2 small points;
D1 video (a standard for digital broadcast video) is defined as
720x486 with a rectangular PAR (pixel aspect ratio). 640x480 is the
original definition of VGA video and has a 1:1 PAR (square pixels).
The conversion to square pixels is understood by a lot of today's
software, but if things start looking squashed either horizontally or
vertically, sombody's forgetting about it.
And that "useless data" in the whites and blacks is what most
cinematographers think makes their images great. 'Schmutz' and
video 'artifacts' in my experience are terms left over from the good
old days of analog video. Here they refer to nothing. Most good
codecs will do some form of basic noise reduction in the highlights
and lowlights in order to increase data rate. I think it's more
important to create an image you are happy with rather than say it
must be modified this way to make the codec happy.
Just my opinon, your mileage may vary...
Mark
--- In videoblogging@yahoogroups.com, Joshua Czikowski <[EMAIL PROTECTED]>
wrote:
> Secrets of Video Compression
>
> Well, I don't know how secret they are, but there are some
techniques you
> can use to get the most out of your web video. A lot of it just has
to do
> with understanding how data (read that movies) is transmitted over
thre net.
> Some of it goes into how we poor humans perceive things like color -
- and
> the differences between TV screens and a computer screen.
>
> Standard D1 video (like what you see on TV) is 640 pixels wide by
480 tall
> and is made up of 30 frames of two fields per second or 60 images
at
> 640x480 per second of transmission. That's 27 Megabytes per second
of data
> to process. Most computers can't even play that straight form the
hard
> drive, they just don't have the processing power to handle all that
data
> that fast. So to make it workable and playable we use a codec for
> compressor/decompressor to squeeze that data down to a manageable
level.
> DV or the Digital Video codec is probably the video codec you're
most
> familiar with. It's the near lossless compression technology that's
used in
> most digital video cameras. It gives us about 5:1 compression. So
our
> original 27 MB of data drops to about a fourth of its size. Even DV
source
> material has a data rate of 3.5 megabytes per second (nearly 30,000
kilobits
> per second), though. So to get that down to a size that can be
streamed over
> the Internet to viewers with broadband connections, or even modems
requires
> a combination of intelligent preprocessing and powerful compression
> technologies.
>
> Basically there are two types of video compressionspatial and
temporal.
> Spatial compression is compression within a single frame of video.
Usually
> spatial compression techniques are based on the idea that computers
are
> stupid. The easiest thing to compress is a solid color square.
That's
> because the computer starts out by recording the square's color by
saying,
> "the pixel with coordinates x=0 and y=0 is black" and then does
that for
> every individual pixel. It's a little like marking your calendar
Monday
> 12:30 Lunch, Tuesday 12:30, Lunch, Wednesday 12:30 Lunch, etc.
A "lossless"
> compressor like LZW (a common compressor for still images in TIFF
format)
> rewrites the data to simply say, "Lunch 12:30 Monday-Friday", or in
this
> case "all the pixels from 0,0 to 100,100 are black". It eliminates
all those
> extra lines of data creating a smaller file without sacrificing any
of the
> original information. Lossless compressors can usually compress up
to about
> 50% of the original file size, sometimes less, but almost never
more.
>
> JPEG is also a spatial compression technology, but its what is
called
> "lossy". It works by throwing out some of the information stored in
the
> image. Ideally it does this intelligently by discarding first that
> information which is less useful. So at lower levels of compression
it
> throws out things like colors that are outside of the range of
human
> perception. As you apply higher levels of compression it relies
more on
> averaging across pixels to flatten out the color range and create
less data
> to store. That's why it's a bad idea to open and resave JPEG or
other lossy
> compressed files. Every time you do they get recompressed, throwing
out more
> data, till you eventually have crap.
>
> Temporal compression is also lossy. It works by compressing across
many
> frames of videocompression across time and temporally compressed
frames of
> video are called "difference frames" or "delta frames". That's
because the
> codec only records the differences between frames, not all the
information
> in each frame. The best codecs use a combination of both spatial
and
> temporal compression. The MPEG4 codec, for example sets a keyframe
(spatial
> compression) every so often as a reference point, then uses delta
frames
> (temporal compression) between keyframes to get even more
compression. The
> combination results in higher quality at smaller sizes. With a
codec like
> MPEG4 you may get upwards of 90-95% compression and still have a
pretty good
> looking video.
