Force even arf group length where possible.

This patch tweaks the calculation of the active maximum GF interval
and also the break out clause for the GF interval loop. The changes
force the maximum and where possible the  break out value to be odd
which in turn will result in an even length ARF group if ARF coding is
selected (vs GF only coding).

The primary aim was to improve coding with multi layer arf groups.
For the single layer case there are small net gains in 3 out of 4 sets
(low,md, hd) and a small net drop for the NF2K set.

For multi-layer the gains (opsnr, ssim, psnr-hvs : -ve = better) were:-

Low res: -0.109, -0.038, -0.036
Mid res: -0.204, -0.171, -0.242
Hd res: -0.330, -0.471, -0.496
NF 2k: -0.165, -0.149, -0.157

Change-Id: I245f8561f5d1bd34312a0133c670c2154a0da23f

diff --git a/vp9/encoder/vp9_firstpass.c b/vp9/encoder/vp9_firstpass.c
index 0bf1a728bc01c727d4bde98b24c0657805eeb59b..58c3a435d9fc119b0501dd315641176fb64253ac 100644 (file)
--- a/vp9/encoder/vp9_firstpass.c
+++ b/vp9/encoder/vp9_firstpass.c
@@ -2513,7 +2513,10 @@ static void define_gf_group(VP9_COMP *cpi, FIRSTPASS_STATS *this_frame) {
     // bits to spare and are better with a smaller interval and smaller boost.
     // At high Q when there are few bits to spare we are better with a longer
     // interval to spread the cost of the GF.
-    active_max_gf_interval = 12 + arf_active_or_kf + VPXMIN(4, q_term);
+    active_max_gf_interval = 11 + arf_active_or_kf + VPXMIN(5, q_term);
+
+    // Force max GF interval to be odd.
+    active_max_gf_interval = active_max_gf_interval | 0x01;
 
     // We have: active_min_gf_interval <=
     // rc->max_gf_interval + arf_active_or_kf.
@@ -2613,7 +2616,7 @@ static void define_gf_group(VP9_COMP *cpi, FIRSTPASS_STATS *this_frame) {
             // Don't break out with a very short interval.
             (i >= active_min_gf_interval) &&
             // If possible dont break very close to a kf
-            ((rc->frames_to_key - i) >= rc->min_gf_interval) &&
+            ((rc->frames_to_key - i) >= rc->min_gf_interval) && (i & 0x01) &&
             (!flash_detected) &&
             ((mv_ratio_accumulator > mv_ratio_accumulator_thresh) ||
              (abs_mv_in_out_accumulator > abs_mv_in_out_thresh) ||