Text objects: make CharInfo mat_nr zero-based

For text objects, the CharInfo mat_nr material index used to start at 1
(not at zero like for meshes or nurbs).
Code was mostly considering this (but not in all places, so material
index handling (removing/moving) could still go wrong.

As an alternative to !109746 (where it was made sure all places would
make the right assumption about mat_nr starting at 1), this PR now
changes the mat_nr to be zero-based.

This is more in line with other places handling material indices.

Versioning code is in place to properly convert old files.

Fixes #109491

Pull Request: https://projects.blender.org/blender/blender/pulls/112954

source/blender/blenkernel/BKE_blender_version.h

@@ -29,7 +29,7 @@ extern "C" {
 
 /* Blender file format version. */
 #define BLENDER_FILE_VERSION BLENDER_VERSION
-#define BLENDER_FILE_SUBVERSION 30
+#define BLENDER_FILE_SUBVERSION 31
 
 /* Minimum Blender version that supports reading file written with the current
  * version. Older Blender versions will test this and cancel loading the file, showing a warning to

source/blender/blenkernel/intern/curve.cc

@@ -5427,11 +5427,7 @@ void BKE_curve_material_remap(Curve *cu, const uint *remap, uint remap_len)
     }
 
     for (i = 0; i <= charinfo_len; i++) {
-      if (strinfo[i].mat_nr > 0) {
-        strinfo[i].mat_nr -= 1;
-        MAT_NR_REMAP(strinfo[i].mat_nr);
-        strinfo[i].mat_nr += 1;
-      }
+      MAT_NR_REMAP(strinfo[i].mat_nr);
     }
   }
   else {

source/blender/blenkernel/intern/vfont.cc

@@ -448,8 +448,8 @@ static void build_underline(Curve *cu,
   nu2->bezt = nullptr;
   nu2->knotsu = nu2->knotsv = nullptr;
   nu2->charidx = charidx + 1000;
-  if (mat_nr > 0) {
-    nu2->mat_nr = mat_nr - 1;
+  if (mat_nr >= 0) {
+    nu2->mat_nr = mat_nr;
   }
   nu2->pntsu = 4;
   nu2->pntsv = 1;
@@ -538,7 +538,7 @@ void BKE_vfont_build_char(Curve *cu,
       nu2->flag = CU_SMOOTH;
       nu2->charidx = charidx;
       if (info->mat_nr > 0) {
-        nu2->mat_nr = info->mat_nr - 1;
+        nu2->mat_nr = info->mat_nr;
       }
       else {
         nu2->mat_nr = 0;

source/blender/blenloader/intern/versioning_400.cc

@@ -18,6 +18,7 @@
 #include "DNA_brush_types.h"
 #include "DNA_camera_types.h"
 #include "DNA_defaults.h"
+#include "DNA_curve_types.h"
 #include "DNA_light_types.h"
 #include "DNA_lightprobe_types.h"
 #include "DNA_modifier_types.h"
@@ -42,6 +43,7 @@
 
 #include "BKE_armature.h"
 #include "BKE_attribute.h"
+#include "BKE_curve.h"
 #include "BKE_effect.h"
 #include "BKE_grease_pencil.hh"
 #include "BKE_idprop.hh"
@@ -1614,6 +1616,20 @@ void blo_do_versions_400(FileData *fd, Library * /*lib*/, Main *bmain)
     }
   }
 
+  if (!MAIN_VERSION_FILE_ATLEAST(bmain, 400, 31)) {
+    LISTBASE_FOREACH (Curve *, curve, &bmain->curves) {
+      const int curvetype = BKE_curve_type_get(curve);
+      if (curvetype == OB_FONT) {
+        CharInfo *info = curve->strinfo;
+        for (int i = curve->len_char32 - 1; i >= 0; i--, info++) {
+          if (info->mat_nr > 0) {
+            /** CharInfo mat_nr used to start at 1, unlike mesh & nurbs, now zero-based. */
+            info->mat_nr--;
+          }
+        }
+      }
+    }
+  }
   /**
    * Versioning code until next subversion bump goes here.
    *

source/blender/editors/curve/editfont.cc

@@ -385,7 +385,7 @@ static int insert_into_textbuf(Object *obedit, uintptr_t c)
     ef->textbuf[ef->pos] = c;
     ef->textbufinfo[ef->pos] = cu->curinfo;
     ef->textbufinfo[ef->pos].kern = 0.0f;
-    ef->textbufinfo[ef->pos].mat_nr = obedit->actcol;
+    ef->textbufinfo[ef->pos].mat_nr = obedit->actcol - 1;
 
     ef->pos++;
     ef->len++;
@@ -418,10 +418,7 @@ static void text_update_edited(bContext *C, Object *obedit, const eEditFontMode
   cu->curinfo = ef->textbufinfo[ef->pos ? ef->pos - 1 : 0];
 
   if (obedit->totcol > 0) {
-    obedit->actcol = cu->curinfo.mat_nr;
-
-    /* since this array is calloc'd, it can be 0 even though we try ensure
-     * (mat_nr > 0) almost everywhere */
+    obedit->actcol = cu->curinfo.mat_nr + 1;
     if (obedit->actcol < 1) {
       obedit->actcol = 1;
     }
@@ -1851,7 +1848,7 @@ static void font_cursor_set_apply(bContext *C, const wmEvent *event)
   cu->curinfo = ef->textbufinfo[ef->pos ? ef->pos - 1 : 0];
 
   if (ob->totcol > 0) {
-    ob->actcol = cu->curinfo.mat_nr;
+    ob->actcol = cu->curinfo.mat_nr + 1;
     if (ob->actcol < 1) {
       ob->actcol = 1;
     }

source/blender/editors/render/render_shading.cc

@@ -347,7 +347,7 @@ static int material_slot_assign_exec(bContext *C, wmOperator * /*op*/)
       if (ef && BKE_vfont_select_get(ob, &selstart, &selend)) {
         for (i = selstart; i <= selend; i++) {
           changed = true;
-          ef->textbufinfo[i].mat_nr = mat_nr_active + 1;
+          ef->textbufinfo[i].mat_nr = mat_nr_active;
         }
       }
     }

source/blender/makesdna/DNA_curve_types.h

@@ -157,7 +157,6 @@ typedef struct Nurb {
 
 typedef struct CharInfo {
   float kern;
-  /** Index start at 1, unlike mesh & nurbs. */
   short mat_nr;
   char flag;
   char _pad[1];

source/blender/makesrna/intern/rna_curve.cc

@@ -318,13 +318,13 @@ static void rna_Curve_material_index_range(
 static int rna_ChariInfo_material_index_get(PointerRNA *ptr)
 {
   CharInfo *info = static_cast<CharInfo *>(ptr->data);
-  return info->mat_nr ? info->mat_nr - 1 : 0;
+  return info->mat_nr ? info->mat_nr : 0;
 }
 
 static void rna_ChariInfo_material_index_set(PointerRNA *ptr, int value)
 {
   CharInfo *info = static_cast<CharInfo *>(ptr->data);
-  info->mat_nr = value + 1;
+  info->mat_nr = value;
 }
 
 static void rna_Curve_active_textbox_index_range(