README.md: add context

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 # pdf2book

-CLI script to rearrange pdf pages for printing book signatures and foldable zines
+CLI script to rearrange pdf pages for printing book signatures and foldable zines
+
+## INSTALL pdf2book in Linux or macOS
+
+### Dependencies: 
+- [`bash`](https://www.gnu.org/software/bash/) (mostly already installed)
+    - check with `bash --version`
+- [`python3`](https://www.python.org/downloads/) (mostly already installed)
+    - check with `python3 --version`
+- [`venv`](https://docs.python.org/3/library/venv.html) (might be installed)
+    - check with `python3 -m venv --help`
+
+```
+PREFIX="$HOME/.local"
+LIBDIR=$PREFIX/lib
+BINDIR=$PREFIX/bin
+SRCDIR=$LIBDIR/pdf2book.git
+mkdir -p $LIBDIR
+git clone https://git.wlankabel.at/onipa/pdf2book.git $SRCDIR
+python3 -m venv $SRCDIR/.venv
+. $SRCDIR/.venv/bin/activate
+pip3 install pypdf
+echo -n '''#!/bin/bash
+SRCDIR='$SRCDIR'
+VENVDIR=$SRCDIR/.venv
+. $VENVDIR/bin/activate
+python3 $SRCDIR/pdf2book.py $@
+deactivate
+'''> $BINDIR/pdf2book
+chmod +x $BINDIR/pdf2book
+echo "you might need to restart your shell"
+```
+
+
+--- 
+
+## pdf2book.py
+This script takes a pdf and reorders pages for printing simple signatures.
+One sheets holds 4 pages. The number of sheets per signature can be varied.
+In the following code snippet the original sequence of pages on a sheet and
+its changed sequence are shown.
+
+```
+this is how the squence is changed in a 1 sheet per signature setting
+._________.   ._________.      ._________.   ._________.
+|    |    |   |    |    |      |    |    |   |    |    |
+| 1  | 2  | + | 3  | 4  |  ->  | 4  | 1  | + | 2  | 3  |
+|    |    |   |    |    |      |    |    |   |    |    |
+'---------'   '---------'      '---------'   '---------'
+
+# 1 sheet per signature
+s=1: 1,2,3,4 -> 04,01.02,03  i=0 
+                08,05.06,07  i=1
+                12,09.10,11  i=2
+                16,13.14,15  i=3
+                20,17.18,19  i=4
+down=(i+1)*4*s
+up=  (i)*4*s+1
+#pages_mod=$(python -c " (int(pages/s)+1)*(pages%s) " ) 
+for i in $(pages%s)
+print (down--,up++,up++,down--)
+
+# 2 sheets per signature
+s=2: 1,2,3,4,5,6,7,8 -> 08,01.02,07 06,03.04,05  i=0
+                        16,09.10,15 14,11.12,13  i=1
+                        24,17.18,23 22,19.20.21  i=2
+down=(i+1)*4*s
+up=  (i)*4*s+1
+for i in range(pages%s):
+    for count in range(2):
+        print (down--,up++,up++,down--)
+
+# 3 sheets per signature
+s=3: 1,2,3,4,5,6,7,8,9,10,11,12 -> 12,01.02,11 10,03.04,09 08,05.06,07  
+                                   24,13.14,23 22,15.16,21 20,17.18,19
+# 4 sheets per signature 
+s=4: 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16 -> 16,01.02,15 14,03.04,13 12,05.06,11 10,07.08,09  
+                                               32,17,18,31 30,19.20.29 28,21.22.27 26,23.24,25
+```
+
+
+
+---
+
+### TODO
+- [ ] alternative names to pdf2book
+- [ ] write install script
+- [ ] test in powershell
+