From 33ab199838791c9b3b7a29a342014ea4716fdd27 Mon Sep 17 00:00:00 2001
From: Jasper van de Gronde <jasper.vandegronde@gmail.com>
Date: Sat, 2 Apr 2011 17:31:36 +0200
Subject: Faster rounding for Gaussian blur

(bzr r10144)
---
 src/display/nr-filter-gaussian.cpp | 43 ++++++++++++++++++++++++++++++++------
 1 file changed, 37 insertions(+), 6 deletions(-)

diff --git a/src/display/nr-filter-gaussian.cpp b/src/display/nr-filter-gaussian.cpp
index 3f7bea35d..9b83c8647 100644
--- a/src/display/nr-filter-gaussian.cpp
+++ b/src/display/nr-filter-gaussian.cpp
@@ -78,15 +78,46 @@ template<typename T> static inline T clip(T const& v, T const& a, T const& b) {
 }
 
 template<typename Tt, typename Ts>
-static inline Tt round_cast(Ts const& v) {
+static inline Tt round_cast(Ts v) {
     static Ts const rndoffset(.5);
     return static_cast<Tt>(v+rndoffset);
 }
 
+template<>
+inline unsigned char round_cast(double v) {
+    // This (fast) rounding method is based on:
+    // http://stereopsis.com/sree/fpu2006.html
+#if G_BYTE_ORDER==G_LITTLE_ENDIAN
+    double const dmr = 6755399441055744.0;
+    v = v + dmr;
+    return ((unsigned char*)&v)[0];
+#elif G_BYTE_ORDER==G_BIG_ENDIAN
+    double const dmr = 6755399441055744.0;
+    v = v + dmr;
+    return ((unsigned char*)&v)[7];
+#else
+    static double const rndoffset(.5);
+    return static_cast<unsigned char>(v+rndoffset);
+#endif
+}
+
+template<typename Tt, typename Ts>
+static inline Tt clip_round_cast(Ts const v) {
+    Ts const minval = std::numeric_limits<Tt>::min();
+    Ts const maxval = std::numeric_limits<Tt>::max();
+    Tt const minval_rounded = std::numeric_limits<Tt>::min();
+    Ts const maxval_rounded = std::numeric_limits<Tt>::max();
+    if ( v < minval ) return minval_rounded;
+    if ( v > maxval ) return maxval_rounded;
+    return round_cast<Tt>(v);
+}
+
 template<typename Tt, typename Ts>
-static inline Tt clip_round_cast(Ts const& v, Tt const minval=std::numeric_limits<Tt>::min(), Tt const maxval=std::numeric_limits<Tt>::max()) {
-    if ( v < minval ) return minval;
-    if ( v > maxval ) return maxval;
+static inline Tt clip_round_cast_varmax(Ts const v, Ts const maxval, Tt const maxval_rounded) {
+    Ts const minval = std::numeric_limits<Tt>::min();
+    Tt const minval_rounded = std::numeric_limits<Tt>::min();
+    if ( v < minval ) return minval_rounded;
+    if ( v > maxval ) return maxval_rounded;
     return round_cast<Tt>(v);
 }
 
@@ -320,7 +351,7 @@ filter2D_IIR(PT *const dest, int const dstr1, int const dstr2,
         dstimg -= dstr1;
         if ( PREMULTIPLIED_ALPHA ) {
             dstimg[PC-1] = clip_round_cast<PT>(v[0][PC-1]);
-            for(unsigned int c=0; c<PC-1; c++) dstimg[c] = clip_round_cast<PT>(v[0][c], std::numeric_limits<PT>::min(), dstimg[PC-1]);
+            for(unsigned int c=0; c<PC-1; c++) dstimg[c] = clip_round_cast_varmax<PT>(v[0][c], v[0][PC-1], dstimg[PC-1]);
         } else {
             for(unsigned int c=0; c<PC; c++) dstimg[c] = clip_round_cast<PT>(v[0][c]);
         }
@@ -335,7 +366,7 @@ filter2D_IIR(PT *const dest, int const dstr1, int const dstr2,
             dstimg -= dstr1;
             if ( PREMULTIPLIED_ALPHA ) {
                 dstimg[PC-1] = clip_round_cast<PT>(v[0][PC-1]);
-                for(unsigned int c=0; c<PC-1; c++) dstimg[c] = clip_round_cast<PT>(v[0][c], std::numeric_limits<PT>::min(), dstimg[PC-1]);
+                for(unsigned int c=0; c<PC-1; c++) dstimg[c] = clip_round_cast_varmax<PT>(v[0][c], v[0][PC-1], dstimg[PC-1]);
             } else {
                 for(unsigned int c=0; c<PC; c++) dstimg[c] = clip_round_cast<PT>(v[0][c]);
             }
-- 
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