HOW
COMPRESSION
WORKS
‐
Fundamentally,
compression
works
by
summarizing
a
sequence
of
images
and
sounds
as
efficiently
as
possible.
Video
that
has
been
compressed
doesn’t
provide
an
identical
representation
of
the
source
file
back
to
the
user—the
data
rates
would
be
far
too
high.
‐
Instead,
the
encoded
files
describe
the
important
details
of
the
content
as
tersely
as
possible
while
still
providing
a
reproduction
that,
to
the
human
sensory
system,
provides
the
experience
of
the
original
as
accurately
as
possible.
‐
A
matched
pair
of
a
compressor
and
a
decompressor
,
better
known
as
a
codec
,
performs
the
compression.
Æ
A
compressor
is
part
of
the
encoding
process,
reducing
the
amount
of
data
required
to
store
the
video.
Æ
A
decompressor
works
on
the
receiving
end,
decoding
the
compressed
data
so
that
it
can
be
presented
to
the
viewer.
‐
It’s
important
that
the
pair
be
matched
because
the
decoder
(decompressor)
needs
to
understand
the
encoder’s
(compressor’s)
summary
of
the
data.
TYPES
OF
COMPRESSION
Video
codecs
can
use
spatial
compression,
temporal
compression,
or
a
combination
of
both.
Spatial
Compression
Æ
Intraframe
Æ
Affects
only
a
single
frame
at
a
time
Æ
Makes
random
access
to
any
point
in
the
video
and
editing
easy
Æ
Finds
redundancy
within
a
frame
(eg.
similar
colours
or
areas
that
look
like
other
areas)
and
encodes
these
similar
areas
by
reference,
rather
than
by
a
literal
description
for
each
and
every
pixel
Æ
Based
on
Discrete
Cosine
Transformation
(DCT)
technique
which
are
good
with
smooth
gradations,
but
struggle
with
random
details
or
sharp
edges
at
lower
bit
rates
Æ
can
cause
visible
degredation
of
the
image
=
artifacts,
‐‐
such
as
ringing
(halo
or
smudge)
and
blocking
(smooth
edges
become
blocky)
Summary of Contents for XL1 3CCD
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