From: cristy <urban-warrior@git.imagemagick.org>
Date: Wed, 17 Apr 2013 21:52:56 +0000 (+0000)
Subject: (no commit message)
X-Git-Tag: 7.0.1-0~3865
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=c29824a8a311026fa6d88675fdbb7fdf28a8b93d;p=imagemagick

---

diff --git a/MagickCore/pixel.c b/MagickCore/pixel.c
index af0952b09..77c76094d 100644
--- a/MagickCore/pixel.c
+++ b/MagickCore/pixel.c
@@ -288,12 +288,71 @@ MagickExport PixelInfo *ClonePixelInfo(const PixelInfo *pixel)
 %    o pixel: the pixel.
 %
 */
+
+static inline double DecodeGamma(const double x)
+{
+  div_t
+    quotient;
+
+  double
+    p,
+    term[9];
+
+  int
+    exponent;
+
+  static const double coefficient[] =  /* terms for x^(7/5), x=1.5 */
+  {
+    1.7917488588043277509,
+    0.82045614371976854984,
+    0.027694100686325412819,
+    -0.00094244335181762134018,
+    0.000064355540911469709545,
+    -5.7224404636060757485e-06,
+    5.8767669437311184313e-07,
+    -6.6139920053589721168e-08,
+    7.9323242696227458163e-09
+  };
+
+  static const double powers_of_two[] =  /* (2^x)^(7/5) */
+  {
+    1.0,
+    2.6390158215457883983,
+    6.9644045063689921093,
+    1.8379173679952558018e+01,
+    4.8502930128332728543e+01
+  };
+
+  /*
+    Compute x^2.4 == x*x^(7/5) == pow(x,2.4).
+  */
+  term[0]=1.0;
+  term[1]=4.0*frexp(x,&exponent)-3.0;
+  term[2]=2.0*term[1]*term[1]-term[0];
+  term[3]=2.0*term[1]*term[2]-term[1];
+  term[4]=2.0*term[1]*term[3]-term[2];
+  term[5]=2.0*term[1]*term[4]-term[3];
+  term[6]=2.0*term[1]*term[5]-term[4];
+  term[7]=2.0*term[1]*term[6]-term[5];
+  term[8]=2.0*term[1]*term[7]-term[6];
+  p=coefficient[0]*term[0]+coefficient[1]*term[1]+coefficient[2]*term[2]+
+    coefficient[3]*term[3]+coefficient[4]*term[4]+coefficient[5]*term[5]+
+    coefficient[6]*term[6]+coefficient[7]*term[7]+coefficient[8]*term[8];
+  quotient=div(exponent-1,5);
+  if (quotient.rem < 0)
+    {
+      quotient.quot-=1;
+      quotient.rem+=5;
+    }
+  return(x*ldexp(powers_of_two[quotient.rem]*p,7*quotient.quot));
+}
+
 MagickExport MagickRealType DecodePixelGamma(const MagickRealType pixel)
 {
   if (pixel <= (0.0404482362771076*QuantumRange))
     return(pixel/12.92f);
-  return((MagickRealType) (QuantumRange*pow((double) (QuantumScale*pixel+
-    0.055)/1.055,2.4)));
+  return((MagickRealType) (QuantumRange*DecodeGamma((double) (QuantumScale*
+    pixel+0.055)/1.055)));
 }
 
 /*
@@ -349,12 +408,78 @@ MagickExport PixelChannelMap *DestroyPixelChannelMap(
 %    o pixel: the pixel.
 %
 */
+
+static inline double EncodeGamma(const double x)
+{
+  div_t
+    quotient;
+
+  double
+    p,
+    term[9];
+
+  int
+    exponent;
+
+  static const double coefficient[] =  /* Chebychevi poly: x^(5/12), x=1.5 */
+  {
+    1.1758200232996901923,
+    0.16665763094889061230,
+    -0.0083154894939042125035,
+    0.00075187976780420279038,
+    -0.000083240178519391795367,
+    0.000010229209410070008679,
+    -1.3400466409860246e-06,
+    1.8333422241635376682e-07,
+    -2.5878596761348859722e-08
+  };
+
+  static const double powers_of_two[] =  /* (2^N)^(5/12) */
+  {
+    1.0,
+    1.3348398541700343678,
+    1.7817974362806785482,
+    2.3784142300054420538,
+    3.1748021039363991669,
+    4.2378523774371812394,
+    5.6568542494923805819,
+    7.5509945014535482244,
+    1.0079368399158985525e1,
+    1.3454342644059433809e1,
+    1.7959392772949968275e1,
+    2.3972913230026907883e1
+  };
+
+  /*
+    Compute x^(1/2.4) == x^(5/12) == pow(x,1.0/2.4).
+  */
+  term[0]=1.0;
+  term[1]=4.0*frexp(x,&exponent)-3.0;
+  term[2]=2.0*term[1]*term[1]-term[0];
+  term[3]=2.0*term[1]*term[2]-term[1];
+  term[4]=2.0*term[1]*term[3]-term[2];
+  term[5]=2.0*term[1]*term[4]-term[3];
+  term[6]=2.0*term[1]*term[5]-term[4];
+  term[7]=2.0*term[1]*term[6]-term[5];
+  term[8]=2.0*term[1]*term[7]-term[6];
+  p=coefficient[0]*term[0]+coefficient[1]*term[1]+coefficient[2]*term[2]+
+    coefficient[3]*term[3]+coefficient[4]*term[4]+coefficient[5]*term[5]+
+    coefficient[6]*term[6]+coefficient[7]*term[7]+coefficient[8]*term[8];
+  quotient=div(exponent-1,12);
+  if (quotient.rem < 0)
+    {
+      quotient.quot-=1;
+      quotient.rem+=12;
+    }
+  return(ldexp(powers_of_two[quotient.rem]*p,5*quotient.quot));
+}
+
 MagickExport MagickRealType EncodePixelGamma(const MagickRealType pixel)
 {
   if (pixel <= (0.0031306684425005883*QuantumRange))
     return(12.92f*pixel);
-  return((MagickRealType) QuantumRange*(1.055*pow((double) QuantumScale*pixel,
-    1.0/2.4)-0.055));
+  return((MagickRealType) QuantumRange*(1.055*EncodeGamma((double) QuantumScale*
+    pixel-0.055)));
 }
 
 /*