International organisation for standardisation organisation internationale de normalisation

5.17Intra prediction and intra mode coding

A BoG coordinated by K. Chono was asked to initially consider the contributions in this area. This was done in two phases as reported by the BoG.

5.17.1.1.1.1.1.1.1JCTVC-H0712 BoG Report on Intra mode coding cleanup and simplification [K. Chono]

It was suggested to use 35 modes overall for all PU sizes. (Decision: Adopt H0166 & H0342)

Alternatives for complexity reduction were considered: to reduce the number of modes or increase the number of MPMs, unify modes over different PU sizes, MPM sorting postponing, bypass coding (as much as possible, for remaining modes).

Both mode reduction and 3 MPM increase have the advantage of enabling fixed-length coding.

3 MPM solutions give a little bit of compression benefit (approximately 0.2-0.3%).

Assessment unveils that neither reduction of number of modes nor increasing the number of MPM provides a significant complexity reduction or simplification.

MPM sorting postponing provides a complexity reduction w.r.t. software implementation (not hardware) and was suggested by BoG (H0098 and H0428 are identical, H0175 is similar but on top of 3 MPM design).

Initial conclusions included the following:

• 
  In spirit, we want to have an approach that uses 3 MPM and fixed-length coding of remaining modes
  
• 
  Use H0075 as a basis, and look for best alternative way to encode the 3 MPMs, remaining mode numbering (such that the table is removed) adopt numbering Similar to H0407 and H0175 but 0 for DC, 1-33 directional, 34 planar and MPM sorting postponing from H0175, H0407, H0435, H0516 and H0563 – proponents report back
  
• 
  Either decide for one of the 3 MPM solutions and use ordering of remaining modes as said above.
  
• 
  or use 2 MPM + escape and ordering of modes planar=0, DC=1, 2-34 directional
  

The BoG reported its conclusions on luma intra mode coding as follows:

• 
  35 modes for all PU sizes
  
• 
  Sequential intra prediction mode numbering (0: Planar, 1: DC, 22-34: Angular)
  
• 
  Postponed MPM-sorting
  
• 
  3 MPM (JCTVC-H0326 (Scheme1) + JCTVC-H0475); "TU bypass" for mpm_idx and "FL bypass" for rem_intra_luma_pred_mode
  

The WD text was provided in "JCTVC-H0712-WDtext-final.doc".

Two solutions for some aspects were considered in BoG as candidates as described below in relation to H0721.

5.17.1.1.1.1.1.1.2JCTVC-H0721 Intra mode coding: Combination of H0075 and H0175 [E. Francois (Canon), C. Yeo (I2R)] [late 02-05]

This contribution summarized the combined proposal from JCTVC-H0075 and JCTVC-H0175 on intra mode coding. The solution is built on top of H0075 as follows:

• 
  3 MPM, one of which is the planar mode
  
• 
  1 context less than in HM5
  
• 
  0.2% bit rate reduction was reported.
  

An alternative was a combination H0407, H0435.

The 3 MPMs are the most probable directional mode and its two closest angular directions.

0.2% bit rate reduction was reported.

Both proposals were requested to be assessed equal in terms of complexity.

Adoption of one of them was determined to be beneficial by consensus of the group.

As it did not seem possible to find technical arguments for one of the sides, and the decision appears to be to some extent random anyway, the coding efficiency was regarded with higher precision (second digit behind the decimal point), which is slightly better for the combination H0407/H0435.

Decision: Adopt combination H0407/H0435 (JCTVC-H0407_3MPM.doc).

5.17.1.1.1.1.1.1.3JCTVC-H0723 Cross Verification of JCTVC-H0721 on Intra Mode Coding [G. Li (Santa Clara Univ.), L. Liu (Huawei)] [late 02-05]