[babl] CIE: Add conversion from "RGBA float" to "CIE Lab float"

[Øyvind Kolås <ok src gnome org>]

commit ea00661dd8986b9a1380c9c500122dec8ca5a0db
Author: Debarshi Ray <debarshir gnome org>
Date:   Sun Oct 29 13:52:26 2017 +0100

    CIE: Add conversion from "RGBA float" to "CIE Lab float"
    
    Conversions from "RaGaBaA float" to "CIE Lab float", as seen when
    using gegl:shadows-highlights" go via:
      "RaGaBaA float" to "RGBA float"
      "RGBA float"    to "RGB float"
      "RGB float"     to "CIE Lab float"
    
    A direct conversion from "RaGaBaA float" to "CIE Lab float" in simple
    C is hindered by the need to check every pixel's alpha value to avoid
    dividing by zero. The pipeline stalls make it lose out to the look-up
    table and SIMD based conversions to unassociated alpha.
    
    However, we can trivially cut out the second step and still reduce
    some memory traffic.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=789695

 extensions/CIE.c |   40 ++++++++++++++++++++++++++++++++++++++++
 1 files changed, 40 insertions(+), 0 deletions(-)
---
diff --git a/extensions/CIE.c b/extensions/CIE.c
index d16d862..a010f0b 100644
--- a/extensions/CIE.c
+++ b/extensions/CIE.c
@@ -647,6 +647,40 @@ rgbf_to_Labf (const Babl *conversion,float *src,
 }
 
 static void
+rgbaf_to_Labf (const Babl *conversion,float *src,
+               float *dst,
+               long   samples)
+{
+  long n = samples;
+
+  while (n--)
+    {
+      float r = src[0];
+      float g = src[1];
+      float b = src[2];
+
+      float xr = 0.43603516f / D50_WHITE_REF_X * r + 0.38511658f / D50_WHITE_REF_X * g + 0.14305115f / 
D50_WHITE_REF_X * b;
+      float yr = 0.22248840f / D50_WHITE_REF_Y * r + 0.71690369f / D50_WHITE_REF_Y * g + 0.06060791f / 
D50_WHITE_REF_Y * b;
+      float zr = 0.01391602f / D50_WHITE_REF_Z * r + 0.09706116f / D50_WHITE_REF_Z * g + 0.71392822f / 
D50_WHITE_REF_Z * b;
+
+      float fx = xr > LAB_EPSILON ? _cbrtf (xr) : (LAB_KAPPA * xr + 16.0f) / 116.0f;
+      float fy = yr > LAB_EPSILON ? _cbrtf (yr) : (LAB_KAPPA * yr + 16.0f) / 116.0f;
+      float fz = zr > LAB_EPSILON ? _cbrtf (zr) : (LAB_KAPPA * zr + 16.0f) / 116.0f;
+
+      float L = 116.0f * fy - 16.0f;
+      float A = 500.0f * (fx - fy);
+      float B = 200.0f * (fy - fz);
+
+      dst[0] = L;
+      dst[1] = A;
+      dst[2] = B;
+
+      src += 4;
+      dst += 3;
+    }
+}
+
+static void
 rgbaf_to_Labaf (const Babl *conversion,float *src,
                 float *dst,
                 long   samples)
@@ -905,6 +939,12 @@ conversions (void)
   );
   babl_conversion_new (
     babl_format ("RGBA float"),
+    babl_format ("CIE Lab float"),
+    "linear", rgbaf_to_Labf,
+    NULL
+  );
+  babl_conversion_new (
+    babl_format ("RGBA float"),
     babl_format ("CIE Lab alpha float"),
     "linear", rgbaf_to_Labaf,
     NULL