            +
            3cm
          }
        }
      #+end_src
  - Initial padding of the text embedded in the vertical bar of the "L".
      #+begin_src latex
      \dim_const:Nn \c__letgutbanner_banner_padding_dim  { .25cm }
      #+end_src
  - Width of the "L" (the width ($w$) of the text zone plus the width ($l$) of
    the left margin minus half the difference between the width of the left
    margin and the width ($d$) of the "L": $=w+l-(l-d)/2=w+(l+d)/2$).
      #+begin_src latex
        \dim_const:Nn \c__letgutbanner_banner_width_dim  {
          \g__letgutbanner_textwidth_dim
          +
          (
          \c__letgutbanner_banner_thickness_dim
          +
          \c__letgutbanner_lmargin_dim
          )/2
        }
      #+end_src
  - Initial width of the text embedded in the "L" vertical bar.
      #+begin_src latex
        \dim_const:Nn
        \c__letgutbanner_banner_vertical_contents_width_dim {
          \c__letgutbanner_banner_thickness_dim
          -
          2\c__letgutbanner_banner_padding_dim
        }
      #+end_src
  - Width of the text embedded in the horizontal bar of the "L" (of the text
    area plus half the difference between that of the left margin and that of
    the thickness of the "L", minus the padding -- and not 2 times the
    padding because this text will be stuck to its left "margin").
      #+begin_src latex
        \dim_const:Nn
        \c__letgutbanner_banner_horizontal_contents_width_dim {
          \g__letgutbanner_textwidth_dim
          - (
          \c__letgutbanner_banner_thickness_dim
          -
          \c__letgutbanner_lmargin_dim
          )/2
          -
          \c__letgutbanner_banner_padding_dim
        }
      #+end_src

*** Boxes
:PROPERTIES:
:CUSTOM_ID: ImplementationDeclarationsBoxes-s83hxqt0fck0
:END:

New boxes, for each of the letters in the word "LETTRE" (!), and then for the
rest of the content listed in the banner.
#+begin_src latex
\box_new:N \g__letgutbanner_L_box
\box_new:N \g__letgutbanner_E_box
\box_new:N \g__letgutbanner_T_box
\box_new:N \g__letgutbanner_R_box
\box_new:N \g__letgutbanner_La_box
\box_new:N \g__letgutbanner_GUTenberg_box
\box_new:N \g__letgutbanner_largest_LETR_box
\box_new:N \g__letgutbanner_g_box
#+end_src

*** Floating point numbers
:PROPERTIES:
:CUSTOM_ID: ImplementationDeclarationsFloatingpointnumbers-e75hxqt0fck0
:END:

- Minimum percentage of the common height of the letters of the word "LETTRE"
  that their line spacing must represent.
#+begin_src latex
\fp_const:Nn \c__letgutbanner_leading_minimum_percent_fp { 0.15 }
#+end_src
- Factor applied to the line spacing between "La" and the 1st letter of
  "LETTRE" compared to that between the letters of "LETTRE".
#+begin_src latex
\fp_const:Nn \c__letgutbanner_leading_La_factor_fp { 1.5 }
#+end_src

** Applying the page dimensions
:PROPERTIES:
:CUSTOM_ID: ImplementationApplyingthepagedimensions-187hxqt0fck0
:END:

#+begin_src latex
  \geometry{
    asymmetric,
    textheight    = \c__letgutbanner_textheight_dim,
    textwidth     = \g__letgutbanner_textwidth_dim,
    lmargin       = \c__letgutbanner_lmargin_dim,
    tmargin       = \c__letgutbanner_tmargin_dim,
    head          = \c__letgutbanner_head_dim,
    headsep       = \c__letgutbanner_headsep_dim,
    marginparwidth= \c__letgutbanner_marginparwidth_dim,
    % showframe,
    % verbose,
  }
#+end_src

** Filling of the boxes
:PROPERTIES:
:CUSTOM_ID: ImplementationFillingoftheboxes-q49hxqt0fck0
:END:
#+begin_src latex
  \hbox_gset:Nn \g__letgutbanner_L_box { \textbf{L} }
  \hbox_gset:Nn \g__letgutbanner_E_box { \textbf{E} }
  \hbox_gset:Nn \g__letgutbanner_T_box { \textbf{T} }
  \hbox_gset:Nn \g__letgutbanner_R_box { \textbf{R} }
  \hbox_gset:Nn \g__letgutbanner_La_box { \textbf{\textit{La}} }
  \hbox_gset:Nn \g__letgutbanner_GUTenberg_box {
    \textbf{\textit{GUTenberg}}
  }
  \hbox_gset:Nn \g__letgutbanner_g_box { \textbf{g} }
#+end_src

** Options of the package
:PROPERTIES:
:CUSTOM_ID: ImplementationOptionsofthepackage-21ri36v0fck0
:END:

We define:

- an option for the color of the text in the "L" (that has to be the same as the
  page color of the document),
- an option that let us choose another letter than “g” as the watermark letter.

