Graphics Reference
In-Depth Information
Tabl e 7. 3
Size of the
elements of a 16
Values in line buffer
Required bits
16 CTU
needed to be kept in a line
buffer
Four motion vectors*
128 bits
4 reference picture indices*
16 bits
8
8/4
8 partitions flags*
2 bits
B-/P-prediction flags in 8
8 partitions* 2 bits
sign3 64 bits
Luma SAO type 2 bits
Chroma SAO type 2 bits
Starting band positions or EO classes 15 bits
Luma and chroma offsets 48 bits
Total elements 279 bits
64 luma samples 512 bits
32 chroma samples 256 bits
Total samples 768 bits
Total bits for CTU 1047 bits
*This information is also used in motion vector derivation
the motion vectors adjacent to the lower CTU boundary, and SAO needs SAO type
and SAO offsets of the upper CTB row. For an 8-bit 4:2:0 video and the smallest
CTU size (16
16 luma and 8
8 chroma samples), the information stored per
CTU is 279 bits (this number may depend on the implementation). Among these
279 bits, 128 bits are for four motion vectors (4
8and8
4 partitions can only use
one motion vector), 16 bits are for four indices of reference pictures in the decoded
picture buffer, two bits are for signaling whether 4
8or8
8 partitions are used,
and two bits are for signaling if one or two MVs are used in 8
8 partitions. Please
note that this information is also needed for other modules, such as motion vector
derivation. Alternatively, 8 bits with four Bs values may be stored if the information
about motion vectors in the next CTU row is available. Then, 64 bits are for “
sign3
”
values of 16 luma and 16 chroma samples, two bits are for luma SAO type, two bits
are for chroma SAO type, 15 bits are from starting band positions or EO classes, and
48 bits are from luma and chroma offsets. The sample line buffers for the 16
16
CTU keep 64 luma samples and 32 chroma samples, which requires 768 bits (see
Tab le
7.3
for details). Therefore, the total line buffer size is about 15K bytes for full
HD (i.e. 1920
1080) videos.
The line buffer size would be somewhat smaller when larger CTU sizes are used
since the same parameters apply to a larger number of samples. The size of the line
buffer can be further reduced by using vertical tiles, hence splitting the horizontal
“span” of CTUs to be processed before the bottom CTU is encoded/decoded.
7.4.4
Error Resilience
In order to provide additional error resilience, HEVC allows an encoder to
disable in-loop filters over tile and slice boundaries. A flag
slice_loop_
