Fix #136396: Metaball Cube can't be selected with solid shading

The metaball selection radius was inside the cube and could only
be selected with wire-frame shading.

Resolve by expanding the radius by the dimensions of the cube.

source/blender/blenkernel/BKE_mball.hh

@@ -4,6 +4,7 @@
 
 #pragma once
 
+#include "BLI_math_vector_types.hh"
 #include "BLI_span.hh"
 
 /** \file
@@ -82,6 +83,15 @@ void BKE_mball_translate(MetaBall *mb, const float offset[3]);
  */
 MetaElem *BKE_mball_element_add(MetaBall *mb, int type);
 
+/**
+ * Calculate & return the display radius & stiffness.
+ */
+blender::float2 BKE_mball_element_display_radius_calc_with_stiffness(const MetaElem *ml);
+/**
+ * Calculate & return the display radius.
+ */
+float BKE_mball_element_display_radius_calc(const MetaElem *ml);
+
 /* *** Select functions *** */
 
 int BKE_mball_select_count(const MetaBall *mb);

source/blender/blenkernel/intern/mball.cc

@@ -228,6 +228,35 @@ MetaElem *BKE_mball_element_add(MetaBall *mb, const int type)
   return ml;
 }
 
+blender::float2 BKE_mball_element_display_radius_calc_with_stiffness(const MetaElem *ml)
+{
+  blender::float2 radius_stiffness = {
+      /* Display radius. */
+      ml->rad,
+      /* Display stiffness. */
+      ml->rad * atanf(ml->s) * float(2.0 / blender::math::numbers::pi),
+  };
+
+  if (ml->type == MB_CUBE) {
+    /* Without this additional size, the cube can't be selected in solid mode.
+     * Use the minimum size so this doesn't become too large because of one large axis.
+     * See: #136396. */
+    const float offset = min_fff(ml->expx, ml->expy, ml->expz) * M_SQRT2;
+    radius_stiffness[0] += offset;
+    radius_stiffness[1] += offset;
+  }
+  return radius_stiffness;
+}
+float BKE_mball_element_display_radius_calc(const MetaElem *ml)
+{
+  float radius = ml->rad;
+  if (ml->type == MB_CUBE) {
+    const float offset = min_fff(ml->expx, ml->expy, ml->expz) * M_SQRT2;
+    radius += offset;
+  }
+  return radius;
+}
+
 bool BKE_mball_is_basis(const Object *ob)
 {
   /* Meta-Ball Basis Notes from Blender-2.5x

source/blender/draw/engines/overlay/overlay_metaball.hh

@@ -8,6 +8,8 @@
 
 #pragma once
 
+#include "BKE_mball.hh"
+
 #include "ED_mball.hh"
 
 #include "overlay_base.hh"
@@ -53,16 +55,16 @@ class Metaballs : Overlay {
     LISTBASE_FOREACH (MetaElem *, ml, mb.editelems) {
       const bool is_selected = (ml->flag & SELECT) != 0;
       const bool is_scale_radius = (ml->flag & MB_SCALE_RAD) != 0;
-      const float stiffness_radius = ml->rad * atanf(ml->s) * 2.0f / math::numbers::pi;
+      blender::float2 radius_stiffness = BKE_mball_element_display_radius_calc_with_stiffness(ml);
       const float3 position = float3(&ml->x);
 
       const select::ID radius_id = res.select_id(ob_ref, MBALLSEL_RADIUS | elem_num);
       color = (is_selected && is_scale_radius) ? col_radius_select : col_radius;
-      circle_buf_.append({ob->object_to_world(), position, ml->rad, color}, radius_id);
+      circle_buf_.append({ob->object_to_world(), position, radius_stiffness[0], color}, radius_id);
 
       const select::ID stiff_id = res.select_id(ob_ref, MBALLSEL_STIFF | elem_num);
       color = (is_selected && !is_scale_radius) ? col_stiffness_select : col_stiffness;
-      circle_buf_.append({ob->object_to_world(), position, stiffness_radius, color}, stiff_id);
+      circle_buf_.append({ob->object_to_world(), position, radius_stiffness[1], color}, stiff_id);
       elem_num += 1 << 16;
     }
   }
@@ -81,7 +83,8 @@ class Metaballs : Overlay {
     LISTBASE_FOREACH (MetaElem *, ml, &mb->elems) {
       const float3 position = float3(&ml->x);
       /* Draw radius only. */
-      circle_buf_.append({ob->object_to_world(), position, ml->rad, color}, select_id);
+      const float radius = BKE_mball_element_display_radius_calc(ml);
+      circle_buf_.append({ob->object_to_world(), position, radius, color}, select_id);
     }
   }