convert a bitmap image to a vectorized PDF using mkbitmap and potrace

it' a snap to install using MacPorts

$ port info potrace
potrace @1.8 (graphics)
Variants:             a4_default, metric_default, universal

Description:          Potrace is a utility for tracing a bitmap, which means, transforming a bitmap into a smooth, scalable image. The
                      input is a bitmap (PBM, PGM, PPM, or BMP), and the default output is one of several vector file formats. A
                      typical use is to create EPS files from scanned data, such as company or university logos, handwritten notes,
                      etc. The resulting image is not jaggy like a bitmap, but smooth. It can then be rendered at any resolution.
Homepage:             http://potrace.sourceforge.net/

Library Dependencies: zlib
Platforms:            darwin
License:              unknown
Maintainers:          nomaintainer@macports.org
manga:~ lup$ port variants potrace
potrace has the variants:
   a4_default: compile potrace with A4 as the default page size.
   metric_default: compile potrace with centimeters as the default unit  instead of inches.
   universal: Build for multiple architectures

to install

sudo port install potrace +a4_default +metric_default

check man pages and open your input for inspection

man mkbitmap
man potrace
open dubout.png

you can use directly this workflow

convert dubout.png ppm:- | mkbitmap -f 2 -s 2 -t 0.48 | potrace -t 5 --progress -b pdf -o dubout.pdf

but convert being what it is, first do

convert dubout.png dubout.ppm

to take some more time fine tuning parameters:

cat dubout.ppm | mkbitmap  -t 0.48 | potrace -t 15 --progress -b pdf -o dubout.pdf

in particular, the -x option resets defaults:

cat dubout.ppm | mkbitmap  -x -s 2 -3 -t 0.5 | potrace -t 25 --progress -b pdf -o dubout.pdf

wait and ... enjoy!