[x265] [PATCH] Disable QP limitations for lowpass subband dct

[mont3z.claro5 at gmail.com]

# HG changeset patch
# User hribeiro
# Date 1510110049 28800
#      Tue Nov 07 19:00:49 2017 -0800
# Node ID e7e951834b57633ae474f50e7878b24271afabd2
# Parent  b6c692f5f5b145eb8d874cfb44db8c85fc2978cc
Disable QP limitations for lowpass subband dct

diff -r b6c692f5f5b1 -r e7e951834b57 doc/reST/cli.rst
--- a/doc/reST/cli.rst	Tue Nov 07 19:15:31 2017 +0530
+++ b/doc/reST/cli.rst	Tue Nov 07 19:00:49 2017 -0800
@@ -2165,10 +2165,10 @@
 
 .. option:: --lowpass-dct
 
-    If enabled, x265 will use low-pass truncated dct approximation instead of the
-    standard dct. This approximation is less computational intesive but it generates
-    truncated coefficient matrixes for the transformed block. Empirical analysis shows
-    this approximation gives good PSNR results for QP>=23. 
+    If enabled, x265 will use low-pass subband dct approximation instead of the
+    standard dct for 16x16 and 32x32 blocks. This approximation is less computational 
+    intesive but it generates truncated coefficient matrixes for the transformed block. 
+    Empirical analysis shows marginal loss in compression and performance gains up to 10%.
 
     This approximation should be considered for platforms with performance and time 
     constrains.
diff -r b6c692f5f5b1 -r e7e951834b57 source/common/primitives.cpp
--- a/source/common/primitives.cpp	Tue Nov 07 19:15:31 2017 +0530
+++ b/source/common/primitives.cpp	Tue Nov 07 19:00:49 2017 -0800
@@ -272,7 +272,7 @@
 
         setupAliasPrimitives(primitives);
 
-        if (param->bLowPassDct && param->rc.qp > 20)
+        if (param->bLowPassDct)
         {
             enableLowpassDCTPrimitives(primitives); 
         }
diff -r b6c692f5f5b1 -r e7e951834b57 source/x265.h
--- a/source/x265.h	Tue Nov 07 19:15:31 2017 +0530
+++ b/source/x265.h	Tue Nov 07 19:00:49 2017 -0800
@@ -1510,9 +1510,8 @@
     /* Disable lookahead */
     int       bDisableLookahead;
 
-    /* Use low-pass truncated dct approximation 
-    *  This DCT approximation is less computational intensive and gives results close to 
-    *  standard DCT for QP >= 23 */
+    /* Use low-pass subband dct approximation 
+    *  This DCT approximation is less computational intensive and gives results close to standard DCT */
     int       bLowPassDct;
 
     /* Sets the portion of the decode buffer that must be available after all the
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