This commit extends the operating range of the forward and inverse
quantization tables in order to reduce distortion at low bit rates.

Danillo Bracco Graziosi authored Jul 31, 2019 and David Flynn committed Aug 12, 2019

This commit exploits the use of fixed quantisation step sizes (due to
QP) to perform quantisation using multiplication by the reciprocal step
size.

Maintains contexts for:
 64->6:    0n, ud, 1n, 2nh, 2nv, 3n
 64->6->5: 0n, ud+1n, 2nh, 2nv, 3n
 64->6->3: 0n, ud+1n+2nh, 2nv+3n

Where ud = up/down,
      1n = single neighbour
      2nh = two horizontal neighbours
      2nv = two vertical neighbours
      3n = three neighbours
      0n = no neighbours

Noritaka Iguchi authored Mar 27, 2019 and David Flynn committed Apr 16, 2019

This commit introduces control of the quantisation step size using
the familiar HEVC | AVC quantisation parameter.

This proposal reduces the number of neighbour configurations used
in occupancy contextualisation from {10, 10, 10, 10, 10, 10, 7, 5}
to {9, 9, 9, 9, 5, 5, 3, 2} by replacing the 64-to-10, 10-to-7,
and 10-to-5 mapping tables with 64-to-9, 9-to-5, and 9-to-3 tables.

This commit updates the geometry octree node occupancy map scan order
to permit better screening of b6 from the right.

This commit fixes a naming error where kMortonCode256X is used for
the z co-ordinate, and similarly for kMortonCode256Z.

Replaces the existing map update tables (generated from (L*p+s)/(L+1),
memory L=10) with an adaptive memory (L=max(5, 1/p, 1/(1-p)) clipped
to 200) represented as delta transitions and quantised to 16 entries.

chi wang authored Oct 09, 2018 and David Flynn committed Nov 01, 2018

When restricted neighbour contextualisation is used, this modification
introduces an alternative neighbour reduction table.

NB: the integrated table has been updated to remove the need to add
additional remapping tables.  This is a non-normative change.

Bitwise geometry occupancy coding uses a mapping table to select entropy
contexts.  This commit employs a dynamic mapping which is updated after
each coding operation, replacing the previous static mapping tables.

NB: the proposed version used a context with a halving period
(max_count) of 64 symbols.  However, this conflicts with another
adoption (512 symbols), and a wholesale replacement of the arithmetic
codec and context model.  To resolve the conflict, the existing halving
period (128) is used.

Khaled Mammou authored Sep 19, 2018 and David Flynn committed Nov 01, 2018

This commit allows bytewise encoding of occupancy symbols using the
dual-lut coder.

This commit provides an implementation of m42522 to code occupancy
using a binary context model rather than a 255-ary model.  A total of
581 binary contexts are used compared to the previous 10 255-ary
contexts.

This commit provides an implementation of m42238 to code occupancy:

 - a node's child occupancy is contextualised based on the adjacent
   neighbours of the parent node.

 - when an isolated node is occupied by only a single child, the
   sub-node address [0,7] is directly coded instead of the 8-bit
   occupancy word.

 - collection of neighbour occupancy is performed using a scatter/
   gather operation upon the insertion of each child node into the
   fifo queue.

 - the feature may be controlled via the command line / config file
   using the --neighbourContextualisation=1|0 option