[gimp] app: improve num. stability of gimp_gradient_segment_range_compress()

[N/A <ell src gnome org>]

commit 1b2a395e889258d844df8bc2efe15067e801ff6b
Author: Ell <ell_se yahoo com>
Date:   Tue Aug 1 13:39:01 2017 -0400

    app: improve num. stability of gimp_gradient_segment_range_compress()
    
    When using gimp_gradient_segment_range_compress() to expand a 0-
    length segment, redistribute the range's endpoints and midpoints
    uniformly, rather than using the regular code path, which would
    result in NaN values.
    
    Make sure that the left and right endpoints of the range are
    *exactly* equal to the new left and right values.  Previously,
    they could be slightly off due to numerical errors.

 app/core/gimpgradient.c |   55 +++++++++++++++++++++++++++++++++++-----------
 1 files changed, 42 insertions(+), 13 deletions(-)
---
diff --git a/app/core/gimpgradient.c b/app/core/gimpgradient.c
index 520ea72..b228c52 100644
--- a/app/core/gimpgradient.c
+++ b/app/core/gimpgradient.c
@@ -1182,7 +1182,6 @@ gimp_gradient_segment_range_compress (GimpGradient        *gradient,
                                       gdouble              new_r)
 {
   gdouble              orig_l, orig_r;
-  gdouble              scale;
   GimpGradientSegment *seg, *aseg;
 
   g_return_if_fail (GIMP_IS_GRADIENT (gradient));
@@ -1196,24 +1195,54 @@ gimp_gradient_segment_range_compress (GimpGradient        *gradient,
   orig_l = range_l->left;
   orig_r = range_r->right;
 
-  scale = (new_r - new_l) / (orig_r - orig_l);
+  if (orig_r - orig_l > EPSILON)
+    {
+      gdouble scale;
 
-  seg = range_l;
+      scale = (new_r - new_l) / (orig_r - orig_l);
 
-  do
+      seg = range_l;
+
+      do
+        {
+          if (seg->prev)
+            seg->left  = new_l + (seg->left - orig_l) * scale;
+          seg->middle  = new_l + (seg->middle - orig_l) * scale;
+          if (seg->next)
+            seg->right = new_l + (seg->right - orig_l) * scale;
+
+          /* Next */
+
+          aseg = seg;
+          seg  = seg->next;
+        }
+      while (aseg != range_r);
+    }
+  else
     {
-      if (seg->prev)
-        seg->left   = new_l + (seg->left - orig_l) * scale;
-      seg->middle = new_l + (seg->middle - orig_l) * scale;
-      if (seg->next)
-        seg->right  = new_l + (seg->right - orig_l) * scale;
+      gint n;
+      gint i;
 
-      /* Next */
+      n = gimp_gradient_segment_range_get_n_segments (gradient,
+                                                      range_l, range_r);
 
-      aseg = seg;
-      seg  = seg->next;
+      for (i = 0, seg = range_l; i < n; i++, seg = seg->next)
+        {
+          if (seg->prev)
+            seg->left  = new_l + (new_r - new_l) * (i + 0.0) / n;
+          seg->middle  = new_l + (new_r - new_l) * (i + 0.5) / n;;
+          if (seg->next)
+            seg->right = new_l + (new_r - new_l) * (i + 1.0) / n;
+        }
     }
-  while (aseg != range_r);
+
+  /* Make sure that the left and right endpoints of the range are *exactly*
+   * equal to new_l and new_r; the above computations can introduce
+   * numerical inaccuracies.
+   */
+
+  range_l->left  = new_l;
+  range_l->right = new_r;
 
   gimp_data_thaw (GIMP_DATA (gradient));
 }