Limit key frame quantizer for forced key frames.

Where a key frame occurs because of a minimum interval
selected by the user, then these forced key frames ideally need
to be more closely matched in quality to the surrounding frame.

Change-Id: Ia55b1f047e77dc7fbd78379c45869554f25b3df7

vp8/encoder/firstpass.c

@@ -980,10 +980,10 @@ static int estimate_max_q(VP8_COMP *cpi, double section_err, int section_target_
     // Restriction on active max q for constrained quality mode.
     if ( (cpi->oxcf.end_usage == USAGE_CONSTRAINED_QUALITY) &&
          (Q < cpi->cq_target_quality) )
-         //(Q < cpi->oxcf.cq_target_quality) )
+         //(Q < cpi->oxcf.cq_level;) )
     {
         Q = cpi->cq_target_quality;
-        //Q = cpi->oxcf.cq_target_quality;
+        //Q = cpi->oxcf.cq_level;
     }
 
     // Adjust maxq_min_limit and maxq_max_limit limits based on
@@ -2286,6 +2286,9 @@ void vp8_find_next_key_frame(VP8_COMP *cpi, FIRSTPASS_STATS *this_frame)
 
     cpi->common.frame_type = KEY_FRAME;
 
+    // is this a forced key frame by interval
+    cpi->this_key_frame_forced = cpi->next_key_frame_forced;
+
     // Clear the alt ref active flag as this can never be active on a key frame
     cpi->source_alt_ref_active = FALSE;
 
@@ -2348,7 +2351,11 @@ void vp8_find_next_key_frame(VP8_COMP *cpi, FIRSTPASS_STATS *this_frame)
         kf_group_err /= 2.0;
         kf_group_intra_err /= 2.0;
         kf_group_coded_err /= 2.0;
+
+        cpi->next_key_frame_forced = TRUE;
     }
+    else
+        cpi->next_key_frame_forced = FALSE;
 
     // Special case for the last frame of the file
     if (cpi->stats_in >= cpi->stats_in_end)

vp8/encoder/onyx_if.c

@@ -2272,6 +2272,8 @@ VP8_PTR vp8_create_compressor(VP8_CONFIG *oxcf)
 
     cpi->frames_since_key = 8;        // Give a sensible default for the first frame.
     cpi->key_frame_frequency = cpi->oxcf.key_freq;
+    cpi->this_key_frame_forced = FALSE;
+    cpi->next_key_frame_forced = FALSE;
 
     cpi->source_alt_ref_pending = FALSE;
     cpi->source_alt_ref_active = FALSE;
@@ -3817,10 +3819,22 @@ static void encode_frame_to_data_rate
            // KEY FRAMES
            else
            {
-               if (cpi->gfu_boost > 600)
-                   cpi->active_best_quality = kf_low_motion_minq[Q];
-               else
-                   cpi->active_best_quality = kf_high_motion_minq[Q];
+                // Special case for key frames forced because we have reached
+                // the maximum key frame interval. Here force the Q to a range
+                // close to but just below the ambient Q to reduce the risk
+                // of popping
+                if ( cpi->this_key_frame_forced )
+                {
+                    cpi->active_worst_quality = cpi->avg_frame_qindex * 7/8;
+                    cpi->active_best_quality = cpi->avg_frame_qindex * 2/3;
+                }
+                else
+                {
+                   if (cpi->gfu_boost > 600)
+                       cpi->active_best_quality = kf_low_motion_minq[Q];
+                   else
+                       cpi->active_best_quality = kf_high_motion_minq[Q];
+                }
            }
         }
         else

vp8/encoder/onyx_int.h

@@ -317,7 +317,8 @@ typedef struct
 
     unsigned int frames_since_key;
     unsigned int key_frame_frequency;
-    unsigned int next_key;
+    unsigned int this_key_frame_forced;
+    unsigned int next_key_frame_forced;
 
     unsigned int mode_check_freq[MAX_MODES];
     unsigned int mode_test_hit_counts[MAX_MODES];