[theora-dev] dead DCSearchPoints code

[Dan Miller]

> From: Mike Melanson [mailto:melanson at pcisys.net]
> 
> > BTW I think I will scrap the group numbering
...
> 	True, it could probably be described without the group 
> numbering.
> However, it will be critical to carefully explain how to make the DC
> prediction bit-exact (think about the HIGHBITDUPPED() macro). 
> I know from
> experience how quickly the data will be messed up if the process isn't
> followed precisely as in the original code.
> 
Absolutely.  That's why I'm doing the Python thing -- whether anyone looks at it or not, it's the only way I can assure myself I really understand what's going on and have described it correctly.

The HIGHBITDUPPED thing looks like a hack to use shift instead of divide -- basically if the value is negative, you add 
2^shift-1 before shifting.  This forces the shift to round towards zero, rather than always rounding down (which would increase the absolute value of a negative number).  For example, without the correction 7>>1 = 3, and -7>>1 = -4.  But if we add 1 to -7 we get -6, and -6>>1 = -3.

I'll get to the part of the code with the bug eventually.  I'm recalling a bit more -- this programmer said that for some reason they decided late in the game to tie chroma motion vectors to Y vectors -- is it possible that while the decoder has logic to unpack these unused bits, the encoder never generates them?  The explanation would be that for a while they had old bitstreams lying around they didn't want to break...

I can always try to get some help from On2, but they're on to VP6 and tend not to remember what was going on 4 years ago.  And of course they're always on deadline and basically have no allowance to give me any time on VP3 anymore.

I still have faith everything will make sense eventually...

-dan

> > Also, sanity check -- DCSearchPointCount is always zero, so 
> the code that looks through the DC search points never gets 
> called, right?  It falls through to Last[WhichFrame], ie last 
> block coded, or zero if no blocks coded...?  Nice if true, 
> less craziness to document.
> 
> 	From my analysis, DCSearchPoints is mooted in the VP3 source.
> 
> > As for that possible bug, I have a vague memory of one of 
> the original VP3 programmers explaining something once about 
> how the color modes are tied to the Y modes, so I suspect 
> it's not really a bug...  but this could be a bad dream...  
> in any case, if it were a bug, I think things would be way 
> screwed up & we would have noticed.  More likely it's just 
> another of the cruelly subtle coding that makes VP3 analysis 
> such a challenging hobby...
> 
> 	For anyone following along, here is the original message:
>   http://www.xiph.org/archives/theora-dev/200305/0002.html
> I still contend that it does not make sense. While C-plane macroblock
> coding data is unpacked, this information is never used in 
> the decoding
> process. The Y-plane macroblock data is applied to the C-planes. Thus,
> while the information is present in the bitstream (using 
> precious bits),
> it does not affect the decoding process.
> 
> > Thanks much for the contribution! Sure you don't wanna do 
> IRC?  Great timewaster, but we sometimes actually talk about 
> this stuff ;*}
> 
> 	Generous offer, but no thanks. I'm much too busy with 
> the general
> multimedia hacking. Besides, as with Wiki, I don't know how 
> IRC works and
> I'm too unmotivated to learn...:)
> 
> --
> 	-Mike Melanson
> 
> 
> 
> 
> 
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