From Wikipedia, the free encyclopedia
Intra-frame coding
is a
data compression
technique used within a
video
frame, enabling smaller file sizes and lower bitrates, with little or no loss in quality. Since neighboring pixels within an image are often very similar, rather than storing each pixel independently, the frame image is divided into blocks and the typically minor difference between each pixel can be encoded using fewer bits.
Intra-frame prediction exploits spatial redundancy, i.e. correlation among pixels within one frame, by calculating prediction values through extrapolation from already coded pixels for effective
delta coding
. It is one of the two classes of predictive coding methods in
video coding
. Its counterpart is inter-frame prediction which exploits temporal redundancy. Temporally independently coded so-called intra frames use only intra coding. The temporally coded
predicted frames
(e.g. MPEG's P- and B-frames) may use intra- as well as inter-frame prediction.
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.
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.
Coding process
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Data is usually read from a video camera or a video card in the
YCbCr
data format (often informally called
YUV
for brevity). The coding process varies greatly depending on which type of encoder is used (e.g.,
JPEG
or
H.264
), but the most common steps usually include: partitioning into
macroblocks
, transformation (e.g., using a
DCT
or
wavelet
),
quantization
and
entropy encoding
.
Applications
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It is used in codecs like
ProRes
: a
group of pictures
codec without
inter frames
.
See also
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External links
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