Film - A Practical Guide to Video and Audio Compression - page 68

The adhesive tape splice was introduced so that the polyester film could be edited.
This was necessary because there wasn't a usable solvent available. Quite a large area of
the film is obscured, but the tape is transparent enough that it is actually harder to detect
than the glued undercut joint.
All of the splices will cause some gate weave and disruption to the smooth motion
of the film through a gate. Specially designed gates or stepper-based movements will
allow splices to go through without impeding the film movement. If you are building a
telecine system, you must consider this issue, because otherwise the film will be displaced
as it passes through the gate.

4.13.2 Fogging

Many amateur filmmakers were caught out when turning over the 8mm film to shoot the
other side. The film occasionally slipped out of the roll, and unless you used a black bag
to change the film, there was a risk of spoiling your movie. This caused many films to be
exposed to some light as the film was exchanged. The result is a yellowish fogging effect
down the first few feet of film. If this fogging is not too severe, some careful restorative
image processing could clean it up once the film has been scanned in. Figure 4-24 shows
what this looks like. It is usually fogged down the sprocketed edge of the film.

4.13.3 Surface Damage

When dealing with film as a source, you must consider the implications of dust and
scratches. There used to be proprietary formulations that you could apply to films to
clean them. They involved mixing all manner of carcinogenic chemicals (carbon tetra-
chloride—carpet cleaner—and paraffin wax for example) and applying them to the
film. The wax dissolved in the carpet cleaner and as that evaporated, it left a waxy
deposit on the film. It is sufficient to make sure the film is scanned in a dust-free envi-
ronment rather than risk damaging your film with chemicals. Passing it between a pair
of lightly sprung felt pads would remove any accretions. Any remaining particulate
matter will become an integral part of the scan. This has to be removed using some
image-processing code.

4.13.4 Physical Structure of Film

Before we consider how dust and scratches affect the film, we need to examine how film
is physically structured. Up to now we've been looking through the film and discussing
the imaging area in its normal viewing aspect. If we turn the film sideways and examine
it from the edge, we can see that it is made up a series of layers sandwiched together.
Figure 4-25 illustrates the structure.
Various depredations have been perpetrated on this piece of film, each one resulting
in some reduction in image quality. The shortcut solution when restoring is just to isolate
the area of damage and fill in the missing fragments by interpolation. A more scientific
approach requires that each kind of damage be identified and treated accordingly.