VMD-L Mailing List

From: Bogdan Costescu (bogdan.costescu_at_IWR.Uni-Heidelberg.De)
Date: Fri Jun 20 2003 - 14:30:24 CDT

[ I've wrote on CCL some weeks ago about similar topic... ]

On Wed, 18 Jun 2003, John Stone wrote:

> I'm assume that your goal is to produce an MPEG-1 movie, at standard
> VCD profile etc. Here are things to be cognizant of when making
> movies for low-bitrate playback (i.e. VideoCD or standard MPEG-1
> rates):

Why is there this association between MPEG-1 and low-bitrate and therefore
low-quality ? Here we create movies for 2 purposes: putting them on a web
site (= viewer should understand what is happening, size is important) and
including them in laptop presentations (= crystal-clear image, attention
to details, size doesn't matter too much). So the web movies are created
with low bitrates at low resolutions and the presentation movies are
created at high bitrates with high resolutions.

> - MPEG is a garbage-in/garbage-out medium.

Well put :-)
I'd like to add something that is obvious but still lots of people fail to
realize it: the rendered frames should have the same resolution as the
final movie. Most (all ?) MPEG creation programs can scale in software the
frames to the movie resolution; however this scaling usually introduces
artefacts in the pictures, before the MPEG encoding takes place. This
adds to the digital noise and contributes to a lower quality of the
result.

> - MPEG-1 targets 352x240 resolution. If you exceed this resolution,
> you're asking for all kinds trouble both in encoding and playback.

Well, let me disagree here :-)
As stated above we want 2 kinds of movies. However, most important is that
these movies should be playable with as few (if any) setup steps needed on
the part of the viewer. By trial-and-error we have discovered that out of
our OSes (Linux, IRIX and Windows) Windows seems to be the most picky
platform when using its default codecs (a.k.a. filters) - it only plays
MPEG-1 files with certain resolutions, which are (surprise !) those
established by the TV standards; the bitrate should be constant; the frame
rate should also ideally be one of the standard (again from TV) ones - if
frames should be played slower, the same frame should be repeated for
several "real" frames.

So the web movies that we create are 320x240 or 352x240, with 1150 Kbps.
The presentation movies are 720x576 using something like 8000 Kbps - as
here we want the best quality, the bitrate might vary as well as the frame
type sequence (some use only I, some IPP, or some more P frames). We've
had no problems playing any of these files in Windows (98, 2000, XP)
without installing any additional codecs or player software; in Linux, we
use mplayer; I forgot the exact name of the IRIX player... Limited trials
with these files on MacOS were also successful.

> The best software-based MPEG encoders can also "rewind"

... or make several passes. CCE (for Windows) allows even editting the
profile that is kept between passes, such that the human can better direct
what regions of the movie should be allocated more bits. But this
discussion becomes too technical :-) There are places on the 'net where
movie creation topics are discussed; one starting point is
http://www.doom9.org 

--
Bogdan Costescu
IWR - Interdisziplinaeres Zentrum fuer Wissenschaftliches Rechnen
Universitaet Heidelberg, INF 368, D-69120 Heidelberg, GERMANY
Telephone: +49 6221 54 8869, Telefax: +49 6221 54 8868
E-mail: Bogdan.Costescu_at_IWR.Uni-Heidelberg.De