[gimp] Issue #3062 - Picking by hue using "Select by Color" goes awry ...

[Ell <ell src gnome org>]

commit 9886b69dacfd200bcd34e7e25f01f8c7c7983b97
Author: Ell <ell_se yahoo com>
Date:   Tue Mar 5 08:41:58 2019 -0500

    Issue #3062 - Picking by hue using "Select by Color" goes awry ...
    
    ... in GIMP 2.10.9 from git
    
    In gimppickable-contiguous-region's pixel_difference() function,
    which is used, among other things, by the select-by-color and
    fuzzy-select tools, when selecting by LCh/HSV hue, treat a pair of
    colors as inifinitely far apart if one of them has positive chroma/
    saturation, and the other has chroma/saturation that's very close
    to 0; conversely, treat a pair of colors as equal if both of them
    have chroma/sautation that's close to 0.
    
    As a result, when the seed color is saturated, gray pixels are
    never selected, while when the seed color is desaturated, all, and
    only, gray pixels are selected.

 app/core/gimppickable-contiguous-region.cc | 54 +++++++++++++++++++++++++++---
 1 file changed, 50 insertions(+), 4 deletions(-)
---
diff --git a/app/core/gimppickable-contiguous-region.cc b/app/core/gimppickable-contiguous-region.cc
index 817ba31e55..de44f25cc1 100644
--- a/app/core/gimppickable-contiguous-region.cc
+++ b/app/core/gimppickable-contiguous-region.cc
@@ -721,8 +721,31 @@ pixel_difference (const gfloat        *col1,
           break;
 
         case GIMP_SELECT_CRITERION_H:
-          max = fabs (col1[0] - col2[0]);
-          max = MIN (max, 1.0 - max);
+          if (col1[1] > EPSILON)
+            {
+              if (col2[1] > EPSILON)
+                {
+                  max = fabs (col1[0] - col2[0]);
+                  max = MIN (max, 1.0 - max);
+                }
+              else
+                {
+                  /* "infinite" difference.  anything >> 1 will do. */
+                  max = 10.0;
+                }
+            }
+          else
+            {
+              if (col2[1] > EPSILON)
+                {
+                  /* "infinite" difference.  anything >> 1 will do. */
+                  max = 10.0;
+                }
+              else
+                {
+                  max = 0.0;
+                }
+            }
           break;
 
         case GIMP_SELECT_CRITERION_S:
@@ -742,8 +765,31 @@ pixel_difference (const gfloat        *col1,
           break;
 
         case GIMP_SELECT_CRITERION_LCH_H:
-          max = fabs (col1[2] - col2[2]) / 360.0;
-          max = MIN (max, 1.0 - max);
+          if (col1[1] > 100.0 * EPSILON)
+            {
+              if (col2[1] > 100.0 * EPSILON)
+                {
+                  max = fabs (col1[2] - col2[2]) / 360.0;
+                  max = MIN (max, 1.0 - max);
+                }
+              else
+                {
+                  /* "infinite" difference.  anything >> 1 will do. */
+                  max = 10.0;
+                }
+            }
+          else
+            {
+              if (col2[1] > 100.0 * EPSILON)
+                {
+                  /* "infinite" difference.  anything >> 1 will do. */
+                  max = 10.0;
+                }
+              else
+                {
+                  max = 0.0;
+                }
+            }
           break;
         }
     }