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#### 12.2 Chroma Prediction {#h-12-02}
The chroma prediction is a little simpler than the luma prediction,
so we treat it first. Each of the chroma modes treats U and V
identically; that is, the U and V prediction values are calculated in
parallel, using the same relative addressing and arithmetic in each
of the two planes.
The modes extrapolate prediction values using the 8-pixel row "A"
lying immediately above the block (that is, the bottom chroma row of
the macroblock immediately above the current macroblock) and the
8-pixel column "L" immediately to the left of the block (that is, the
rightmost chroma column of the macroblock immediately to the left of
the current macroblock).
Vertical prediction (chroma mode `V_PRED`) simply fills each 8-pixel
row of the 8x8 chroma block with a copy of the "above" row (A). If
the current macroblock lies on the top row of the frame, all 8 of the
pixel values in A are assigned the value `127`.
Similarly, horizontal prediction (`H_PRED`) fills each 8-pixel column
of the 8x8 chroma block with a copy of the "left" column (L). If the
current macroblock is in the left column of the frame, all 8 pixel
values in `L` are assigned the value `129`.
DC prediction (`DC_PRED`) fills the 8x8 chroma block with a single
value. In the generic case of a macroblock lying below the top row
and right of the leftmost column of the frame, this value is the
average of the 16 (genuinely visible) pixels in the (union of the)
above row `A` and left column `L`.
Otherwise, if the current macroblock lies on the top row of the
frame, the average of the 8 pixels in `L` is used; if it lies in the
left column of the frame, the average of the 8 pixels in `A` is used.
Note that the averages used in these exceptional cases are not the
same as those that would be arrived at by using the out-of-bounds `A`
and `L` values defined for `V_PRED` and `H_PRED`. In the case of the
leftmost macroblock on the top row of the frame, the 8x8 block is
simply filled with the constant value `128`.
For `DC_PRED`, apart from the exceptional case of the top-left
macroblock, we are averaging either 16 or 8 pixel values to get a
single prediction value that fills the 8x8 block. The rounding is
done as follows:
int sum; /* sum of 8 or 16 pixels at (at least) 16-bit precision */
int shf; /* base 2 logarithm of the number of pixels (3 or 4) */
Pixel DCvalue = (sum + (1 << (shf-1))) >> shf;
Because the summands are all valid pixels, no "clamp" is necessary in
the calculation of `DCvalue`.
The remaining "True Motion" (`TM_PRED`) chroma mode gets its name from
an older technique of video compression used by On2 Technologies, to
which it bears some relation. In addition to the row "A" and column
"L", `TM_PRED` uses the pixel "P" above and to the left of the chroma
The following figure gives an example of how `TM_PRED` works:
| P | A0 | A1 | A2 | A3 | A4 | A5 | A6 | A7 |
| L0 | X00 | X01 | X02 | X03 | X04 | X05 | X06 | X07 |
| L1 | X10 | X11 | X12 | X13 | X14 | X15 | X16 | X17 |
| L2 | X20 | X21 | X22 | X23 | X24 | X25 | X26 | X27 |
| L3 | X30 | X31 | X32 | X33 | X34 | X35 | X36 | X37 |
| L4 | X40 | X41 | X42 | X43 | X44 | X45 | X46 | X47 |
| L5 | X50 | X51 | X52 | X53 | X54 | X55 | X56 | X57 |
| L6 | X60 | X61 | X62 | X63 | X64 | X65 | X66 | X67 |
| L7 | X70 | X71 | X72 | X73 | X74 | X75 | X76 | X77 |
Where P, As, and Ls represent reconstructed pixel values from
previously coded blocks, and X00 through X77 represent predicted
values for the current block. `TM_PRED` uses the following equation
to calculate X_ij:
X_ij = L_i + A_j - P (i, j=0, 1, 2, 3)
The exact algorithm is as follows:
void TMpred(
Pixel b[8][8], /* chroma (U or V) prediction block */
const Pixel A[8], /* row of already-constructed pixels
above block */
const Pixel L[8], /* column of "" just to the left of
block */
const Pixel P /* pixel just to the left of A and
above L*/
) {
int r = 0; /* row */
do {
int c = 0; /* column */
do {
b[r][c] = clamp255( L[r]+ A[c] - P);
} while( ++c < 8);
} while( ++r < 8);
Note that the process could equivalently be described as propagating
the vertical differences between pixels in L (starting from P), using
the pixels from A to start each column.
An implementation of chroma intra-prediction may be found in the
reference decoder file `predict.c` (Section 20.14).
Unlike `DC_PRED`, for macroblocks on the top row or left edge, `TM_PRED`
does use the out-of-bounds values of `127` and `129` (respectively)
defined for `V_PRED` and `H_PRED`.