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#### 18.1 Bounds on, and Adjustment of, Motion Vectors {#h-18-01}
Since each motion vector is differentially encoded from a neighboring
block or macroblock and the only clamp is to ensure that the
referenced motion vector represents a valid location inside a
reference frame buffer, it is technically possible within the VP8
format for a block or macroblock to have arbitrarily large motion
vectors, up to the size of the input image plus the extended border
areas. For practical reasons, VP8 imposes a motion vector size range
limit of -4096 to 4095 full pixels, regardless of image size (VP8
defines 14 raw bits for width and height; 16383x16383 is the maximum
possible image size). Bitstream-compliant encoders and decoders
shall enforce this limit.
Because the motion vectors applied to the chroma subblocks have
1/8-pixel resolution, the synthetic pixel calculation, outlined in
Section 5 and detailed below, uses this resolution for the luma
subblocks as well. In accordance, the stored luma motion vectors are
all doubled, each component of each luma vector becoming an even
integer in the range -2046 to +2046, inclusive.
The vector applied to each chroma subblock is calculated by averaging
the vectors for the 4 luma subblocks occupying the same visible area
as the chroma subblock in the usual correspondence; that is, the
vector for U and V block 0 is the average of the vectors for the Y
subblocks { 0, 1, 4, 5}, chroma block 1 corresponds to Y blocks { 2,
3, 6, 7}, chroma block 2 to Y blocks { 8, 9, 12, 13}, and chroma
block 3 to Y blocks { 10, 11, 14, 15}.
In detail, each of the two components of the vectors for each of the
chroma subblocks is calculated from the corresponding luma vector
components as follows:
int avg( int c1, int c2, int c3, int c4)
int s = c1 + c2 + c3 + c4;
/* The shift divides by 8 (not 4) because chroma pixels
have twice the diameter of luma pixels. The handling
of negative motion vector components is slightly
cumbersome because, strictly speaking, right shifts
of negative numbers are not well-defined in C. */
return s >= 0 ? (s + 4) >> 3 : -( (-s + 4) >> 3);
Furthermore, if the version number in the frame tag specifies only
full-pel chroma motion vectors, then the fractional parts of both
components of the vector are truncated to zero, as illustrated in the
following pseudocode (assuming 3 bits of fraction for both luma and
chroma vectors):
x = x & (~7);
y = y & (~7);
Earlier in this document we described the `vp8_clamp_mv()` function
to limit "nearest" and "near" motion vector predictors inside
specified margins within the frame boundaries. Additional clamping
is performed for `NEWMV` macroblocks, for which the final motion
vector is clamped again after combining the "best" predictor and the
differential vector decoded from the stream.
However, the secondary clamping is not performed for `SPLITMV`
macroblocks, meaning that any subblock's motion vector within the
`SPLITMV` macroblock may point outside the clamping zone. These non-
clamped vectors are also used when determining the decoding tree
context for subsequent subblocks' modes in the `vp8_mvCont()` function.