User guide

DivX 10
46
DivX, LLC User Guide
1-PASS, 2-PASS,
AND MULTIPASS
1-PASS ENCODING
2-PASS/MULTIPASS ENCODING
In 1-Pass mode the encoder will output a working video stream
as it receives the source video. This is both the easiest and fastest
method of encoding video while targeting an average bitrate,
and is particularly suited to video capture applications where it is
impossible to recreate the source video identically more than once.
However, 1-Pass mode is the least consistent of all available modes
in terms of video quality.
During 1-Pass encoding, if you are using the DivX Codec in a
third-party application, it is possible to modify the birate in real-time
by selecting the bitrate option from the bottom panel of the encoder
feedback window. This feature is likely to be of particular interest
when capturing from a live source. For example, if capturing
from television you might want to reduce the bitrate during
advertisements.
This mode is particularly useful when capturing from a live source
and you desire control over the bitrate/file size. When capturing
from a live source and attempting to maintain a consistent quality
throughout the video you should use 1-Pass Quality Based mode.
When working from static sources (for example re-processing
existing stored video) you will achieve more consistent quality
by using Multipass mode.
As opposed to 1-Pass encoding where all the encoding is done in one
single pass or on the fly, in 2-Pass or Multipass (2 or more passes)
encoding the file is analyzed completely during the first pass and
a sort of intermediate or temporary file is created then the second
pass is used by the encoder looks up the intermediate file and uses
that information to decide how to allocate all the bits. So the actual
encoding takes place in the second pass with the information of
having fully analyzed the entire file beforehand.
DivX uses a rate control algorithm to vary the bitrate throughout the
video to achieve a consistent quality while targeting the specified
average bitrate. To do this effectively, the codec has to be able to
examine the entire video in order to determine where bandwidth
should be allocated before it spends any bits at all.
At a given file size, 2-Pass encoding usually has better quality than
single pass encoding but it will take longer to encode the file. This
also has the potential to give smaller files with better quality at lower
bitrates than the same file converted in single pass. Single pass can
also have quality but with less control over the output file size.
One of the reasons this is used is the variation in the scenes during
a video, especially in the example of movie. There might be long,
black scenes in one area and complex scenes with action or lots
of movement in another. In a 1-pass encode, the exact same data is
used on both these scenes. When using multi-pass encoding, which
is made possible through Variable Bit Rate, the encoder has fully
analyzed the entire file before encoding so it knows that some
sections need a lower bitrate and then others can use more for
a better overall quality by a more efficient use of bitrate.