Computer Concepts Eagle M2, Руководство пользователя

Страница: 46 / 66

7. Acorn movie compression routines
and RCompress
Acorn movie compression routines
These are an alternative to the optional Eidos ESCaPE routines.
The Acorn routines compress movies in two stages. They operate
on the stored movie. (Unlike ESCaPE, which can compress a 12.5fps
movie as it is stored to disc.) However, they let you compress 25fps
movies. The routines are accessed using !RCompress (described
later).
Stage 1 — compression
This stage compresses the movie, frame-by-frame, into a series of
files in a specially created directory. The result of this stage is called
an exploded movie (because it has exploded into several files). For
details of these files, see 9. Technical information.
This stage uses a module inside !ARMovie called Differ: Compres-
sion is slow – each frame takes several second to process.
The first frame in the movie is as a complete frame. For subsequent
frames, only the differences from the preceding frame are stored.
Thus, a slowly changing or static picture has little difference between
frames and so good compression is achieved. A rapidly changing
image has more differences and so is compressed less. A dialogue box
(described later) specifies the minimum permitted compression
for each frame. If necessary, Differ reduces the image quality for
individual frames to meet this compression. Usually this reduced
quality is unnoticeable because subsequent frames have fewer
changes. This gives better compression and so they can be displayed
at higher quality.
Stage 2 — combining the files
This stage merges the exploded movie files into a single com-
pressed Replay movie file.
This stage uses a module inside !ARMovie called Join. Combining
is much quicker than compression and takes only a few seconds to
complete.
39