EXPLANATION
compression method used by the
MPEG standard. In a motion sequence, individual frames of pictures are grouped together (called a
group of pictures, or
GOP) and played back so that the viewer registers the video��s spatial motion. P-frames follow
I-frames
and contain only the data that have changed from the preceding I-frame
(such as color or content changes). Because of this, P-frames depend on
the I-frames to fill in most of the data.
P-frames and
B-frames are also referred to as
delta frames.
Also see
I-frame and
B-frame.
MPEG4 is going to be more
space-efficient, but given enough space MJPEG will look just as good.
The important difference is that editing MJPEG is easier because every
frame is a keyframe, so you can cut and paste at any point. Editing
MPEG4 is more difficult because of the alpha frames (the "difference"
frames that come between keyframes). If your friend is just recording
the video to watch later without doing anything to it, then she should
just record to MPEG4. If this for home movies that she'll be messing
with, then MJPEG.
Typically, pictures (frames) are segmented into
macroblocks,
and individual prediction types can be selected on a macroblock basis
rather than being the same for the entire picture, as follows:
- I-frames can contain only intra macroblocks
- P-frames can contain either intra macroblocks or predicted macroblocks
- B-frames can contain intra, predicted, or bi-predicted macroblocks
Furthermore, in the
H.264 video coding standard, the frame can be segmented into sequences of macroblocks called
slices,
and instead of using I, B and P-frame type selections, the encoder can
choose the prediction style distinctly on each individual slice. Also in
H.264 are found several additional types of frames/slices:
- SI‑frames/slices (Switching I): Facilitates switching between
coded streams; contains SI-macroblocks (a special type of intra coded
macroblock).
- SP‑frames/slices (Switching P): Facilitates switching between coded streams; contains P and/or I-macroblocks
- Multi‑frame motion estimation (up to 16 reference frames or 32 reference fields)
Multi‑frame motion estimation increases the quality of the video,
while allowing the same compression ratio. SI and SP frames (defined
for the Extended Profile) improve
error correction. When such frames are used along with a smart decoder, it is possible to recover the broadcast streams of damaged DVDs.
Intra-frame coding is used in video coding (compression). It is part of an intra-frame codec like ... It is one of the two classes of predictive coding methods in video coding. ... The temporally coded predicted frames (e.g. MPEG's P- and B-frames) may use intra- as ... Wikimedia Commons has media related to Intra-frame coding.
Usually only few of the spatially closest known samples are used for
the extrapolation. Formats that operate sample by sample like
Portable Network Graphics
(PNG) can usually use one of four adjacent pixels (above, above left,
above right, left) or some function of them like e.g. their average.
Block-based (frequency transform) formats prefill whole blocks with
prediction values extrapolated from usually one or two straight lines of
pixels that run along their top and left borders.
The term intra-frame coding refers to the fact that the various lossless and
lossy compression techniques are performed relative to information that is contained only within the
current frame, and
not relative
to any other frame in the video sequence. In other words, no temporal
processing is performed outside of the current picture or frame.
Non-intra coding techniques are extensions to these basics. It turns out
that this block diagram is very similar to that of a JPEG still image
video encoder, with only slight implementation detail differences.
Inter frame has been specified by the
CCITT in 1988–1990 by
H.261 for the first time. H.261 was meant for teleconferencing and ISDN telephoning.
