[Matroska-devel] RFC 1134 - New EBML Block design

Moritz Bunkus moritz at bunkus.org
Tue Jan 20 23:10:50 CET 2004


Heya,

Yay, a new block design. I strongly feel we should discuss this.

> The order of the headers are
> 
> 1. TrackNumber
> 2. Time
> 3. Bitflags

That's good. Having the same order certainly lesses the overhead/amount
of new code.

> The Time is an EBML SINT that is used to calculate the timecode of the Block2.
> Exact method depends on the bitflags used below.

Certainly an improvement over a fixed 16bit sint, IMHO.

> Bitflags are as follows.
> Bit
> 0-1    Lacing flags
> 2       Time flag
> 3-6    Unused
> 7       Gap flag

If we be radical we can also think about using simple flags for track
types. We could use the bits 3 and 4 like this:

00: I frame
01: P frame
10: B frame
11: invalid

The ReferenceBlocks would then be _optional_ for such a block. If
they're not present the references are implicitely given. However, if
some special application needs some special references it may still add
them. The number / types of references must then comply to the flags
used.

This would save us a lot of space.

> Lacing flags layout is identical to the original Block layout.

Good.

> 1. By introducing a completely new Block structure with a new ID, we can be sure
> that current parsers won't b0rk on the data.  It is better for the parser to
> simply skip the data than to break while trying to read data.

I agree.

> 1. Two of the bit flags could be used to indicate forward or backward
> references(as originally planned).  This would remove accuracy in the case of a
> damaged file, but would decrease overhead by no longer needing to use the
> ReferenceBlock element in most cases.  3 bytes for P frames and 6 bytes for B
> frames would be saved if using this.

Maybe I should just read the mail completely before I start spitting out
stuff that you've already said ;)

All in all I like this more flexible block layout very much.

Mosu

-- 
If Darl McBride was in charge, he'd probably make marriage
unconstitutional too, since clearly it de-emphasizes the commercial
nature of normal human interaction, and probably is a major impediment
to the commercial growth of prostitution. - Linus Torvalds



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