📄 advanced video compression - part 1.mht
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Subject: Advanced Video Compression - Part 1
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<HTML><HEAD><TITLE>Advanced Video Compression - Part 1</TITLE>
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<H1 align=3Dcenter><B><FONT size=3D+1>Advanced Video Compression - =
Part=20
1<BR>Various Codecs, Containers and their Pros and =
Cons</FONT></B></H1>
<P>Now that you know the basic concepts and terminology behind =
video=20
compression, we can talk about various containers and codecs, and =
which=20
one is right for you. First off, we need to point out what the =
difference=20
between a Codec and a Container is. Codec stands for=20
Compressor/Decompressor, meaning it is something which can both =
encode and=20
decode something, in this case video. Codecs include things like =
MJPEG,=20
Sorenson, Cinepak, and DivX. Some codecs may have specific =
containers=20
associated with them, such as MPEG. Some other containers you are =
probably=20
familiar with include AVI, Quicktime, or Matroska. A container =
simply=20
holds everything together. For instance, the AVI container holds =
both the=20
video stream (which is compressed with a Codec) and an audio =
stream.=20
Without the container, the video and audio would be in seperate =
files!=20
More advanced containers like Matroska can allow for additional =
things=20
like subtitle streams, multiple audio and video steams, chapters, =
menus,=20
and more.</P>
<P>A container does not necessarily mean anything about the video =
quality,=20
it only dictates the underlying structure of a file. I always =
scratch my=20
head when people say things like "I like AVIs better than MPEGs =
because=20
they're smaller," which is a statement that makes absolutely no =
sense.=20
I've had an AVI file that's 26GB large, orders of magnitude larger =
than=20
most MPEG files. The fact is that containers like AVI and =
Quicktime have=20
little to nothing to do with the underlying Codec, except that the =
limitations of a container must be adhered to in the codec. For =
instance,=20
AVI does not support many of the features required for h.264 =
(MPEG-4 AVC)=20
compressed video. </P>
<P>Now on to the various codecs that we'll be talking about. We'll =
be=20
splitting the various codecs into two sections - one for codecs =
which are=20
good when used for editing, and one for codecs that should be used =
for=20
distribution purposes.</P>
<P><FONT size=3D+1><B>I. Common Editable Codecs</B></FONT></P>
<TABLE cellPadding=3D0 width=3D"100%" border=3D0>
<TBODY>
<TR>
<TD vAlign=3Dtop><B>1)</B></TD>
<TD><B>MJPEG</B> - <B>M</B>otion=20
<B>J</B>oint-<B>P</B>icture-<B>E</B>xperts-<B>G</B>roup =
Compression=20
<P><B>Origins:</B> All of you know what JPEG is, I'm sure. =
If you've=20
surfed the web, then you've seen a JPEG image. JPEG was a=20
compression scheme developed by the Joint Picture Experts =
Group for=20
the compression of images. Now imagine instead of =
compressing single=20
images as JPEGs, compressing 24 to 30 images a second and =
storing=20
them in sequence. That's essentially what MJPEG is - it's =
the JPEG=20
compression algorithm applied to frames of video.</P>
<P><B>How it Works:</B> MJPEG is a lossy codec. JPEG uses =
what's=20
called a psycho-optical algorithm, or a visual-perception =
algorithm.=20
Basically, what this means is that JPEG exploits flaws and =
short=20
comings in the human perceptual system. The human eye has a =
much=20
harder time differentiating between slight differences in =
color than=20
differences in brightness. Also, JPEG plays other various =
tricks,=20
but what you wind up with is an image which looks about the =
same to=20
the human eye.</P>
<P><B>Benefits:</B> MJPEG is blazing fast. You probably wont =
find a=20
speedier codec! MJPEG can achieve decent compression rates =
and good=20
image quality for most natural images like live video.</P>
<P><B>Disadvantages:</B> Here's where MJPEG starts looking =
much less=20
attractive. First off, JPEG does not compress anime well. =
Due to the=20
way that JPEG works, abrupt changes in color and brightness =
(for=20
instance, thin black lines in between different blocks of =
color) are=20
not handled well by JPEG, which means MJPEG suffers from the =
same=20
problem. JPEG has a problem with sharp edges which is =
unavoidable,=20
partly because the standard was meant to compress natural =
images=20
like scanned photographs.<BR>There's also the problem of=20
recompression. While the human eye can be fooled by JPEG's=20
compression, computers aren't. Thus, when you compress from =
MJPEG=20
into another format to use for distribution, you will =
receive a=20
lower-quality picture than if you had a lossless source =
because=20
there is data missing which, while it's not important to the =
visual=20
cortex, is very important to a mathematical analysis of =
things such=20
as color data.</P>
<P><B>Recommendations:</B> MJPEG is an old standard. There =
are much=20
more refined and better Discrete Cosine Transform-based =
compression=20
algorithms out there (like DV) that I would recommend over =
MJPEG.=20
About the only time you should use MJPEG is for fast-editing =
temporary files, but DON'T use them as your final=20
source.<BR><BR></P></TD></TR>
<TR>
<TD vAlign=3Dtop><B>2)</B></TD>
<TD>
<P><B>DV</B> - <B>D</B>igital <B>V</B>ideo</P>
<P><B>Origins:</B> First launched in 1996, DV became a =
standard for=20
use in both consumer and semi-professional digital video=20
compression. DV (or one of its many varients) is used by =
many video=20
cameras on the market today. It is also well supported by =
many video=20
editing applications and video hardware devices.</P>
<P><B>How it Works:</B> DV is a lossy codec. I've heard lots =
and=20
lots of misinformation floating around about how "DV is =
lossless,"=20
which it definitely is <B>not</B>. DV is a different =
approach=20
towards a Discrete Cosine Transform algorithm than what was =
taken=20
with MJPEG, partly because DV is a newer standard. I won't =
bore you=20
with all the gory details, but lets just say that in almost =
every=20
respect, DV is technically superior. The main disadvantage =
is that=20
DV is fixed to 720x480 in 29.97 FPS (or 720x576 in 25 FPS) =
at=20
25MBit/second. That means very little flexibility if you =
want to do=20
low-quality previews of your video or you're pressed for =
disk space=20
(25MBit/second is 3.4MB/sec, or roughly 5 minutes per =
GigaByte!).=20
Some variants of DV exist that may support different =
resolutions, or=20
allow for progressive (non-interlaced) video. But for the =
most part,=20
DV severely limits what you can put into it. Like MJPEG, DV =
is a=20
good editing codec because it doesn't use any inter-frame=20
compression, thus every frame is a "keyframe."</P>
<P><B>Benefits:</B> DV looks very good, better than MJPEG in =
every=20
respect. DV is good enough that it's used by both consumers =
and many=20
professionals as a compression standard. If you are only =
going to be=20
doing a recompression once or twice, the quality shouldn't =
be much=20
of an issue. With repeated recompression though, you can =
start=20
really losing quality.<BR>DV is also cross-platform, meaning =
it=20
works the same on PCs, Macs, AVID video editing =
workstations, you=20
name it. You can also create a DV project, and then pipe it =
out=20
through FireWire to DV tape and get a complete, lossless =
digital=20
copy of your master as a backup.</P>
<P><B>Disadvantages:</B> As stated above, it's rather =
restrictive in=20
terms of framerate, resolution, and datarate. Also, its not=20
lossless, but the quality is still VERY good.
<P>
<P><B>Recommendations:</B> If you don't plan to do any=20
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