#+begin_src latex
  \keys_define:nn { letgut-banner }
  {
    , pagecolor .clist_gset:N = \g__letgutbanner_pagecolor_clist
    , pagecolor .initial:n = { 1,1,1 }
    , watermark~ letter .code:n = {
      \hbox_gset:Nn \g__letgutbanner_g_box { \textbf{#1} }
    }
    , watermark~ scale .tl_gset:N = \g__letgutbanner_watermark_scale_tl
    , watermark~ scale .initial:n = 1
  }
  \ProcessKeyOptions [ letgut-banner ]
#+end_src

What follows has to be done at this end of the preamble, otherwise the font set
with ~\setmainfont~ in the preamble isn't taken in account in the banner.

#+begin_src latex
\AddToHook{begindocument}{
#+end_src


** Computations
:PROPERTIES:
:CUSTOM_ID: ImplementationComputations-r7bhxqt0fck0
:END:

Determination of the widths of each of the letter boxes of the word "LETTRE".
#+begin_src latex
  \dim_const:Nn \c__letgutbanner_L_width_dim {
    \box_wd:N \g__letgutbanner_L_box
  }
  \dim_const:Nn \c__letgutbanner_E_width_dim {
    \box_wd:N \g__letgutbanner_E_box
  }
  \dim_const:Nn \c__letgutbanner_T_width_dim {
    \box_wd:N \g__letgutbanner_T_box
  }
  \dim_const:Nn \c__letgutbanner_R_width_dim {
    \box_wd:N \g__letgutbanner_R_box
  }
#+end_src
Determination of the largest of these widths, stored in the scratch dimension
~\g_tmpa_dim~.
#+begin_src latex
\dim_gset:Nn \g_tmpa_dim {
  \fp_eval:n {
    max (
    \c__letgutbanner_L_width_dim,
    \c__letgutbanner_E_width_dim,
    \c__letgutbanner_T_width_dim,
    \c__letgutbanner_R_width_dim,
    )
  }pt
}
#+end_src
Determination of the (1st) box whose width is the largest of all (~argmax~, as
it were).
#+begin_src latex
\box_gset_eq:NN \g__letgutbanner_largest_LETR_box \dim_case:nnF
{ \g_tmpa_dim }
{
  { \c__letgutbanner_L_width_dim } { \g__letgutbanner_L_box }
  { \c__letgutbanner_E_width_dim } { \g__letgutbanner_E_box }
  { \c__letgutbanner_T_width_dim } { \g__letgutbanner_T_box }
  { \c__letgutbanner_R_width_dim } { \g__letgutbanner_R_box }
}
{ No~ idea! }
#+end_src
Resize the box containing "GUTenberg" to the width of the text embedded in the
horizontal bar of the "L".
#+begin_src latex
  \box_gresize_to_wd:Nn \g__letgutbanner_GUTenberg_box {
    \c__letgutbanner_banner_horizontal_contents_width_dim
  }
#+end_src
Determination of the height and depth of the box containing "GUTenberg" resized.
#+begin_src latex
  \dim_const:Nn \c__letgutbanner_GUTenberg_height_dim {
    \box_ht:N \g__letgutbanner_GUTenberg_box
  }
  \dim_const:Nn \c__letgutbanner_GUTenberg_depth_dim  {
    \box_dp:N \g__letgutbanner_GUTenberg_box
  }
#+end_src
Determining the size of the vertical offset of the box containing the resized
"GUTenberg", so that it is vertically centered in the horizontal bar of the "L":
the box positioned in $0$ would be at the very bottom of the box containing the
banner so with base line at the bottom the horizontal bar of the "L" so we raise
it first by its depth ($d$) and then by half the difference between the heights
($H$) of the horizontal bar of the "L" and ($h+d$) total of the box containing
"GUTenberg" resized ($=d+H/2-(h+d)/2=(H-h+d)/2$).
#+begin_src latex
  \dim_const:Nn \c__letgutbanner_GUTenberg_yoffset_dim {
    (
    \c__letgutbanner_banner_thickness_dim
    -
    \c__letgutbanner_GUTenberg_height_dim
    +
    \c__letgutbanner_GUTenberg_depth_dim
    )/2
  }
#+end_src
Once the height of the zone devoted to "La LETTRE" in the vertical bar of the
"L" has been fixed, the line spacing between the letters of the word "LETTRE"
depends in fact only on the width occupied by the widest of them, by definition
equal to the width allocated to the text embedded in the vertical bar of the
"L". The latter, stored in ~\g__letgutbanner_banner_vertical_contents_width_dim~, is
initially fixed:
#+begin_src latex
  \dim_gset:Nn
  \g__letgutbanner_banner_vertical_contents_width_dim
  \c__letgutbanner_banner_vertical_contents_width_dim
#+end_src
but, to avoid this line spacing being too small (or even negative), we test that
it is greater than a certain threshold, equal to a certain minimum percentage of
the height common to the letters of the word "LETTRE", stored in:

