Perspectra

Software and corresponding workflow to scan documents and books with as little hardware as possible.

Check out github:adius/awesome-scanning for and extensive list of alternative solutions.

Best Practices for Taking the Photos

Camera Settings

# Rule of thumb is the inverse of your focal length,
# but motion blur is pretty much the worst for readable documents,
# therefore use at least half of it and never less than 1/50.
shutter: 1/50 - 1/200 s

# The whole document must be sharp even if you photograph it from an angle.
# Therefore at least 8 f.
aperture: 8-12 f

# Noise is less bad than motion blur => relative high ISO
# Should be the last thing you set:
# As high as necessary as low as possible
iso: 800-6400

When using Tv (Time Value) or Av (Aperture Value) mode use exposure compensation to set lightness value below 0. You really don't want to overexpose your photos as the bright pages are the first thing that clips.

On the other hand, it doesn't matter if you loose background parts because they are to dark.

Generating the Photos from a Video

A good tool for this purpose is PySceneDetect. It's a Python/OpenCV-based scene detection program, using threshold/content analysis on a given video.

For easy installation you can use the docker image

Find good values for threshold:

docker run \
  --rm \
  --volume (pwd):/video \
  handflucht/pyscenedetect
  --input /video/page-turning.mp4 \
  --downscale-factor 2 \
  --detector content \
  --statsfile page-turning-stats.csv

To launch the image run:

docker run \
  --interactive \
  --tty \
  --volume=(pwd):/video \
  --entrypoint=bash \
  handflucht/pyscenedetect

Then run in the shell:

cd /video
scenedetect \
  --input page-turning.mp4 \
  --downscale-factor 2 \
  --detector content \
  --threshold 3 \
  --min-scene-length 80 \
  --save-images

TODO: The correct way to do this: (after Breakthrough/PySceneDetect#45 is implemented)

docker run \
  --rm \
  --volume (pwd):/video \
  handflucht/pyscenedetect \
  --input /video/page-turning.mp4 \
  --downscale-factor 2 \
  --detector content \
  --threshold 3 \
  --min-scene-length 80 \
  --save-images <TODO: path>

Aim for a low threshold and a long minimun scene length. I.e. turn the page really fast and show it for a long time.

TODO

• Add a border around the image so that watershed starts to flood the image from all directions and to improve recognition for clipped documents
• Implement scikit-image/scikit-image#2212