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### Section 12: Intraframe Prediction {#h-12-00}
Intraframe prediction uses already-coded macroblocks within the
current frame to approximate the contents of the current macroblock.
It applies to intra-coded macroblocks in an interframe and to all
macroblocks in a key frame.
Relative to the current macroblock "M", the already-coded macroblocks
include all macroblocks above M together with the macroblocks on the
same row as, and to the left of, M, though at most four of these
macroblocks are actually used: the block "A" directly above M, the
blocks immediately to the left and right of A, and the block
immediately to the left of M.
Each of the prediction modes (i.e., means of extrapolation from
already-calculated values) uses fairly simple arithmetic on pixel
values whose positions, relative to the current position, are defined
by the mode.
The chroma (U and V) and luma (Y) predictions are independent of each
The relative addressing of pixels applied to macroblocks on the upper
row or left column of the frame will sometimes cause pixels outside
the visible frame to be referenced. Usually such out-of-bounds
pixels have an assumed value of `129` for pixels to the left of the
leftmost column of the visible frame and `127` for pixels above the
top row of the visible frame (including the special case of the pixel
above and to the left of the top-left pixel in the visible frame).
Exceptions to this (associated to certain modes) will be noted below.
The already-coded macroblocks referenced by intra-prediction have
been "reconstructed", that is, have been predicted and residue-
adjusted (as described in Section 14), but have not been loop-
filtered. While it does process the edges between individual
macroblocks and individual subblocks, loop filtering (described in
Section 15) is applied to the frame as a whole, after all of the
macroblocks have been reconstructed.