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Prevent parameters that can cause invalid ARF groups.

Having a very low "lag_in_frames" value could cause the encoder to create
incorrect / corrupt ARF groups including displayed frames that update the
ARF buffer and false overlay frames that are coded at low rate but are not
actually overlays of a real ARF frame.

This is linked to a reported unit test "slow down" where the chosen parameters
(lag of 3 frames) gave rise to such "broken" ARF group(s).

See also BUG=webm:1454

Change-Id: If52d0236243ed5552537d1ea9ed3fed8c867232c

--- a/vp9/encoder/vp9_encoder.h

+++ b/vp9/encoder/vp9_encoder.h

@@ -867,9 +867,10 @@

 }

 #endif

+#define MIN_LOOKAHEAD_FOR_ARFS 4

 static INLINE int is_altref_enabled(const VP9_COMP *const cpi) {

   return !(cpi->oxcf.mode == REALTIME && cpi->oxcf.rc_mode == VPX_CBR) &&

-         cpi->oxcf.lag_in_frames > 0 &&

+         cpi->oxcf.lag_in_frames >= MIN_LOOKAHEAD_FOR_ARFS &&

          (cpi->oxcf.enable_auto_arf &&

           (!is_two_pass_svc(cpi) ||

            cpi->oxcf.ss_enable_auto_arf[cpi->svc.spatial_layer_id]));

--- a/vp9/vp9_cx_iface.c

+++ b/vp9/vp9_cx_iface.c

@@ -191,6 +191,13 @@

                 (MAX_LAG_BUFFERS - 1));

   }

+  // For formation of valid ARF groups lag_in _frames should be 0 or greater

+  // than the max_gf_interval + 2

+  if (cfg->g_lag_in_frames > 0 && extra_cfg->max_gf_interval > 0 &&

+      cfg->g_lag_in_frames < extra_cfg->max_gf_interval + 2) {

+    ERROR("Set lag in frames to 0 (low delay) or >= (max-gf-interval + 2)");

+  }

+

   if (cfg->rc_resize_allowed == 1) {

     RANGE_CHECK(cfg, rc_scaled_width, 0, cfg->g_w);

     RANGE_CHECK(cfg, rc_scaled_height, 0, cfg->g_h);