From a014e9c9cd50c56948637be8b6f5222b2bfcbea7 Mon Sep 17 00:00:00 2001
From: skal <pascal.massimino@gmail.com>
Date: Wed, 30 Oct 2013 23:57:23 +0100
Subject: [PATCH] tune quantization biases toward higher precision

* raise U/V quantization bias to more neutral values
* also raise the non-zero AC bias for Y1/Y2 matrices

(we need all the precision we can for U/V leves, which are often empty)
This will increase quality in the higher range (q >= 90) mostly.
Files size is exacted to raise a little (5-7%). and SSIM accordingly of course.

Change-Id: I8a9ffdb6d8fb6dadb959e3fd392e66dc5aaed64e
---
 src/enc/quant.c | 19 ++++---------------
 1 file changed, 4 insertions(+), 15 deletions(-)

diff --git a/src/enc/quant.c b/src/enc/quant.c
index 9cc6ab1f..0cab861b 100644
--- a/src/enc/quant.c
+++ b/src/enc/quant.c
@@ -169,20 +169,8 @@ static const uint16_t kCoeffThresh[16] = {
   30, 30, 30, 30
 };
 
-// TODO(skal): tune more. Coeff thresholding?
-static const uint8_t kBiasMatrices[3][16] = {  // [3] = [luma-ac,luma-dc,chroma]
-  { 96, 96, 96, 96,
-    96, 96, 96, 96,
-    96, 96, 96, 96,
-    96, 96, 96, 96 },
-  { 96, 96, 96, 96,
-    96, 96, 96, 96,
-    96, 96, 96, 96,
-    96, 96, 96, 96 },
-  { 96, 96, 96, 96,
-    96, 96, 96, 96,
-    96, 96, 96, 96,
-    96, 96, 96, 96 }
+static const uint8_t kBiasMatrices[3][2] = {  // [luma-ac,luma-dc,chroma][dc,ac]
+  { 96, 110 }, { 96, 112 }, { 112, 120 }
 };
 
 // Sharpening by (slightly) raising the hi-frequency coeffs.
@@ -205,7 +193,8 @@ static int ExpandMatrix(VP8Matrix* const m, int type) {
     m->q_[i] = m->q_[1];
   }
   for (i = 0; i < 16; ++i) {
-    const int bias = kBiasMatrices[type][i];
+    const int is_ac_coeff = (i > 0);
+    const int bias = kBiasMatrices[type][is_ac_coeff];
     m->iq_[i] = (1 << QFIX) / m->q_[i];
     m->bias_[i] = BIAS(bias);
     // TODO(skal): tune kCoeffThresh[]