tune=ssimulacra2 efficiency tweaks

- Adjusted QP -> QM level formula. Small efficiency gains in
  SSIMULACRA 2 scores (~0.2%) for several QP regions (e.g.
  QP 25, QP 40-50).
- Further increased chroma delta q boost for 4:2:0 chroma
  subsampling mode. Similar average SSIMULACRA 2 scores, slightly
  better 10th percentile scores.

Daala subset1 was used to benchmark improvements.

Bug: aomedia:401234702
Change-Id: I93b481abc1068e66fe4cdbebe05f7cad1c4c5fa7

av1/common/quant_common.h

diff --git a/av1/common/quant_common.h b/av1/common/quant_common.h
index 965a746..6dc82c0 100644
--- a/av1/common/quant_common.h
+++ b/av1/common/quant_common.h
@@ -116,17 +116,17 @@
     qm_level = 10;
   } else if (qindex <= 60) {
     qm_level = 9;
-  } else if (qindex <= 100) {
+  } else if (qindex <= 90) {
     qm_level = 8;
   } else if (qindex <= 120) {
     qm_level = 7;
-  } else if (qindex <= 140) {
+  } else if (qindex <= 130) {
     qm_level = 6;
-  } else if (qindex <= 160) {
+  } else if (qindex <= 140) {
     qm_level = 5;
-  } else if (qindex <= 200) {
+  } else if (qindex <= 160) {
     qm_level = 4;
-  } else if (qindex <= 220) {
+  } else if (qindex <= 200) {
     qm_level = 3;
   } else {
     qm_level = 2;

av1/encoder/av1_quantize.c

diff --git a/av1/encoder/av1_quantize.c b/av1/encoder/av1_quantize.c
index 1673d53..c0a1227 100644
--- a/av1/encoder/av1_quantize.c
+++ b/av1/encoder/av1_quantize.c
@@ -902,7 +902,13 @@
         // The ramp-down of chroma increase was determined by generating the
         // convex hull of SSIMULACRA 2 scores (for all boosts from 0-16), and
         // finding a linear equation that fits the convex hull.
-        chroma_dc_delta_q = -clamp((quant_params->base_qindex / 2) - 14, 0, 16);
+        // For the SSIMULACRA 2 tune only: a convex hull of SSIMULACRA 2 scores
+        // (mean, 10th percentile) was generated by decreasing chroma delta_q
+        // by 3 different values (16, 20 and 24). 20 was selected as it had the
+        // best 10th percentile performance.
+        int offset = (tuning == AOM_TUNE_SSIMULACRA2) ? 20 : 16;
+        chroma_dc_delta_q =
+            -clamp((quant_params->base_qindex / 2) - 14, 0, offset);
         chroma_ac_delta_q = chroma_dc_delta_q;
       } else if (cm->seq_params->subsampling_x == 1 &&
                  cm->seq_params->subsampling_y == 0) {