[gimp] app: add handling of CIE Lab blend space to gradient

[Øyvind Kolås <ok src gnome org>]

commit 6d6da684f1cb4e07bce4883f6d37b1865d58d942
Author: Øyvind Kolås <pippin gimp org>
Date:   Sat Apr 14 18:11:57 2018 +0200

    app: add handling of CIE Lab blend space to gradient
    
    Using CIE Lab yields gradients that more closely resemble the perceptual
    gradients but without the gamma based blending problems of linear-RGB / CIE
    XYZ.

 app/core/gimpgradient.c |   38 ++++++++++++++++++++++++++++++--------
 1 files changed, 30 insertions(+), 8 deletions(-)
---
diff --git a/app/core/gimpgradient.c b/app/core/gimpgradient.c
index 4d36d4e..3a269b4 100644
--- a/app/core/gimpgradient.c
+++ b/app/core/gimpgradient.c
@@ -100,6 +100,8 @@ G_DEFINE_TYPE_WITH_CODE (GimpGradient, gimp_gradient, GIMP_TYPE_DATA,
 
 static const Babl *fish_srgb_to_linear_rgb = NULL;
 static const Babl *fish_linear_rgb_to_srgb = NULL;
+static const Babl *fish_srgb_to_cie_lab    = NULL;
+static const Babl *fish_cie_lab_a_to_srgb  = NULL;
 
 
 static void
@@ -128,6 +130,10 @@ gimp_gradient_class_init (GimpGradientClass *klass)
                                        babl_format ("RGB double"));
   fish_linear_rgb_to_srgb = babl_fish (babl_format ("RGBA double"),
                                        babl_format ("R'G'B'A double"));
+  fish_srgb_to_cie_lab    = babl_fish (babl_format ("R'G'B' double"),
+                                       babl_format ("CIE Lab double"));
+  fish_cie_lab_a_to_srgb  = babl_fish (babl_format ("CIE Lab alpha double"),
+                                       babl_format ("R'G'B'A double"));
 }
 
 static void
@@ -514,22 +520,38 @@ gimp_gradient_get_color_at (GimpGradient                *gradient,
 
   if (seg->color == GIMP_GRADIENT_SEGMENT_RGB)
     {
-      if (blend_color_space == GIMP_GRADIENT_BLEND_RGB_LINEAR)
+      switch (blend_color_space)
         {
-          babl_process (fish_srgb_to_linear_rgb,
-                        &left_color, &left_color, 1);
-          babl_process (fish_srgb_to_linear_rgb,
-                        &right_color, &right_color, 1);
+          case GIMP_GRADIENT_BLEND_CIE_LAB:
+            babl_process (fish_srgb_to_cie_lab,
+                          &left_color, &left_color, 1);
+            babl_process (fish_srgb_to_cie_lab,
+                          &right_color, &right_color, 1);
+            break;
+          case GIMP_GRADIENT_BLEND_RGB_LINEAR:
+            babl_process (fish_srgb_to_linear_rgb,
+                          &left_color, &left_color, 1);
+            babl_process (fish_srgb_to_linear_rgb,
+                          &right_color, &right_color, 1);
+          case GIMP_GRADIENT_BLEND_RGB_PERCEPTUAL:
+            break;
         }
 
       rgb.r = left_color.r + (right_color.r - left_color.r) * factor;
       rgb.g = left_color.g + (right_color.g - left_color.g) * factor;
       rgb.b = left_color.b + (right_color.b - left_color.b) * factor;
 
-      if (blend_color_space == GIMP_GRADIENT_BLEND_RGB_LINEAR)
+      switch (blend_color_space)
         {
-          babl_process (fish_linear_rgb_to_srgb,
-                        &rgb, &rgb, 1);
+          case GIMP_GRADIENT_BLEND_CIE_LAB:
+            babl_process (fish_cie_lab_a_to_srgb,
+                          &rgb, &rgb, 1);
+            break;
+          case GIMP_GRADIENT_BLEND_RGB_LINEAR:
+            babl_process (fish_linear_rgb_to_srgb,
+                          &rgb, &rgb, 1);
+          case GIMP_GRADIENT_BLEND_RGB_PERCEPTUAL:
+            break;
         }
     }
   else