function IndexToStateImageMask(I: Longint): Longint;
begin
  Result := I shl 12;
end;

procedure TTreeNode.SetStateIndex(Value: Integer);
var
  Item: TTVItem;
begin
  if not Deleting and (Value <> StateIndex) then
  begin
    FStateIndex := Value;
    if Value >= 0 then Dec(Value);
    with Item do
    begin
      mask := TVIF_STATE or TVIF_HANDLE;
      stateMask := TVIS_STATEIMAGEMASK;
      hItem := ItemId;
      state := IndexToStateImageMask(Value + 1);
    end;
    TreeView_SetItem(Handle, Item);
  end;
end;

if Value < 0 { = -1} then Value := 0;