Graphics Reference
In-Depth Information
Motion estimation for each inter PU partition is done based on the minimization
of inter prediction cost shown in (4.12).
mp
D
arg min
mp - M P
D
mp
C
pred
R
mp
(9.12)
For a given reference picture list, set
MP
, over which the minimization is
carried out, consists of all possible motion parameters, namely motion vectors
and associated reference indices. The minimization task in (
9.12
)isbrokeninto
two parts: integer-sample precision and sub-sample precision. For integer-sample
precision, distortion term
D
mp
corresponds to the SAD between original PU block
and its motion compensated reference block. For sub-sample motion search however
distortion term
D
mp
represents SATD of the block difference between the original
and sub-sample motion compensated reference block.
R
mp
term represents an
estimate of the number of coded bits required to transmit
mp
.
For bi-prediction, cost function minimization in (
9.12
) becomes a joint optimiza-
tion problem and is solved by the application of an iterative algorithm [
5
]. The
algorithm is initialized first with the two best motion parameters that are obtained
independently, for each reference list (L0 and L1). Iteration for further refinement
and combined cost minimization is performed by keeping motion parameter of L0
list constant while performing sub-pixel motion search on the complementary list
(L1). Once minimum cost is achieved, motion parameter associated with L1 list
is held constant and motion parameter of the L0 list is adjusted for computing
minimum combined cost. This “ping-pong” like iteration process is continued until
convergence is reached.
For intra PU prediction, a two-stage minimization process is performed:
At the first stage, a fix number
1
of candidate intra prediction modes with lowest
prediction cost are chosen according to the minimization of the prediction cost
function in (
9.13
).
p
D
arg min
p-P
D
p
C
pred
R
p
(9.13)
Distortion term
D
p
in (
9.13
) represents the SATD between the original block
and its prediction block using intra prediction mode
p
and
R
p
represents number of
coded bits required for signaling mode
p
.Set
P
, over which minimization is carried
out, consists of planar, DC and all the 33 angular prediction directions.
In the second stage, the list containing the candidate intra prediction modes from
the first stage is augmented with the three most probable modes if not already
present in the list. The best intra prediction mode is the one that gives the minimum
J
mode
among candidate intra prediction modes in this augmented list.
Note that HEVC allows PCM coding of a CU block if the block size is greater
or equal to a signaled minimum PCM coding block size. For PCM
J
mode
evaluation,
1
These fix values are pre-determined and they depend on PU size.
