commit 4e8acca92533d969892d7236d812899e45b9f904
author Wei-ting Lin <weitinglin@google.com> Mon Aug 22 11:19:30 2016 -0700
committer Wei-ting Lin <weitinglin@google.com> Mon Aug 22 17:00:41 2016 -0700
tree d2ecee33eddfcca280c132f688342269f059f017
parent 3464aff41f6bd8fb2316e4bb72a545451804e658

Allow LF_UPDATE type of frames to use BWDREF

Originally, only bi-pred type of frames can use BWDREF. When
extra alt-refs are inserted in a gf group, the closest alt-ref
serves as ALTREF for the frames within the corresponding
subgroup. Therefore, the original alt-ref can be used as BWDREF
for the LF_UPDATE type of frames.

This patch further swaps the virtual indices of BWDREF and ALTREF
for those frames whose BWDREF is farther than ALTREF. As a result,
the BWDREF is always the closet backward reference frame, and the
ALTREF is the farther one.

It improves the average RD performance by

0.132% in lowres, and
0.030% in midres.

The overall gains for the ext-refs compared to the baseline are

5.486% in lowres, and
4.666% in midres.

Change-Id: I22e4e5f378f19c4c89196a0a5e9214adb46c3428