Antiflicker Filter for Virtual Dub
Version 1.0

[Place this text file in the VirtualDub plugins directory to make
it available via the Help button on the filter configuration dialog
box. The computer must have a program called Notepad available in
its search path.]

When video at one frame rate is projected and then recorded from the
projection screen at a different frame rate, a temporal moire pattern,
or flicker, is produced. This filter removes the flicker, thereby making
this a viable process. For example, it becomes feasible to project 18 fps
films and record them with a 25 fps PAL camcorder.

The process for using the filter is a bit tricky. Here is the step-by-step
process. Warning: deviations can result in crashes of VirtualDub. 

1. Start VirtualDub.

2. Open the input video.

3. Add the deflicker filter. Set the window size, softening threshold, and
interlacing mode as described below. Check "Prepare video" and save the
filter with OK, OK.

4. Now on the VirtualDub timeline, play the video through to completion
using the >o button. The output pane will remain black. Do not rewind or
anything else yet! (You may NOT move around on the timeline in "Prepare
video" mode. If you rewind, you have to play through again until completion.
Never rewind and then change to "Process video" mode.)

5. Reconfigure the deflicker filter for "Process video".

7. Use the VirtualDub timeline |< button to rewind to the start of the clip.

8. Play the video to see the filtered result. You can rewind and replay if you
want, or rewind and then Save AVI. You can also freely move around on the
timeline in "Process video" mode (but NOT in "Prepare video" mode).

The following options are available:

Window size: This is the size of the moving average of frame luminance values.
This produces the luminance value that frames are adjusted to. The idea is that
you want it as small as possible while still removing flicker. Ideally, it is equal to
the number of frames that one flicker peirod spans. For example, if the video
frame rate is 25 fps and the flicker is at 7 fps (example of 18 fps shot at 25 fps;
25 -18 = 7), then one flicker period is 25/7 = 3.57 frames. We want our window
to exactly cover an integral number of flicker cycles. So a good choice here would
be a window size of 7 (~ 2 * 3.57). You can always set a long window size if in
doubt, but doing so will spread out luminance changes in the clip. A window size
of 25 is a good general-purpose compromise.

Softening: This is the threshold for the final temporal softening phase. Often, the
physical process that creates the flicker also creates within-frame illumination changes,
causing adjacent frames to differ. This softening phase greatly reduces this effect.
A higher number causes greater temporal softening but can leave motion trails. If the
physical process does not produce within-frame changes, this phase can be disabled
by setting the softening to 0.

Interlaced source: If the source video clip is interlaced, check this option to enable
field-mode processing.

Technical notes. The first phase of the filter performs a moving average over the
video and rescales the pixels in the frames to approach this average. The second phase
applies a temporal softening. A two-pass algorithm is used to avoid having to buffer
an inordinate number of frames (equal to the window size).

Thanks to Niels Basjes for suggesting the creation of this filter, providing
test files, and giving valuable feedback. The final temporal softening phase
is adapted from code by Steven Don. Thanks to Jeff Gonion for valuable
theoretical discussions on digital filtering. 

For additional information, version updates, and other filters, please
go to the following web site:

Filters for VirtualDub
http://www.geocities.com/Colosseum/Pressbox/8699/index.html

Donald Graft
May 9, 2000
(C) Copyright 2000, All Rights Reserved