Visual Effects
Composting Effects are the primary device used in all aspects of visual effects
from television commercials to big budget features that bring dinosaurs to San
Diego. This topic works well as a starting point due to the fact that for almost
all of the other techniques I have listed a composite is the final product.
Simply put, compositing is the act of layering multiple elements through time. The
best analogy would be working with multiple layers in a Adobe Photoshop document. I
will try to provide a couple of examples which give a hint at the many applications
compositing can see.
The form which is aguably most familiar to all audiences can be seen
every evening on the news. Digital weather maps are inserted behind walking
talking newscasters in a process termed blue screening, which will be covered
in a later section. The effect produced however is that your local weather person
is actually standing in front of a large intereactive wall of information. If
you were present in the studio during the taping, what you would actually
see is that the newscaster is standing in front of a large blue screen; actually there
are many colors that can be used but blue is most common. The weather person is
able to interact with and point to specific areas by looking at a live NTSC / PAL
(televison) monitor which shows a composite image with the blue color removed
wherever it has occured in the video feed and replaced with the image of
the radar map or predicted forcast. Incidentally, this monitor would commonly
be located very near to the camera which is doing the taping; this would explain
that uncanny ability newscasters have to point at somthing on the map without
looking.
The nightly weather is actually a very complicated example because there are a
lot of things going on which make it possible. Since it is so common, it is a
good place to start. Composites do not have to incoportate video or
film footage at all. In fact, many composites seen in commercials consist entirely
of animated text and still images. Any number of layers of footage, images, text,
or even 3D objects may be combined over time in the digital age of compositing
through tools from Adobe After Effects to Discreet Logic's Inferno, depening upon
the amount of time available to get the job done and the size of the job.
In order to understand how composites are generated it is necessary to understand
how two different layers of footage can be combined in a meaningful way. In our
first example, the weather person pointing to the digital map would be wholly
useless unless we could consistantly keep track of where to show the weather map
and where to show the person doing the pointing. What is ultimately generated
for this task is called a matte or alpha channel (known as a mask to Adobe Photoshop
users these three terms are essentially interchangable). The
matte is a black and white image geneated for each frame of footage.
For explanation purposes consider that video, like film, is made up of many still
images called frames which, when shown in rapid succession, fool the brain into
perceiving motion (more detailed nuances are covered later). The matte that
is generated must indicate the areas in the foreground footage that we would like
to sit on top of our bacground footage in either black or white and then the rest
of each fame is filled with the opposing color. The area of a footage file
that stores this information is know as the alpha channel. This information is then
used by the compositing software to punch a hole through the foreground footage so
that the background is visable where the alpha designates. These layers of footage
are commonly refered to as plates.
Of some intrest is the method for compositing before affordable desktop workstations
roamed the earth. One would have to look no further than Star Wars to see an excellent
example of the handcrafted version
of this art. Mattes at the time were actually hand-painted pieces of glass through
which a scene was filmed. Imagine placing a rectangular
pice of glass with one half painted completely black in front of a film camera this
would allow only one side of the film negative to be exposed. The same film could then
be re-shot with the black area on the opposite side so that the other portion of the
negative could be exposed at a later time or under different conditions, thus the origin
of the black and white colors mentioned above. This sort of handpainting could also be
done on a clear negative and used in a similar fashion durring printing.
For an explanation of how these mattes are generated in the digital world check out the
rotoscoping section.
Back to the Visual Effects area.
Feedback is always welcome drop a line to:
mrehrer@csua.berkeley.edu.
|