[gimp/gimp-2-10] Issue #3588 - Add Yu'v' (CIE 1976 UCS) to GIMP color picker

[Michael Natterer <mitch src gnome org>]

commit 07b3acd69b5a59dc8608f5b7fe812666a2ce987c
Author: Elle Stone <ellestone ninedegreesbelow com>
Date:   Sat Jun 29 12:16:55 2019 -0400

    Issue #3588 - Add Yu'v' (CIE 1976 UCS) to GIMP color picker
    
    Which is a linear transform of xyY that is more perceptually
    uniform, and so well-suited for eventually adding chromaticity
    diagrams to GIMP color tools. ACES documentation uses this color
    space instead of xyY for showing chromaticity diagrams. Moving
    forward I expect other venues also will start using Yu'v' as
    the advantages over xyY chromaticity diagrams are fairly obvious.
    
    (cherry picked from commit bb660c5821f591b7fc17bff3f0c32f4ded1a7b3f)

 app/core/core-enums.c        |  2 ++
 app/core/core-enums.h        |  3 ++-
 app/widgets/gimpcolorframe.c | 33 +++++++++++++++++++++++++++++++++
 3 files changed, 37 insertions(+), 1 deletion(-)
---
diff --git a/app/core/core-enums.c b/app/core/core-enums.c
index df40ea99a8..f8ecb14a29 100644
--- a/app/core/core-enums.c
+++ b/app/core/core-enums.c
@@ -142,6 +142,7 @@ gimp_color_pick_mode_get_type (void)
     { GIMP_COLOR_PICK_MODE_LAB, "GIMP_COLOR_PICK_MODE_LAB", "lab" },
     { GIMP_COLOR_PICK_MODE_CMYK, "GIMP_COLOR_PICK_MODE_CMYK", "cmyk" },
     { GIMP_COLOR_PICK_MODE_XYY, "GIMP_COLOR_PICK_MODE_XYY", "xyy" },
+    { GIMP_COLOR_PICK_MODE_YUV, "GIMP_COLOR_PICK_MODE_YUV", "yuv" },
     { 0, NULL, NULL }
   };
 
@@ -155,6 +156,7 @@ gimp_color_pick_mode_get_type (void)
     { GIMP_COLOR_PICK_MODE_LAB, NC_("color-pick-mode", "CIE LAB"), NULL },
     { GIMP_COLOR_PICK_MODE_CMYK, NC_("color-pick-mode", "CMYK"), NULL },
     { GIMP_COLOR_PICK_MODE_XYY, NC_("color-pick-mode", "CIE xyY"), NULL },
+    { GIMP_COLOR_PICK_MODE_YUV, NC_("color-pick-mode", "CIE Yu'v'"), NULL },
     { 0, NULL, NULL }
   };
 
diff --git a/app/core/core-enums.h b/app/core/core-enums.h
index a1a31ef4d9..8826145266 100644
--- a/app/core/core-enums.h
+++ b/app/core/core-enums.h
@@ -104,8 +104,9 @@ typedef enum  /*< pdb-skip >*/
   GIMP_COLOR_PICK_MODE_LAB,         /*< desc="CIE LAB"      >*/
   GIMP_COLOR_PICK_MODE_CMYK,        /*< desc="CMYK"         >*/
   GIMP_COLOR_PICK_MODE_XYY,         /*< desc="CIE xyY"      >*/
+  GIMP_COLOR_PICK_MODE_YUV,         /*< desc="CIE Yu'v'"      >*/
 
-  GIMP_COLOR_PICK_MODE_LAST = GIMP_COLOR_PICK_MODE_XYY /*< skip >*/
+  GIMP_COLOR_PICK_MODE_LAST = GIMP_COLOR_PICK_MODE_YUV /*< skip >*/
 } GimpColorPickMode;
 
 
diff --git a/app/widgets/gimpcolorframe.c b/app/widgets/gimpcolorframe.c
index e5338839e3..5b96c1dfc6 100644
--- a/app/widgets/gimpcolorframe.c
+++ b/app/widgets/gimpcolorframe.c
@@ -158,6 +158,7 @@ gimp_color_frame_init (GimpColorFrame *frame)
                                            GIMP_COLOR_PICK_MODE_LCH,
                                            GIMP_COLOR_PICK_MODE_LAB,
                                            GIMP_COLOR_PICK_MODE_XYY,
+                                           GIMP_COLOR_PICK_MODE_YUV,
                                            GIMP_COLOR_PICK_MODE_CMYK);
   frame->combo = gimp_enum_combo_box_new_with_model (GIMP_ENUM_STORE (store));
   g_object_unref (store);
@@ -971,6 +972,38 @@ gimp_color_frame_update (GimpColorFrame *frame)
         }
       break;
 
+    case GIMP_COLOR_PICK_MODE_YUV:
+      /* TRANSLATORS: Y from Yu'v' color space */
+      names[0] = C_("Yu'v' color space", "Y:");
+      /* TRANSLATORS: u' from Yu'v' color space */
+      names[1] = C_("Yu'v' color space", "u':");
+      /* TRANSLATORS: v' from Yu'v' color space */
+      names[2] = C_("Yu'v' color space", "v':");
+
+      if (has_alpha)
+        /* TRANSLATORS: A for Alpha (color transparency) */
+        names[3] = C_("Alpha channel", "A:");
+
+      if (frame->sample_valid)
+        {
+          static const Babl *fish = NULL;
+          gfloat             Yuv[4];
+
+          if (G_UNLIKELY (! fish))
+            fish = babl_fish (babl_format ("R'G'B'A double"),
+                              babl_format ("CIE Yuv alpha float"));
+
+          babl_process (fish, &frame->color, Yuv, 1);
+
+          values = g_new0 (gchar *, 5);
+
+          values[0] = g_strdup_printf ("%1.6f  ",  Yuv[0]);
+          values[1] = g_strdup_printf ("%1.6f  ",  Yuv[1]);
+          values[2] = g_strdup_printf ("%1.6f  ",  Yuv[2]);
+          values[3] = g_strdup_printf ("%.01f %%", Yuv[3] * 100.0);
+        }
+      break;
+
     case GIMP_COLOR_PICK_MODE_CMYK:
       /* TRANSLATORS: C for Cyan (CMYK) */
       names[0] = C_("CMYK", "C:");