Add support for cutting guides

foliator.py

@@ -2,6 +2,8 @@
 
 import argparse
 from pypdf import PaperSize, PdfReader, PdfWriter, Transformation
+from pypdf.annotations import Line
+from pypdf.generic import ArrayObject, FloatObject, NameObject
 import math
 
 # Interprets input_pdf as a list of A6 pages, and appends to append_to with
@@ -50,6 +52,14 @@ def blit_to_A4(input_pdf, append_to=PdfWriter()):
                                                height=PaperSize.A4.height)
             back_page = writer.add_blank_page(width=PaperSize.A4.width,
                                               height=PaperSize.A4.height)
+
+            midline = Line(rect = (0, PaperSize.A4.height/2, PaperSize.A4.width, PaperSize.A4.height/2),
+                           p1=(10, PaperSize.A4.height/2),
+                           p2=(PaperSize.A4.width-10, PaperSize.A4.height/2))
+            midline.flags = 4 # print the midline
+            midline[NameObject("/C")] = ArrayObject([FloatObject(0.5)])
+            writer.add_annotation(front_page, midline)
+
         front = input_pdf.pages[k]
         back = input_pdf.pages[k+1] if (k+1<L) else create_blank_page(None, PaperSize.A5.width, PaperSize.A5.height)