>
> Video codecs are generally broken down to transfer codecs and
delivery
> codecs. Transfer codecs are used to transfer files form one place
to
> another, for example from one editing station to another, or to
store files
> using less drive space. These codecs are generally lossless or
minimally
> lossy and afford minimal compression, but very high quality.
Apple's new
> Pixlet codec was developed at Pixar Studios (the people that did
Toy Story
> and Finding Nemo) to do just that move and store files. Delivery
codecs
> are just what they sound like, a way to deliver compressed video
usually on
> the web or CD. Delivery codecs are generally lossy and sacrifice
quality for
> playability.
>
> In all, we can reduce video sizes in three ways, reduce the data-
rate
> (increase the compression), reduce the physical size (number of
pixels), or
> reduce the number of frames per second. Play with any combination
of those
> numbers and you've got a smaller file, more easily playable over
the web or
> from CD ROM.
> ------------------------------
>
> Data Rate
>
> A 56Kilobits per second (Kbps) modem transfers data at 56 kilobits
per
> second -- it doesn't really. Its actually rated at 53 Kb, and in
reality
> it'll probably get between 4 and 5 Kilobytes (KB) of actual data.
See that
> -- small "b" bit -- big "B" byte. The files on your hardrive are
measured in
> Kilobytes and megabytes. Data throughput is measured in kilobits
per second.
>
> The rule of thumb is to divide by eight (there are eight bits to a
byte).
> That'll get you in the ball park. 53 ÷ 8 = 6.6, a little high, but
not bad
> for government work. Wanna' be safe, cut that in half. Shoot for
four or
> five and you'll be covered for those nasty dips and spikes that are
> inevitable in an imperfect system. So really, to get a movie that
will play
> in real-time on a 56K modem you're only looking at getting about
4Kb of data
> for BOTH video and audio.
>
> So your data rate is going to depend on your target audience. In
their
> training materials, Sorenson recommends using the following formula
to
> calculate data rate:
>
> datarate = (width x hieght x frames per second)÷ 48000
>
> Most codecs perform well at no less than half the resulting number,
and no
> more than twice that. For example, if you have a 320 x 240 movie at
30fps,
> you'll get a baseline datarate of 48KB/s. So a pretty static clip,
one
> without a lot of motion will probably still look okay at 24KB (half
that),
> while an action clip with a lot of jerky camera movements and fast
cuts will
> probably need about twice that, or somewhere around 96KB. You'll
still need
> to test to get the best results, but that gives you a place to
start
> looking.
>
> Another good rule, if you're doing progressive download is to give
your
> viewers some options. If you've been to the Apple QuickTime movie
trailer
> site, you've probably seen some of the trailers offered in small,
medium and
> large (even full screen) options. For the viewer the choices are
small
> crappy video that starts playing quickly, or big beautiful video
that
> requires a wait to get it started. My advice is to let them decide.
You
> should be aware though, that according to Apple's numbers something
like
> half of all the movie viewers on their site choose the biggest
movie --
> regardless of their connection. That means you have people on dial-
up
> waiting hours while they download a 40-50MB movie. The next most
popular --
> the smallest size.
> ------------------------------
>
> Make it Pretty
>
> So we've covered the science, now let's discuss the art. The real
trick to
> making it pretty is not to try to do too much at once. First you
clean, then
> you compress. Its called pre-processing and most compressionists
agree it is
> the secret to getting great web video.
>
> Personally, my favorite codec at this time is still Sorenson Video
3. I'm
> not a huge fan of MPEG4 video, despite Apple's marketing push. Love
the AAC
> sound compressor, but the video codec just isn't quite there yet.
Even
> still, the following recommendations will work with just about any
of the
> current crop of codecs. As I've explained, most of these codecs use
> algorithms that try to squeeze the video by interpolating and
saving only
> the data that is different between one frame and the next. So the
more alike
> each frame is, the better we can compress. That means high motion
shots,
> fast movements and lots of jerky camera work are harder to compress
than
> static headshots. These are called "interframe" codecs.
