Fix T81529: Can't move with constraint to a zero-size local axis

To fix the problem, it was necessary to create a fallback for the
zero-sized axis in local orientation.

This also affects the gizmos.

source/blender/editors/transform/transform_constraints.c

@@ -425,7 +425,8 @@ static void applyAxisConstraintVec(
         else if (t->con.mode & CON_AXIS1) {
           copy_v3_v3(c, t->spacemtx[1]);
         }
-        else if (t->con.mode & CON_AXIS2) {
+        else {
+          BLI_assert(t->con.mode & CON_AXIS2);
           copy_v3_v3(c, t->spacemtx[2]);
         }
 

source/blender/editors/transform/transform_convert_object.c

@@ -50,6 +50,7 @@
 
 /* Own include. */
 #include "transform_convert.h"
+#include "transform_orientations.h"
 
 /* -------------------------------------------------------------------- */
 /** \name Object Mode Custom Data
@@ -183,8 +184,7 @@ static void ObjectToTransData(TransInfo *t, TransData *td, Object *ob)
   }
 
   /* axismtx has the real orientation */
-  copy_m3_m4(td->axismtx, ob->obmat);
-  normalize_m3(td->axismtx);
+  transform_orientations_create_from_axis(td->axismtx, UNPACK3(ob->obmat));
 
   td->con = ob->constraints.first;
 

source/blender/editors/transform/transform_orientations.c

@@ -241,6 +241,54 @@ static TransformOrientation *createMeshSpace(bContext *C,
   return addMatrixSpace(C, mat, name, overwrite);
 }
 
+bool transform_orientations_create_from_axis(float mat[3][3],
+                                             const float x[3],
+                                             const float y[3],
+                                             const float z[3])
+{
+  bool is_zero[3] = {true, true, true};
+  zero_m3(mat);
+  if (x) {
+    is_zero[0] = normalize_v3_v3(mat[0], x) == 0.0f;
+  }
+  if (y) {
+    is_zero[1] = normalize_v3_v3(mat[1], y) == 0.0f;
+  }
+  if (z) {
+    is_zero[2] = normalize_v3_v3(mat[2], z) == 0.0f;
+  }
+
+  int zero_axis = is_zero[0] + is_zero[1] + is_zero[2];
+  if (zero_axis == 0) {
+    return true;
+  }
+
+  if (zero_axis == 1) {
+    int axis = is_zero[0] ? 0 : is_zero[1] ? 1 : 2;
+    cross_v3_v3v3(mat[axis], mat[(axis + 1) % 3], mat[(axis + 2) % 3]);
+    if (normalize_v3(mat[axis]) != 0.0f) {
+      return true;
+    }
+  }
+  else if (zero_axis == 2) {
+    int axis, a, b;
+    axis = !is_zero[0] ? 0 : !is_zero[1] ? 1 : 2;
+    a = (axis + 1) % 3;
+    b = (axis + 2) % 3;
+
+    mat[a][a] = 1.0f;
+    mat[b][b] = 1.0f;
+    project_plane_v3_v3v3(mat[a], mat[a], mat[axis]);
+    project_plane_v3_v3v3(mat[b], mat[b], mat[axis]);
+    if ((normalize_v3(mat[a]) != 0.0f) && (normalize_v3(mat[b]) != 0.0f)) {
+      return true;
+    }
+  }
+
+  unit_m3(mat);
+  return false;
+}
+
 bool createSpaceNormal(float mat[3][3], const float normal[3])
 {
   float tangent[3] = {0.0f, 0.0f, 1.0f};
@@ -493,8 +541,7 @@ short ED_transform_calc_orientation_from_type_ex(const bContext *C,
           ED_getTransformOrientationMatrix(C, ob, obedit, pivot_point, r_mat);
         }
         else {
-          copy_m3_m4(r_mat, ob->obmat);
-          normalize_m3(r_mat);
+          transform_orientations_create_from_axis(r_mat, UNPACK3(ob->obmat));
         }
         return V3D_ORIENT_LOCAL;
       }
@@ -607,7 +654,7 @@ void transform_orientations_current_set(TransInfo *t, const short orient_index)
 
   BLI_strncpy(t->spacename, spacename, sizeof(t->spacename));
   copy_m3_m3(t->spacemtx, t->orient[orient_index].matrix);
-  invert_m3_m3(t->spacemtx_inv, t->spacemtx);
+  invert_m3_m3_safe_ortho(t->spacemtx_inv, t->spacemtx);
   t->orient_curr = orient_index;
 }
 

source/blender/editors/transform/transform_orientations.h

@@ -34,6 +34,10 @@ const char *transform_orientations_spacename_get(struct TransInfo *t, const shor
 void transform_orientations_current_set(struct TransInfo *t, const short orient_index);
 
 /* Those two fill in mat and return non-zero on success */
+bool transform_orientations_create_from_axis(float mat[3][3],
+                                             const float x[3],
+                                             const float y[3],
+                                             const float z[3]);
 bool createSpaceNormal(float mat[3][3], const float normal[3]);
 bool createSpaceNormalTangent(float mat[3][3], const float normal[3], const float tangent[3]);