Introduction - Computer Animation: Algorithms and Techniques - page 18

have been done. Key lights are set to establish the basic lighting environment. Subtler lighting
particular to an individual shot refines this in order to establish the correct mood and bring focus
to the action.
￿ The Camera Department is responsible for actually rendering the frames. During Toy Story ,
Pixar used a dedicated array of hundreds of processors called the Render Farm . The term render
farm is now commonly used to refer to any such collection of processors for image rendering.
1.4.2 Digital editing
A revolution swept the film and video industries in the 1990s: the digital representation of images. Even
if computer graphics and digital effects are not a consideration in the production process, it has become
commonplace to store program elements in digital form instead of using the analog film and videotape
formats. Digital representations have the advantage of being able to be copied with no image degra-
dation. So, even if the material was originally recorded using analog means, it is often cost-effective to
transcribe the images to digital image store. And, of course, once the material is in digital form, digital
manipulation of the images is a natural capability to incorporate in any system.
In the old days ...
The most useful and fundamental digital image manipulation capability is that of editing sequences of
images together to create a new presentation. Originally, film sequences were edited together by phys-
ically cutting and splicing tape. This is an example of nonlinear editing , in which sequences can be
inserted in any order at any time to assemble the final presentation. However, splicing is a time-
consuming process, and making changes in the presentation or trying different alternatives can place
a heavy burden on the stock material as well.
Electronic editing 8 allows the manipulation of images as electronic signals rather than using a phys-
ical process. The standard configuration uses two source videotape players, a switching box, and an
output videotape recorder. More advanced configurations include a character generator (text overlays)
and special effects generator (wipes, fades, etc.) on the input side, and the switching box is replaced by
an editing station (see Figure 1.8 ). The two source tapes are searched to locate the initial desired
sequence; the tape deck on which it is found is selected for recording on the output deck and the
sequence is recorded. The tapes are then searched to locate the next segment, the deck is selected
for input, and the segment is recorded on the output tape. This continues until the new composite
sequence has been created on the output tape. The use of two source tapes allows multiple sources
to be more easily integrated into the final program. Because the output is assembled in sequential order,
this is referred to as linear editing . The linear assembly of the output is considered the main drawback
of this technique. Electronic editing also has the drawback that the material is copied in the editing
process, introducing some image degradation. Because the output tape is commonly used to master
the tapes that are sent out to be viewed, these tapes are already third generation. Another drawback
is the amount of wear on the source material as the source tapes are repeatedly played and rewound
as the next desired sequence is searched for. If different output versions are required (called
8 To simplify the discussion and make it more relevant to the capabilities of the personal computer, the discussion here focuses
on video editing, although much of it is directly applicable to digital film editing, except that film standards require much
higher resolution and therefore more expensive equipment.