Screencaps: What are they good for?

[Anthony P]

One question: what do the "I," "B," and "P" stand for?

I-frame = Intraframe
P-frame = Predicted frame
B-frame = Bi-directional frame
assuming you meant more then just the words,

I frame = whole frame (i.e. like the pictures that make up the film

a P frame and B frame are almost the same thing, a P frame uses the previous frame and only tells the video processor what to change, to get the new one, and B frame will also have info on the next one.

for example let's say we have the three frames of a film of a guy walking F1=upright, F2 = leg bends to take step and F3 is the guy after the step

F1

|
|

F2

|
|>

F3

|
/\



if F2 is an I frame it would have the whole image

|
|>

or
1.1=
2.1 = |
3.1 = |
3.2 = >


if F2 is a P-frame it will only have
add > to third row of F1
i.e.
3.2 = >

if F2 is a B frame then
add > to third row of F1; replace |> by /\ for row 3 for F3

i.e.
3.2 = >

3.1=/
3.2=\

so your decoder = gets F1

|
|

then it uses the info in the P or B frame to make F2
but it can make a mistake and get

|
| >

or


|
>


which will cause drift and jitter

that is why you need i frames every so often (to correct the decompressing artefacts caused by miscalculations)

but (to make it simple and go back to the initial discussion

F2 as a P frame would only need info on that one element (right leg) while if it is an I frame it would need 4. So a lot more info would be needed to have an I frame then a P (or B) frame, and the encoder will decide to use I, P or B depending on what is allowed and what is most compressible. That is why I frames end up being extremely compressed and why videos use P and B frames to get reasonable BW.

now why P and B? P are good for play but B is needed for stuff like reverse (ergo the name bi-directional)