~\c__letgutbanner_leading_minimum_percent_fp~

and fixed at 0.15. If this is not the
case, we reduce (by 1pt):

~\g__letgutbanner_banner_vertical_contents_width_dim~

and loop until it is the case.
#+begin_src latex
  \dim_do_until:nNnn {
    \g__letgutbanner_between_letters_vertical_space_dim
  }
  >
  {
    \fp_use:N\c__letgutbanner_leading_minimum_percent_fp
    \g__letgutbanner_largest_LETR_box_height_dim
  }{
#+end_src
Resize the largest of these boxes to the width of the text embedded in the "L".
#+begin_src latex
  \box_gresize_to_wd:Nn \g__letgutbanner_largest_LETR_box {
    \g__letgutbanner_banner_vertical_contents_width_dim
  }
#+end_src
Height of this resized larger box.
#+begin_src latex
  \dim_gset:Nn \g__letgutbanner_largest_LETR_box_height_dim {
    \box_ht:N \g__letgutbanner_largest_LETR_box
  }
#+end_src
Resize the boxes for each of the letters in the word "LETTRE" so that they are
all the same height as the widest of them.
#+begin_src latex
  \box_gresize_to_ht:Nn \g__letgutbanner_L_box {
    \g__letgutbanner_largest_LETR_box_height_dim
  }
  \box_gresize_to_ht:Nn \g__letgutbanner_E_box {
    \g__letgutbanner_largest_LETR_box_height_dim
  }
  \box_gresize_to_ht:Nn \g__letgutbanner_T_box {
    \g__letgutbanner_largest_LETR_box_height_dim
  }
  \box_gresize_to_ht:Nn \g__letgutbanner_R_box {
    \g__letgutbanner_largest_LETR_box_height_dim
  }
#+end_src
Resize the box containing "La" to the width of the text embedded in the "L"
vertical bar.
#+begin_src latex
  \box_gresize_to_wd:Nn \g__letgutbanner_La_box {
    \g__letgutbanner_banner_vertical_contents_width_dim
  }
#+end_src
Determine the height ($h$) and depth ($d$) of the resized box containing "La".
#+begin_src latex
  \dim_gset:Nn \g__letgutbanner_La_height_dim {
    \box_ht:N \g__letgutbanner_La_box
  }
  \dim_gset:Nn \g__letgutbanner_La_depth_dim  {
    \box_dp:N \g__letgutbanner_La_box
  }
#+end_src
Determining the size of the vertical offset of the resized box containing "La"
so that its top is, relative to the top of the "L", offset by the same amount as
the bottom of the box containing "GUTenberg" is offset from the bottom of the
"L" (i.e., the distance from the top of the page to the top of "La" and the
distance from the bottom of the page to the baseline of "GUTenberg" are
identical).
#+begin_src latex
  \dim_gset:Nn \g__letgutbanner_La_yoffset_dim {
    \c__letgutbanner_banner_height_dim
    -
    \c__letgutbanner_GUTenberg_yoffset_dim
    -
    \g__letgutbanner_La_height_dim
    +
    \g__letgutbanner_La_depth_dim
  }
#+end_src
For the following, the area containing the letters of the word "LETTRE" aligned
vertically is fixed, defined as follows:
  - its top is the baseline of "La",
  - its bottom is the baseline of "GUTenberg".
Determination of the height of this area.
#+begin_src latex
  \dim_gset:Nn \g__letgutbanner_area_height_dim {
    \g__letgutbanner_La_yoffset_dim
    -
    \g__letgutbanner_La_depth_dim
    -
    \c__letgutbanner_GUTenberg_yoffset_dim
  }
#+end_src
Determining the vertical jump ($s$) between the letters in the word "LETTRE"
with the constraints that:
  - the top of the 1st letter ("L") is $3e/2$ below the top of the area (where
    $e$ is the common vertical space separating the letters of the word
    "LETTRE"),
  - the baseline of the last letter ("E") is merged with the bottom of the area,
  - this jump must not be less than 20 % of the common height $h$ to all these
    letters i.e.:

    ~\g__letgutbanner_largest_LETR_box_height_dim~

    otherwise the size of the largest of these boxes is reduced to less than the
    width of the text embedded in the "L".

