Film - A Practical Guide to Video and Audio Compression - page 66

unnecessary here because our exposure time happens under software control. The film
must be moved through the gate, advancing by one frame's worth of distance each time.
Attach a stepper motor to the sprocket drive to provide the motive force. Stepper
motors are obtained by dismantling old floppy-disk drives to remove the head-position-
ing motor. An interface is required to contrive some kind of electrical connection to the
computer, and one of the servo control interfaces used by amateur robotics enthusiasts
would do. Some of these I/O controllers are now available as USB connected devices.
Controlling this machinery should not involve any particularly complex driver writing.

4.12.3 Software Support

Once the whole machine is assembled, two sets of utility code must be provided.
The first one will control the stepper motor. This must be moved one step forward at
a time. The code is called a number of times depending on the distance from one frame to
the next, measured in steps. Hopefully the step distance will be small enough that there is
an integer relationship between the two.
The second part of the utility software is designed to drive the film scanner in
order to capture a frame. Scanning the whole 35mm film surface area is not necessary,
as the device was likely a 35mm slide scanner to start with. The API support in the
TWAIN interface will provide the support necessary to frame the scanning area so it
just covers the imaged part of the film. This is known to be feasible here because it is
done all the time with paper scanners during a preview and crop. They use the same
TWAIN driver code.

4.12.4 Running the Process

Scanning the full 8mm width will include the sprocket holes. That provides a reference
that will reduce the software-process tracking errors later on.
An application uses these utilities to drive the stepper motor and take a snapshot.
That application could just capture the frames as a series of still images or make a movie
file one frame at a time.
After a successful run, the application will have created a file containing a digitized
version of the film at probably as good a resolution as it is possible to achieve (depending
on the quality of the film scanner you start with). Some remedial work to stabilize any
camera gate weave and crop the output will be necessary.
Some variants on this theme are possible if the scanner is driven one line of pixels at
a time. In that case alternating steps of the film transport and the scanning head will yield
a continuous series of scan lines that will then need to be sliced into frames. This will
deliver a slightly more stable scanned result. The quality of the whole project is depend-
ent on your mechanical skills and how well you conceive the software to process the
results. This design will eliminate the gate weave you normally get with a projector
because the film is being moved far more gently through the gate. None of the scanners
can eliminate the gate weave from the camera because that is an integral part of the film
once it has been exposed. We stabilize the footage in software to remove that problem.