WebM presets should use two-pass encoding

[Hooloovoo]

Two-pass encoding allows the encoder to scan the file first to find where the bits are best allocated for perceived quality. As Arista is aimed at transcoding in non-real time situations, it should use two-pass encoding to improve quality. This is slightly slower, but that should not be a problem in the normal use-case, where quality is more important than instant output.

http://www.webmproject.org/tools/encoder-parameters/#one-pass_vs_two-pass

The particular preset that I have looked at is Computer > WebM, though I assume that there are others such as Computer (Advanced) > WebM and Web > WebM.

[Hooloovoo]

It looks as though this needs the "multipass-mode" property: http://gstreamer.freedesktop.org/data/doc/gstreamer/head/gst-plugins-bad-plugins/html/gst-plugins-bad-plugins-vp8enc.html I also saw this: https://github.com/danielgtaylor/arista/issues/77 which looks like it is only relevant for H.264.

[danielgtaylor]

While Arista supports multi-pass encoding I generally opt to not use it in favor of constant-quality encoding. That is, rather than target a specific bitrate we target a perceived quality factor and allow the bitrate to change to accommodate that quality. In practice this means we need only a single (slightly longer) pass and we don't have to sacrifice quality when the video is too complex, nor do we sacrifice space when the video is not complex.

[Hooloovoo]

Oh right, thanks--guess I'll leave it to the experts!