Invisible wrote:
>>> I'm not really comprehending the significance of the distinction 
>>> between P frames and B frames...
>>
>> P frames are differences from the previous I frame.
>>
>> B frames are differences from the preceding and following frames, 
>> whether they be I or P or B.
>>
>> In other words, you can't play a B frame at all without all the data 
>> between one I frame and the next.
> 
> I get that there are full frames and difference frames. I'm just not 
> really understanding why there are two different kinds of difference 
> frames...

One difference frame (P) is a difference from the past. They're 
"progressive" changes since the last full frame.

The other difference frame (B) is a difference from the past *and* the 
future. In other words, if you have B-frames, the decoder has to buffer 
several uncompressed frames, reading ahead in the stream past where it's 
actually playing. B frames have forward references to the content of 
frames that come later in the stream. You can't play a B-frame as soon 
as you read it. You have to keep reading to the next I-frame [1], then 
decompress the B-frames you've buffered up.

[1] Maybe it's P frames you read ahead to. It's been years since I 
looked at the details.

> Yeah, I figured that out a long time ago. ;-) Presumably otherwise you'd 
> only have one I frame, right at the very start.

Well, it also lets you tune in to the middle of a program. Remember, it 
was for "video on demand" all this got invented. Otherwise you'd just 
mail an analog tape.

-- 
Darren New / San Diego, CA, USA (PST)

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