From d788f5231e81ae2be363f877d04ab93ddc396fa6 Mon Sep 17 00:00:00 2001
From: Antonioya <blendergit@gmail.com>
Date: Wed, 12 Jun 2019 15:51:51 +0200
Subject: Fix T65741: Removing a GPencil Object's Material Slot deletes the
 strokes assigned to it

This was a design decision, but now we have decided to change it using the active material for the strokes using deleted material.

If the material slot is empty a new material is created to keep the strokes visible.
---
 source/blender/blenkernel/intern/gpencil.c  | 30 ++++++-----------------------
 source/blender/blenkernel/intern/material.c | 13 +++++++++----
 2 files changed, 15 insertions(+), 28 deletions(-)

(limited to 'source/blender/blenkernel/intern')

diff --git a/source/blender/blenkernel/intern/gpencil.c b/source/blender/blenkernel/intern/gpencil.c
index 439005ca1b4..8b74400e660 100644
--- a/source/blender/blenkernel/intern/gpencil.c
+++ b/source/blender/blenkernel/intern/gpencil.c
@@ -1640,33 +1640,15 @@ float BKE_gpencil_multiframe_falloff_calc(
   return value;
 }
 
-/* remove strokes using a material */
-void BKE_gpencil_material_index_remove(bGPdata *gpd, int index)
+/* reassign strokes using a material */
+void BKE_gpencil_material_index_reassign(bGPdata *gpd, int totcol, int index)
 {
-  bGPDstroke *gps, *gpsn;
-
   for (bGPDlayer *gpl = gpd->layers.first; gpl; gpl = gpl->next) {
     for (bGPDframe *gpf = gpl->frames.first; gpf; gpf = gpf->next) {
-      for (gps = gpf->strokes.first; gps; gps = gpsn) {
-        gpsn = gps->next;
-        if (gps->mat_nr == index) {
-          if (gps->points) {
-            MEM_freeN(gps->points);
-          }
-          if (gps->dvert) {
-            BKE_gpencil_free_stroke_weights(gps);
-            MEM_freeN(gps->dvert);
-          }
-          if (gps->triangles) {
-            MEM_freeN(gps->triangles);
-          }
-          BLI_freelinkN(&gpf->strokes, gps);
-        }
-        else {
-          /* reassign strokes */
-          if (gps->mat_nr > index) {
-            gps->mat_nr--;
-          }
+      for (bGPDstroke *gps = gpf->strokes.first; gps; gps = gps->next) {
+        /* reassign strokes */
+        if ((gps->mat_nr > index) || (gps->mat_nr > totcol - 1)) {
+          gps->mat_nr--;
         }
       }
     }
diff --git a/source/blender/blenkernel/intern/material.c b/source/blender/blenkernel/intern/material.c
index 1e5a2d53bfa..e845271d802 100644
--- a/source/blender/blenkernel/intern/material.c
+++ b/source/blender/blenkernel/intern/material.c
@@ -361,9 +361,6 @@ static void material_data_index_remove_id(ID *id, short index)
     case ID_MB:
       /* meta-elems don't have materials atm */
       break;
-    case ID_GD:
-      BKE_gpencil_material_index_remove((bGPdata *)id, index);
-      break;
     default:
       break;
   }
@@ -1050,12 +1047,20 @@ bool BKE_object_material_slot_remove(Main *bmain, Object *ob)
   }
 
   /* check indices from mesh */
-  if (ELEM(ob->type, OB_MESH, OB_CURVE, OB_SURF, OB_FONT, OB_GPENCIL)) {
+  if (ELEM(ob->type, OB_MESH, OB_CURVE, OB_SURF, OB_FONT)) {
     material_data_index_remove_id((ID *)ob->data, actcol - 1);
     if (ob->runtime.curve_cache) {
       BKE_displist_free(&ob->runtime.curve_cache->disp);
     }
   }
+  /* check indices from gpencil */
+  else if (ob->type == OB_GPENCIL) {
+    /* need one color */
+    if (ob->totcol == 0) {
+      BKE_gpencil_object_material_ensure_from_active_input_material(bmain, ob);
+    }
+    BKE_gpencil_material_index_reassign((bGPdata *)ob->data, ob->totcol, actcol - 1);
+  }
 
   return true;
 }
-- 
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