[gimp] libgimpwidgets: better algorithm for GimpScaleEntry default increments.

[Jehan <jehanp src gnome org>]

commit ad8b4178718bee79e042fa82f4e609b9f062100a
Author: Jehan <jehan girinstud io>
Date:   Fri Oct 30 17:33:53 2020 +0100

    libgimpwidgets: better algorithm for GimpScaleEntry default increments.
    
    For very small ranges, just dividing by 10 and 100 is not very good. You
    could end up with weird step values. It is often better to use 10^(-x)
    values just below your range.
    I.e for a 0.5 range, a step of 0.1 and page of 0.01 are probably fine
    (better than 0.05 and 0.005).
    
    Of course as usual these are default values only and setting custom
    increments is possible through available API.
    
    Also fixing a small bug in gimp_scale_entry_set_increments() added in
    commit 0f05825a29.

 libgimpwidgets/gimpscaleentry.c | 28 +++++++++++++++++++++++++---
 1 file changed, 25 insertions(+), 3 deletions(-)
---
diff --git a/libgimpwidgets/gimpscaleentry.c b/libgimpwidgets/gimpscaleentry.c
index d40f9d0fd2..12b4c2bb86 100644
--- a/libgimpwidgets/gimpscaleentry.c
+++ b/libgimpwidgets/gimpscaleentry.c
@@ -445,9 +445,31 @@ gimp_scale_entry_update_steps (GimpScaleEntry *entry)
 
   range = upper - lower;
 
-  if (range <= 1.0)
+  if (range > 0 && range <= 1.0)
     {
-      gimp_scale_entry_set_increments (entry, range / 10.0, range / 100.0);
+      gdouble places = 10.0;
+      gdouble step;
+      gdouble page;
+
+      /* Compute some acceptable step and page increments always in the
+       * format `10**-X` where X is the rounded precision.
+       * So for instance:
+       *  0.8 will have increments 0.01 and 0.1.
+       *  0.3 will have increments 0.001 and 0.01.
+       *  0.06 will also have increments 0.001 and 0.01.
+       */
+      while (range * places < 5.0)
+        places *= 10.0;
+
+
+      step = 0.1 / places;
+      page = 1.0 / places;
+
+      gimp_scale_entry_set_increments (entry, step, page);
+    }
+  else if (range <= 2.0)
+    {
+      gimp_scale_entry_set_increments (entry, 0.01, 0.1);
     }
   else if (range <= 40.0)
     {
@@ -917,7 +939,7 @@ gimp_scale_entry_set_increments (GimpScaleEntry *entry,
   gtk_adjustment_set_page_increment (gtk_range_get_adjustment (scale), page);
   gtk_adjustment_set_page_increment (gtk_spin_button_get_adjustment (spinbutton), page);
 
-  g_object_set (entry,
+  g_object_set (entry->priv->spinbutton,
                 "climb-rate", step,
                 NULL);
 }