>
> Whether you're using Discreet's cleaner, Hip Flicks, Squeeze, or
even just
> QuickTime Pro, you've got several options for adjusting brightness,
> contrast, and hue. Depending on the software you've probably got a
couple
> more options-- like white and black restore. The trick is to take
your
> uncompressed video into your favorite program and NOT compress it,
at least
> not on the first pass. Your basic uncompressed video has schmutz --
pros
> call these "artifacts". So we're going to raise the contrast and
brightness
> to get rid of some or all of that. We're looking to blow out the
bright
> areas and crush the blacks. By eliminating some of the useless
detail in
> those areas, we eliminate some of the differences between frames.
> Differences that interframe codecs would interpret as data and try
to
> encode, thereby creating artifacts.
>
> Cleaning up the audio by normalizing it, adjusting the volume, and
applying
> whatever filters your software provides. The real key is to do all
these
> things, clean, crop, adjust -- whatever you need to do OTHER than
compress,
> in your first pass. Save your movie with audio and video
compression of
> "none". Your going to get a darn big file, but it'll be absolutely
pristine
> so once you start compression your end result will be as high a
quality as
> possible given you other variables.
>
> Certain codecs calculate a certain way, learn where your specific
codec's
> sweet spot is and exploit it. For example, Sorenson in particular
samples in
> blocks of eight. So cropping to a size divisible by eight is going
to give
> you the best result. And always, always crop. Not only will it get
rid of
> edge noise sometimes picked and magnified by the compression
process, but
> shaving down the physical dimensions will also save you bandwidth,
even if
> it's just a few pixels.
>
> Most any compression software you use will have a way to preview
the results
> of your filtering, so find the darkest areas of the video and the
lightest
> and do a little testing. Also, frame out your video at the smallest
crops
> possible and scrub through the timeline to make sure you're not
cropping out
> pieces of a face, or other important information. You might even
need to
> crop separate parts of the movie individually, then paste them
together
> later. I've sometimes found that one scene might be framed a bit
high,
> another a bit low. Don't be afraid to split the movie up and handle
it this
> way. The splitting and stitching can all be handled right in
QuickTime Pro.
> ------------------------------
>
> Compress Pass
>
> Now that you have your big-fat video that's been cropped, crushed,
and
> otherwise scrubbed clean of annoying stray pixels that'll never be
noticed
> anyway, its time to do what you really came for in the first place
and
> compress the living crap out of the file.
>
> Remember the rules, you can reduce file size and make the video
more
> deliverable in three ways; change the height and width dimensions
of the
> video, change the frame rate, or change the data rate. It'll always
be some
> combination of the three that will give you the best results, but
don't
> forget that everything is a trade off. Despite what your girlfriend
tells
> you, bigger is better. Reducing the dimensions decreases the
perceived value
> to your audience and may also interfere with the clear
communication of your
> message. Nobody wants to watch a postage stamp-sized video, no
matter how
> pretty it is. Set the frame rate too low and you'll get something
that looks
> more like a slide-show than a video, and squeeze the data-rate down
too much
> and the video will be blocky, blurry and otherwise unattractive.
>
> So what are the secret settings that make the perfect video every
time?
> Unfortunately, they change depending on the video, and in fact the
best
> results might come from using different codecs on different parts
of your
> video and stitching it all up later. As I noted earlier, QuickTime
can
> handle mixed codecs, multiple tracks, and even different bits with
different
> data-rates without missing a step. Don't be afraid to experiment
for the
> best results.
>
> It's important to realize that the best codecs are interframe
codecs, so
> they are looking for changes from scene to scene. If you take
nothing else
> away from this section and the one on camera techniques, take that.
Let it
> roll around in your head like a BB in a basketball until you start
to
> understand the implications of just what that means.
>
> Ultimately, at least in terms of compression, change is bad.
>
> So scenes with little change, talking heads (newscasts for example)
are easy
> to compress. Solid color backgrounds, flat lighting and slow
deliberate
> movements, all yield better compression results. They also make for
a pretty
> darn boring piece of video. The things that make your movie
engaging are the
> things that are hardest to compress, namely action. Running,
jumping, quick
> pans, that cool jerky hand-held technique like they used to do on
the TV
> show Homicide, all make lousy candidates for compression.
>
> With QuickTime we can compensate for this by working the
compression program
> and massaging our three variables till we get to the best possible
> compromise, and that's going to be different every time for every
piece of
> video. Even for experienced compressionists, it's really a matter
of trial
> and error. Like negotiating a divorce, it's all about what you're
willing to
> give up.
>
>
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