MORE than two million films languish in vaults around the world, some of them
so badly damaged they can no longer be screened. If nothing is done, they could
disappear for good, like half the films made in the US before 1950. But research
at the University of La Rochelle in France and Monash University in Clayton,
Victoria, promises an automated technique to restore these old films to their
former glory.
Films can be cleaned chemically or by using ultrasound. But this is
time-consuming and expensive, and chemicals can also damage the original.
Another approach is to digitise the film and then clean up the digital version
frame by frame鈥攁 daunting job. When Disney cleaned up Snow White for
release on DVD, for instance, graphic artists worked in shifts on 40 computers
day and night for 18 weeks.
These computer-aided techniques remain very expensive, says Samia Boukir, who
started work on an automated system in La Rochelle but has now moved to Monash.
Only an automated system can hope to salvage the archive of detriorating
films.
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Once a film has been scanned in, the first stage of the process is to correct
flicker caused by the film slipping鈥攖he result of damage to its
perforations. The software picks out distinct features in a pair of frames, such
as prominent pieces of background, and measures movement of the features between
the frames. It then moves on to the next pair of frames and again notes the
difference. Using this information, the system works out if two frames are
unusually close together or far apart, and corrects any anomalies.
Dust spots usually occur only on single frames, so the system looks for small
specks that are visible on one frame and absent on its neighbours. The software
repairs the image by sampling the unspoilt area of the image on the adjacent
frames and replacing the dust spot with an average of the sampled pixels.
Detecting scratches is more complicated because they may run over several
frames and can be confused with vertical lines that are part of the film. But a
telltale pattern often gives them away: scratches are normally caused by the
mechanical parts of a projector rubbing on the film, so they tend to repeat at
regular intervals.
The system looks for a periodic pattern of this type from frame to frame,
predicting when it should occur in the subsequent frames. When the scratch
finishes, the pattern breaks down. Having identified the scratch and its
duration, the software then repairs the damage by taking pixels from undamaged
frames before and after the scratch. This smoothing effect avoids any sharp
edges in the restored image, says Boukir. Finally, film grain is added back.
When detecting dust particles, says Boukir, 95 per cent accuracy is
acceptable. But with line scratches it has to be better. 鈥淭he removal of
significant scene details would have disastrous consequential effects on the
resulting restored movie,鈥� she explains. Because of this, some level of human
intervention will be needed, but it is likely to be little more than an operator
accepting or rejecting a change.
Boukir says that instead of trying to enhance the whole image on a frame, it
is better to concentrate on repairing damage. 鈥淔or good preservation of texture,
it is necessary to look beyond the use of existing global filters, which are
applied to the entire image,鈥� she says.
Once restored, the digitised films can be viewed by future generations
without risking more damage to the original by running it through a projector.
And films are very vulnerable. 鈥淔ilm has a hundred-year history, but the
knowledge of how to store it properly is only 15 years old,鈥� says Godfrey Pye of
Sunset Digital, a Hollywood-based company that specialises in cleaning up old
movies.
