MOD_curve: do not compute mesh when not needed.

This modifier only uses mesh to get vgroup, which is only needed in case
modified object is indeed a mesh! Building a mesh from curve here is not
only useless and time-consuming, it will also easily fail the assert
about same number of vertices!

Also, use MOD_get_vgroup() helper in modifier code itself and pass
reluting MDeformVert & index to BKE_curve's curve_deform_verts(),
this is simpler and avoids duplicating vgroup handling code.

Also fixes crash when used on lattice.

Related to T57972.

3 files changed, 16 insertions, 31 deletions

diff --git a/source/blender/blenkernel/BKE_lattice.h b/source/blender/blenkernel/BKE_lattice.h
index 2e093ba11a2..b2911129525 100644
--- a/source/blender/blenkernel/BKE_lattice.h
+++ b/source/blender/blenkernel/BKE_lattice.h
@@ -63,9 +63,8 @@ bool object_deform_mball(struct Object *ob, struct ListBase *dispbase);
 void outside_lattice(struct Lattice *lt);
 
 void curve_deform_verts(
-        struct Object *cuOb, struct Object *target,
-        struct Mesh *mesh, float (*vertexCos)[3],
-        int numVerts, const char *vgroup, short defaxis);
+        struct Object *cuOb, struct Object *target, float (*vertexCos)[3],
+        int numVerts, struct MDeformVert *dvert, const int defgrp_index, short defaxis);
 void curve_deform_vector(
         struct Object *cuOb, struct Object *target,
         float orco[3], float vec[3], float mat[3][3], int no_rot_axis);
diff --git a/source/blender/blenkernel/intern/lattice.c b/source/blender/blenkernel/intern/lattice.c
index a04f32adece..0fa675639ea 100644
--- a/source/blender/blenkernel/intern/lattice.c
+++ b/source/blender/blenkernel/intern/lattice.c
@@ -704,14 +704,12 @@ static bool calc_curve_deform(Object *par, float co[3],
 }
 
 void curve_deform_verts(
-        Object *cuOb, Object *target, Mesh *mesh, float (*vertexCos)[3],
-        int numVerts, const char *vgroup, short defaxis)
+        Object *cuOb, Object *target, float (*vertexCos)[3],
+        int numVerts, MDeformVert *dvert, const int defgrp_index, short defaxis)
 {
 	Curve *cu;
 	int a;
 	CurveDeform cd;
-	MDeformVert *dvert = NULL;
-	int defgrp_index = -1;
 	const bool is_neg_axis = (defaxis > 2);
 
 	if (cuOb->type != OB_CURVE)
@@ -732,26 +730,6 @@ void curve_deform_verts(
 		cd.dmax[0] = cd.dmax[1] = cd.dmax[2] =  0.0f;
 	}
 
-	/* Check whether to use vertex groups (only possible if target is a Mesh or Lattice).
-	 * We want either a Mesh/Lattice with no derived data, or derived data with deformverts.
-	 */
-	if (vgroup && vgroup[0] && ELEM(target->type, OB_MESH, OB_LATTICE)) {
-		defgrp_index = defgroup_name_index(target, vgroup);
-
-		if (defgrp_index != -1) {
-			/* if there's derived data without deformverts, don't use vgroups */
-			if (mesh) {
-				dvert = CustomData_get_layer(&mesh->vdata, CD_MDEFORMVERT);
-			}
-			else if (target->type == OB_LATTICE) {
-				dvert = ((Lattice *)target->data)->dvert;
-			}
-			else {
-				dvert = ((Mesh *)target->data)->dvert;
-			}
-		}
-	}
-
 	if (dvert) {
 		MDeformVert *dvert_iter;
 		float vec[3];
diff --git a/source/blender/modifiers/intern/MOD_curve.c b/source/blender/modifiers/intern/MOD_curve.c
index 73a541090e8..308cb79b74a 100644
--- a/source/blender/modifiers/intern/MOD_curve.c
+++ b/source/blender/modifiers/intern/MOD_curve.c
@@ -112,15 +112,23 @@ static void deformVerts(
         int numVerts)
 {
 	CurveModifierData *cmd = (CurveModifierData *) md;
-	Mesh *mesh_src = MOD_get_mesh_eval(ctx->object, NULL, mesh, NULL, false, false);
+	Mesh *mesh_src = NULL;
 
-	BLI_assert(mesh_src->totvert == numVerts);
+	if (ctx->object->type == OB_MESH) {
+		/* mesh_src is only needed for vgroups. */
+		mesh_src = MOD_get_mesh_eval(ctx->object, NULL, mesh, NULL, false, false);
+		BLI_assert(mesh_src->totvert == numVerts);
+	}
+
+	struct MDeformVert *dvert = NULL;
+	int defgrp_index = -1;
+	MOD_get_vgroup(ctx->object, mesh_src, cmd->name, &dvert, &defgrp_index);
 
 	/* silly that defaxis and curve_deform_verts are off by 1
 	 * but leave for now to save having to call do_versions */
-	curve_deform_verts(cmd->object, ctx->object, mesh_src, vertexCos, numVerts, cmd->name, cmd->defaxis - 1);
+	curve_deform_verts(cmd->object, ctx->object, vertexCos, numVerts, dvert, defgrp_index, cmd->defaxis - 1);
 
-	if (mesh_src != mesh) {
+	if (!ELEM(mesh_src, NULL, mesh)) {
 		BKE_id_free(NULL, mesh_src);
 	}
 }