Fix T80077: Objects disappear when joining with a zero scaled axis

Use invert_m4_m4_safe_ortho when joining objects so zero scaled axis
doesn't cause all points to be scaled to zero.

Instead geometry is left un-scaled on degenerate axes.

Report a warning in this case since users may want to adjust the
active objects scale.

1 files changed, 38 insertions, 13 deletions

diff --git a/source/blender/editors/object/object_add.c b/source/blender/editors/object/object_add.c
index a59d8136f47..72180d58ecb 100644
--- a/source/blender/editors/object/object_add.c
+++ b/source/blender/editors/object/object_add.c
@@ -3167,20 +3167,45 @@ static int object_join_exec(bContext *C, wmOperator *op)
     }
   }
 
+  int ret = OPERATOR_CANCELLED;
   if (ob->type == OB_MESH) {
-    return ED_mesh_join_objects_exec(C, op);
-  }
-  if (ELEM(ob->type, OB_CURVE, OB_SURF)) {
-    return ED_curve_join_objects_exec(C, op);
-  }
-  if (ob->type == OB_ARMATURE) {
-    return ED_armature_join_objects_exec(C, op);
-  }
-  if (ob->type == OB_GPENCIL) {
-    return ED_gpencil_join_objects_exec(C, op);
-  }
-
-  return OPERATOR_CANCELLED;
+    ret = ED_mesh_join_objects_exec(C, op);
+  }
+  else if (ELEM(ob->type, OB_CURVE, OB_SURF)) {
+    ret = ED_curve_join_objects_exec(C, op);
+  }
+  else if (ob->type == OB_ARMATURE) {
+    ret = ED_armature_join_objects_exec(C, op);
+  }
+  else if (ob->type == OB_GPENCIL) {
+    ret = ED_gpencil_join_objects_exec(C, op);
+  }
+
+  if (ret & OPERATOR_FINISHED) {
+    /* Even though internally failure to invert is accounted for with a fallback,
+     * show a warning since the result may not be what the user expects. See T80077.
+     *
+     * Failure to invert the matrix is typically caused by zero scaled axes
+     * (which can be caused by constraints, even if the input scale isn't zero).
+     *
+     * Internally the join functions use #invert_m4_m4_safe_ortho which creates
+     * an inevitable matrix from one that has one or more degenerate axes.
+     *
+     * In most cases we don't worry about special handling for non-inevitable matrices however for
+     * joining objects there may be flat 2D objects where it's not obvious the scale is zero.
+     * In this case, using #invert_m4_m4_safe_ortho works as well as we can expect,
+     * joining the contents, flattening on the axis that's zero scaled.
+     * If the zero scale is removed, the data on this axis remains un-scaled
+     * (something that wouldn't work for #invert_m4_m4_safe). */
+    float imat_test[4][4];
+    if (!invert_m4_m4(imat_test, ob->obmat)) {
+      BKE_report(op->reports,
+                 RPT_WARNING,
+                 "Active object final transform has one or more zero scaled axes");
+    }
+  }
+
+  return ret;
 }
 
 void OBJECT_OT_join(wmOperatorType *ot)