[theora-dev] Updated VP3 Document

[Timothy B. Terriberry]

> Feel free to look it over and point out any glaring mistakes. I can't wait
> to see how much sense it makes to me in the morning. But I completely
> revised the DC prediction section so that it is almost entirely separated
> from any specific implementation details (earlier weakness).

Didn't read too deeply yet, but a few comments on the Theora differences 
section:

1) We plan to fix the frame orientation to conform to that used with VP3 
before the beta (most likely in an alpha3). We want to preserve the 
ability to transcode from VP3 losslessly, and there's no real reason to 
keep two different orientations around.

2) Theora stores the prefix-free codes directly, in the form of a full 
binary tree (this is a change from alpha 1, where the histograms used to 
generate the prefix-free codes were stored).

4) Theora removes some of the unused bits in the VP3 frame header. There 
are still 3 0 bits in keyframe headers (keyframe type, which is always 
0, and 2 unused bits), but the others are gone.

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