ezyang's blog

Annotating slides

A little trick for your toolbox: after you’ve generated your slide deck and printed it out to PDF, you might want to annotate the slides with comments. These is a good idea for several reasons:

• If you’ve constructed your slides to be text light, they might be optimized for presentation but not for reading later on. (“Huh, here is this diagram, I sure wish I knew what the presenter was saying at that point.”)
• Writing out a dialog to go along the slides is a nonvocal way of practicing your presentation!

But how do you interleave the slide pages with your annotations? With the power of enscript and pdftk, you can do this entirely automatically, without even having to leave your terminal! Here’s the recipe.

1. Create an “annotations” text file (we’ll refer to it as annot.txt). This will contain your text commentary to accompany the slides. Write the text explaining your first slide, and then insert a form feed (^L, you can do so by pressing C-l in vim (insert mode) or C-q C-l in emacs.) Write the text for your second slide. Rinse and repeat.

2. We now want to render this into a PDF file, with the same dimensions as your slide deck. Figure out what the size of your slides are in pixels, and then edit your ~/.enscriptrc to contain the following line:

   Media: Slide width height llx lly urx ury
   

   where ll stands for lower left and ur stands for upper right: these four numbers denote the bounding box for the text. One possible combination for these might be:

   Media: Slide 576 432 18 17 558 415
   

   We can now invoke enscript to generate a nicely formatted PostScript file of our annotations in the right dimensions, with enscript annot.txt -p annot.ps -M Slide -B -f Palatino-Roman14 (pick a different font, if you like.)

3. Convert the resulting PostScript file into a PDF, with ps2pdf annot.ps.

4. Now, with pdftk, we will split our annotations PDF and our slides PDF into individual pages, and then merge them back together into one PDF. We can use burst to output the pages, suggestively naming the output files so they interleave correctly:

   mkdir stage
   pdftk slides.pdf burst output stage/%02da.pdf
   pdftk annot.pdf burst output stage/%02db.pdf
   

   and then we join them back together:

   pdftk stage/*.pdf cat output annotated-slides.pdf
   

Here’s the full script:

#!/bin/sh
set -e
ANNOT="$1"
SLIDES="$2"
OUTPUT="$3"
if [ -z "$3" ]
then
    echo "usage: $0 annot.txt slides.pdf output.pdf"
    exit 1
fi
TMPDIR="$(mktemp -d)"
enscript "$ANNOT" -p "$ANNOT.ps" -M Slide -B -f Palatino-Roman14
ps2pdf "$ANNOT.ps" "$ANNOT.pdf"
pdftk "$SLIDES" burst output "$TMPDIR/%03da.pdf"
pdftk "$ANNOT.pdf" burst output "$TMPDIR/%03db.pdf"
pdftk "$TMPDIR"/*.pdf cat output "$OUTPUT"
rm -Rf "$TMPDIR"

Don’t forget to define Slide in your .enscriptrc, and happy annotating!