Fix T86427 Exact solver does not apply target material.

I had done some experiments to see what Fast boolean did for material
mapping and thought it just used the same slot in the target as the
slot in the source. The truth is more complicated: if the target material
exists in any slot of the destination, we need to remap to whatever
slot has the matching material. I fixed Exact Boolean to do this.
Since the materials may be in the object, this means that BKE_mesh_boolean
had to get another argument, the remapping arrays.

I will note that the current behavior of Fast, and now Exact, is not ideal.
Ideally, if the source material does not exist in the target, a new material
slot should be created in the target and the source material copied there
(and incrementing the material's reference count). Maybe a future project,
but for now, I want the behavior of Exact to match that of Fast.

1 files changed, 37 insertions, 3 deletions

diff --git a/source/blender/modifiers/intern/MOD_boolean.c b/source/blender/modifiers/intern/MOD_boolean.c
index 6ffbf518dd1..cc42c89a60e 100644
--- a/source/blender/modifiers/intern/MOD_boolean.c
+++ b/source/blender/modifiers/intern/MOD_boolean.c
@@ -615,6 +615,23 @@ static Mesh *collection_boolean_exact(BooleanModifierData *bmd,
 }
 
 #ifdef WITH_GMP
+
+/* Get a mapping from material slot numbers in the src_ob to slot numbers in the dst_ob.
+ * If a material doesn't exist in the dst_ob, the mapping just goes to the same slot
+ * or to zero if there aren't enough slots in the destination.
+ * Caller must MEM_freeN the returned array. */
+static short *get_material_remap(Object *dest_ob, Object *src_ob)
+{
+  short *remap;
+  int n = dest_ob->totcol;
+  if (n <= 0) {
+    n = 1;
+  }
+  remap = MEM_mallocN(n * sizeof(short), __func__);
+  BKE_object_material_remap_calc(dest_ob, src_ob, remap);
+  return remap;
+}
+
 /* New method: bypass trip through BMesh. */
 static Mesh *exact_boolean_mesh(BooleanModifierData *bmd,
                                 const ModifierEvalContext *ctx,
@@ -622,25 +639,32 @@ static Mesh *exact_boolean_mesh(BooleanModifierData *bmd,
 {
   Mesh *result;
   Mesh *mesh_operand;
+  short *remap;
   Mesh **meshes = NULL;
   const float(**obmats)[4][4] = NULL;
+  short **material_remaps = NULL;
   BLI_array_declare(meshes);
   BLI_array_declare(obmats);
+  BLI_array_declare(material_remaps);
 
 #  ifdef DEBUG_TIME
   TIMEIT_START(boolean_bmesh);
 #  endif
 
+  if ((bmd->flag & eBooleanModifierFlag_Object) && bmd->object == NULL) {
+    return mesh;
+  }
+
   BLI_array_append(meshes, mesh);
   BLI_array_append(obmats, &ctx->object->obmat);
+  BLI_array_append(material_remaps, NULL);
   if (bmd->flag & eBooleanModifierFlag_Object) {
-    if (bmd->object == NULL) {
-      return mesh;
-    }
     mesh_operand = BKE_modifier_get_evaluated_mesh_from_evaluated_object(bmd->object, false);
     BKE_mesh_wrapper_ensure_mdata(mesh_operand);
     BLI_array_append(meshes, mesh_operand);
     BLI_array_append(obmats, &bmd->object->obmat);
+    remap = get_material_remap(ctx->object, bmd->object);
+    BLI_array_append(material_remaps, remap);
   }
   else if (bmd->flag & eBooleanModifierFlag_Collection) {
     Collection *collection = bmd->collection;
@@ -652,6 +676,8 @@ static Mesh *exact_boolean_mesh(BooleanModifierData *bmd,
           BKE_mesh_wrapper_ensure_mdata(collection_mesh);
           BLI_array_append(meshes, collection_mesh);
           BLI_array_append(obmats, &ob->obmat);
+          remap = get_material_remap(ctx->object, ob);
+          BLI_array_append(material_remaps, remap);
         }
       }
       FOREACH_COLLECTION_OBJECT_RECURSIVE_END;
@@ -662,6 +688,7 @@ static Mesh *exact_boolean_mesh(BooleanModifierData *bmd,
   const bool hole_tolerant = (bmd->flag & eBooleanModifierFlag_HoleTolerant) != 0;
   result = BKE_mesh_boolean((const Mesh **)meshes,
                             (const float(**)[4][4])obmats,
+                            (const short **)material_remaps,
                             BLI_array_len(meshes),
                             use_self,
                             hole_tolerant,
@@ -669,6 +696,13 @@ static Mesh *exact_boolean_mesh(BooleanModifierData *bmd,
 
   BLI_array_free(meshes);
   BLI_array_free(obmats);
+  for (int i = 0; i < BLI_array_len(material_remaps); i++) {
+    remap = material_remaps[i];
+    if (remap) {
+      MEM_freeN(remap);
+    }
+  }
+  BLI_array_free(material_remaps);
 
 #  ifdef DEBUG_TIME
   TIMEIT_END(boolean_bmesh);