The sum of :
  - the 5 vertical spaces ($5e$) between the letters in the word "LETTRE",
  - the 6 common heights ($6h$) to all these letters,
  - $3e/2$,
must equal the height ($H$) of the zone. So we have $H=3e/2+5e+6h$, that is
$e=2(H-6h)/13$. Once initialized, the vertical jump between the letters is
$s=e+h$ that is $s=(2H+h)/13$. Therefore:

1. Determine the vertical half-space between the letters.
   #+begin_src latex
     \dim_set:Nn \l_tmpa_dim {
       (
       \g__letgutbanner_area_height_dim
       -
       6\g__letgutbanner_largest_LETR_box_height_dim
       )/13
     }
   #+end_src
2. Vertical space between letters.
   #+begin_src latex
     \dim_gset:Nn \g__letgutbanner_between_letters_vertical_space_dim {
       2\l_tmpa_dim
     }
   #+end_src
3. If this space does not check the condition explained above, we reduce:

   ~\g__letgutbanner_banner_vertical_contents_width_dim~

   and loop.
   #+begin_src latex
     \dim_gsub:Nn \g__letgutbanner_banner_vertical_contents_width_dim {
       1pt
       }
     }
   #+end_src
The width allocated to the text embedded in the vertical bar of the "L" may have
been re-evaluated, so the padding (which must be symmetrical) may have changed
as well.
   #+begin_src latex
   \dim_gset:Nn \g__letgutbanner_banner_padding_dim {
     (
     \c__letgutbanner_banner_thickness_dim
     -
     \g__letgutbanner_banner_vertical_contents_width_dim
     )/2
   }
   #+end_src
4. Determination of the vertical offset of the 1st letter ("L").
   #+begin_src latex
     \dim_const:Nn \c__letgutbanner_first_letter_yoffset_dim {
       \g__letgutbanner_La_yoffset_dim
       -
       \g__letgutbanner_largest_LETR_box_height_dim
       -
       \fp_use:N \c__letgutbanner_leading_La_factor_fp
       \g__letgutbanner_between_letters_vertical_space_dim
     }
   #+end_src
5. Determination of the vertical jump between letters.
   #+begin_src latex
     \dim_gset:Nn \g__letgutbanner_between_letters_yoffset_dim {
       \g__letgutbanner_between_letters_vertical_space_dim
       +
       \g__letgutbanner_largest_LETR_box_height_dim
     }
   #+end_src
Resize the box containing "g" to the width of the text.
#+begin_src latex
  \box_gresize_to_wd:Nn \g__letgutbanner_g_box {
    \fp_to_dim:n {
      \g__letgutbanner_watermark_scale_tl
      *
      \g__letgutbanner_textwidth_dim
    }
  }
#+end_src
Calculates the size by which the height of the text box on the 1st page must be
reduced so that the banner does not encroach on the text. Relative to the top of
the page:
  - the top of the horizontal bar of "L" is at a distance equal to the sum of :
    - the distance between this top of page and the top of the vertical bar of
      "L", equal to the height ($p$) of the paper minus the height ($l$) of "L"
#+begin_src latex
  \dim_gset:Nn \g__letgutbanner_first_page_shrink_dim {
    \c__letgutbanner_tmargin_dim
    +
    \c__letgutbanner_textheight_dim
    +
    \c__letgutbanner_banner_thickness_dim
    -
    .5\paperheight
    -
    .5\c__letgutbanner_banner_height_dim
    +
    .5\baselineskip
  }
#+end_src

