opencv_trail_effect is an experiment about recreating the "trail effect"[1],
which can be seen in some cool music videos, automatically and in real-time,
and even without the use of a green-screen.

[1] http://dailypsychedelicvideo.com/tag/trail-effect/

Some examples of it are:

  The Jacksons - Blame It On The Boogie - https://www.youtube.com/watch?v=mkBS4zUjJZo
  OK Go - WTF? - https://www.youtube.com/watch?v=12zJw9varYE
  Elisa - L'Anima Vola - https://www.youtube.com/watch?v=MqhxIQD16EA

opencv_trail_effect draws a trail after performing foreground segmentation
using OpenCV.

Segmentation can done using either thresholding or background subtraction.

A few conditions are required to create a decent effect:

  1. the camera should not move;
  2. the lighting in the scene should be quite stable (when doing background
     subtraction it is recommended to disable auto-gain in the camera, and
     avoid any artifact introduced by the power line frequency);
  3. the subject should enter the scene only after the background learning
     phase has finished, i.e. after opencv_trail_effect has showed the preview
     window.


Examples of use
---------------

The effect in "Blame It On The Boogie" could be described as a short faded
trail and can be achieved with this command line:

  $ ./opencv_trail_effect -l 12 -s background -d fadeaccumulate


The effect in "WTF?" is equivalent to an infinite trail in which the isolated
foreground is copied on the background and stays there, it can be achieved
with this command line:

  $ ./opencv_trail_effect -l -1 -s background -d copy


The effect in "L'Anima Vola" seems to have a "catch-up" behavior; that one can
be done by drawing the trail in reverse, with this command line:

  $ ./opencv_trail_effect -l 30 -s background -d copy -r


Another effect seen in movies can be obtained by doing the average of the
frames in the trail, possibly with the most recent frame drawn on top:

  $ ./opencv_trail_effect -l 10 -s background -d average -B


Side note
-------------------

These effects can bring to mind other cool effects in music videos, like the
frame shuffling effect in Peter Gabriel's Sledgehammer
(https://www.youtube.com/watch?v=OJWJE0x7T4Q), that one can be done with the
nervousTV plugin from effectv
(https://fukuchi.org/research/effectv/index.html.en) or frei0r filters
(https://www.dyne.org/software/frei0r/), for example with:

  $ gst-launch-1.0 -v v4l2src ! videoconvert ! frei0r-filter-nervous ! videoconvert ! autovideosink