Spatial Preprocessing - A Practical Guide to Video and Audio Compression - page 552

Scratched and dusty Restored

Figure 34-2 Dust particles before and after media filtering.

34.3 Removing Dust and Scratches

If you have sourced your material from film, it may suffer from scratches as well as dust
particles. The analog noise removal that averages pixels over a couple of frames may not
remove the speckles. There are image-processing filters that are designed to remove dust,
and the median filter would be appropriate here. You could also run the digitized film
through After Effects to remove the dust. Figure 34-2 shows dust and scratches on a piece
of film before and after filtering has been applied.
Scratches are a bit more difficult to deal with. Because this is mechanical damage to
the film, it might have a repeating pattern that is cyclic but not in sync with the frames. If
the pixel maps are arranged as they would be on the film, applying some kind of matte
and repair algorithm might help. It depends on how bad the damage is. The dust filter
might remove fine scratches, but tears and severe damage might need manual repairs.
Badly repaired film breaks, and old splices may also require some special attention.
Figure 34-3 shows a cyclic scratch on the left and a torn film on the right.
Speckles and scratches may be lighter or darker depending on whether the dust is
on a positive or negative master and whether a scratch damages the substrate or the
emulsion.
This all falls under the heading of restoration, and it is arguable whether that kind of
work should be done at this stage or earlier on before the cropping takes place. It is best
to do this as early in the process as possible, because the more you scale and process the
video that you do want, the more you are blurring the edges of any dust and damage. A
blurred dust particle is going to be much harder to remove than one that is crisp and well
focused, so whenever you choose to deal with dust and scratches, you should always do
it before the scaling step. Figure 34-4 illustrates the effect of scaling on dust particles,
which makes them much harder to remove. Which of the three dust particles shown do
you think will be the easiest to remove?