Automatic addition on the 1st page (only) of the banner.
#+begin_src latex
\AddToHookNext{shipout/background}{
#+end_src
We want the material added to this page to be not selectable in order to let the
reader who wants to copy the interesting text be able to select it.
#+begin_src latex
  \BeginAccSupp{ActualText={}}
#+end_src
We put now the big "L" at the right place.
#+begin_src latex
  \put(
  .5\c__letgutbanner_lmargin_dim-.5\c__letgutbanner_banner_thickness_dim
  ,
  -.5\paperheight-.5\c__letgutbanner_banner_height_dim
  ){
    \put(0,0){
      \rule{
        \c__letgutbanner_banner_width_dim
      }{
        \c__letgutbanner_banner_thickness_dim
      }
    }
    \put(0,0){
      \rule{
        \c__letgutbanner_banner_thickness_dim
      }{
        \c__letgutbanner_banner_height_dim
      }
    }
    #+end_src
The color should not be systematically white, but should be identical to the one
chosen for the page background.
#+begin_src latex
    % \color_fill:nV {rgb}{ \g__letgutbanner_pagecolor_clist }
    \color[rgb]{ \g__letgutbanner_pagecolor_clist }
    \put(
    \g__letgutbanner_banner_padding_dim
    ,
    \g__letgutbanner_La_yoffset_dim
    ){
      \box_use:N \g__letgutbanner_La_box
    }
    \put(
    \g__letgutbanner_banner_padding_dim
    ,
    \c__letgutbanner_first_letter_yoffset_dim
    -0\g__letgutbanner_between_letters_yoffset_dim
    ){
      \box_use:N \g__letgutbanner_L_box
    }
    \put(
    \g__letgutbanner_banner_padding_dim
    ,
    \c__letgutbanner_first_letter_yoffset_dim
    -1\g__letgutbanner_between_letters_yoffset_dim
    ){
      \box_use:N \g__letgutbanner_E_box
    }
    \put(
    \g__letgutbanner_banner_padding_dim
    ,
    \c__letgutbanner_first_letter_yoffset_dim
    -2\g__letgutbanner_between_letters_yoffset_dim
    ){
      \box_use:N \g__letgutbanner_T_box
    }
    \put(
    \g__letgutbanner_banner_padding_dim
    ,
    \c__letgutbanner_first_letter_yoffset_dim
    -3\g__letgutbanner_between_letters_yoffset_dim
    ){
      \box_use:N \g__letgutbanner_T_box
    }
    \put(
    \g__letgutbanner_banner_padding_dim
    ,
    \c__letgutbanner_first_letter_yoffset_dim
    -4\g__letgutbanner_between_letters_yoffset_dim
    ){
      \box_use:N \g__letgutbanner_R_box
    }
    \put(
    \g__letgutbanner_banner_padding_dim
    ,
    \c__letgutbanner_first_letter_yoffset_dim
    -5\g__letgutbanner_between_letters_yoffset_dim
    ){
      \box_use:N \g__letgutbanner_E_box
    }
    \put(
    \c__letgutbanner_banner_thickness_dim
    ,
    \c__letgutbanner_GUTenberg_yoffset_dim
    ){
      \box_use:N \g__letgutbanner_GUTenberg_box
    }
    % \color_fill:n {black!10}
    \color{ black!10 }
    \put(
    .5\c__letgutbanner_lmargin_dim
    +
    .5\c__letgutbanner_banner_thickness_dim
    +
    \fp_to_dim:n {
      (1-\g__letgutbanner_watermark_scale_tl)/2*\g__letgutbanner_textwidth_dim
    }
    ,
    .5\paperheight
    +
    .5\c__letgutbanner_banner_height_dim
    -
    \c__letgutbanner_tmargin_dim
    ){
      \box_move_down:nn {.5\c__letgutbanner_textheight_dim}
      {\box_use:N \g__letgutbanner_g_box}
    }
  }
  #+end_src
  We end the part which isn't selectable.
  #+begin_src latex
  \EndAccSupp{}
  }
#+end_src

We close ~\AddToHook{begindocument}~.

#+begin_src latex
}
#+end_src

If the package is used with another class than \class{letgut}, we reduce the
textheight of the first page of the height of the horizontal bar of the "L".

#+begin_src latex
\@ifclassloaded{letgut}{
}{
  \AddToHook{begindocument/end}{%
    \enlargethispage{
      - \g__letgutbanner_first_page_shrink_dim
    }
    \thispagestyle{empty}
  }
}
#+end_src

* Example file (=letgut-banner-example.tex=)
:PROPERTIES:
:CUSTOM_ID: Examplefileletgutbannerexampletex-heehxqt0fck0
:END:

#+begin_src latex :tangle ../../../doc/lualatex/letgut/letgut-banner-example.tex :exports none
  \documentclass[twoside]{article}
  \usepackage{fontspec}
  \usepackage[a4paper]{geometry}
  \usepackage{fancyhdr}
  \usepackage{letgut-banner}
  \usepackage{lipsum}
  \setmainfont{TeX Gyre Schola}
  \fancyhf{}
  \fancyhead[RO,LE]{\thepage}
  \fancyhead[RE,LO]{Test~\textsf{letgut-banner}}
  \pagestyle{fancy}
  \begin{document}
  \lipsum[1-20]
  \end{document}
#+end_src