[gimp/gimp-2-10] app: Fix Mandala Symmetry Kaleidoscope mode - brush transform and stroke order

[Ell <ell src gnome org>]

commit 817e83bb27347f93a497d599519718f6568c80d2
Author: Elad Shahar <dawn ever gmail com>
Date:   Sun Jan 19 10:41:15 2020 +0200

    app: Fix Mandala Symmetry Kaleidoscope mode - brush transform and stroke order
    
    Changed the reflection method to find the middle of the slice the
    user is drawing on, and reflecting on that. This reflects the slice
    in place, making it easier to rotate it to the intended location.
    This fix preserves the order the dabs are drawn on the
    slices, to be counterclockwise.
    
    Fixed the brush transform, by setting the reflect output variable,
    and calculating accordingly.
    
    (cherry picked from commit ed6517f094425693df0c578199c3c012f25d6220)

 app/core/gimpsymmetry-mandala.c | 51 +++++++++++++++++++++++++++++++++++------
 1 file changed, 44 insertions(+), 7 deletions(-)
---
diff --git a/app/core/gimpsymmetry-mandala.c b/app/core/gimpsymmetry-mandala.c
index 6c575b02ce..94374ac66a 100644
--- a/app/core/gimpsymmetry-mandala.c
+++ b/app/core/gimpsymmetry-mandala.c
@@ -158,8 +158,8 @@ gimp_mandala_class_init (GimpMandalaClass *klass)
 
   GIMP_CONFIG_PROP_BOOLEAN (object_class, PROP_ENABLE_REFLECTION,
                             "enable-reflection",
-                            _("Enable reflection"),
-                            _("Enable reflection"),
+                            _("Kaleidoscope"),
+                            _("Reflect consecutive strokes"),
                             FALSE,
                             GIMP_PARAM_STATIC_STRINGS |
                             GIMP_SYMMETRY_PARAM_GUI);
@@ -457,6 +457,8 @@ gimp_mandala_update_strokes (GimpSymmetry *sym,
   GimpCoords  *coords;
   GimpMatrix3  matrix;
   gint         i;
+  gdouble slice_angle;
+  gdouble mid_slice_angle = 0.0;
 
   g_list_free_full (sym->strokes, g_free);
   sym->strokes = NULL;
@@ -464,6 +466,20 @@ gimp_mandala_update_strokes (GimpSymmetry *sym,
   coords = g_memdup (sym->origin, sizeof (GimpCoords));
   sym->strokes = g_list_prepend (sym->strokes, coords);
 
+  /* The angle of each slice, in radians. */
+  slice_angle = 2.0 * G_PI / mandala->size;
+
+  if (mandala->enable_reflection)
+    {
+      /* Find out in which slice the user is currently drawing. */
+      gdouble angle = atan2 (sym->origin->y - mandala->center_y,
+                             sym->origin->x - mandala->center_x);
+      gint slice_no = (int) floor(angle/slice_angle);
+
+      /* Angle where the middle of that slice is. */
+      mid_slice_angle = slice_no * slice_angle + slice_angle / 2.0;
+    }
+
   for (i = 1; i < mandala->size; i++)
     {
       gdouble new_x, new_y;
@@ -473,14 +489,16 @@ gimp_mandala_update_strokes (GimpSymmetry *sym,
       gimp_matrix3_translate (&matrix,
                               - mandala->center_x,
                               - mandala->center_y);
-      if ( mandala->enable_reflection && i % 2 == 1 )
+      if (mandala->enable_reflection && i % 2 == 1)
         {
-          gimp_matrix3_scale (&matrix, -1, 1);
-          gimp_matrix3_rotate (&matrix, G_PI - (i+1) * 2.0 * G_PI / (gdouble) mandala->size);
+          /* Reflecting over the mid_slice_angle axis, reflects slice without changing position. */
+          gimp_matrix3_rotate(&matrix, -mid_slice_angle);
+          gimp_matrix3_scale (&matrix, 1, -1);
+          gimp_matrix3_rotate(&matrix, mid_slice_angle - i * slice_angle);
         }
       else
         {
-          gimp_matrix3_rotate (&matrix, - i * 2.0 * G_PI / (gdouble) mandala->size);
+          gimp_matrix3_rotate (&matrix, - i * slice_angle);
         }
       gimp_matrix3_translate (&matrix,
                               mandala->center_x,
@@ -507,11 +525,30 @@ gimp_mandala_get_transform (GimpSymmetry *sym,
                             gboolean     *reflect)
 {
   GimpMandala *mandala = GIMP_MANDALA (sym);
+  gdouble     slice_angle;
 
   if (mandala->disable_transformation)
     return;
 
-  *angle = 360.0 * stroke / mandala->size;
+  slice_angle = 360.0 / mandala->size;
+
+  if (mandala->enable_reflection && stroke % 2 == 1)
+    {
+      /* Find out in which slice the user is currently drawing. */
+      gdouble origin_angle = gimp_rad_to_deg (atan2 (sym->origin->y - mandala->center_y,
+                                                     sym->origin->x - mandala->center_x));
+      gint slice_no = (int) floor(origin_angle/slice_angle);
+
+      /* Angle where the middle of that slice is. */
+      gdouble mid_slice_angle = slice_no * slice_angle + slice_angle / 2.0;
+
+      *angle = 180.0 - (-2 * mid_slice_angle + stroke * slice_angle) ;
+      *reflect = TRUE;
+    }
+  else
+    {
+      *angle = stroke * slice_angle;
+    }
 }
 
 static void