How to avoid cutting corners with A* pathfinding?

Question

On my second try with A* I managed to calculate all values needed for traceback. Also marked the starting cell with S, blocked with B and the goal cell with F.

Now for the backtracking I would simply follow the cells with lowest G value, starting from goal cell.

Here I would follow G=24 => G=10 => S.

As you can see this solution would create a path which is not valid because it leads throught a wall in this case.

This hangs together with corner cutting when calculating the values for a grid. As you can see here. One would backtrace: G=50 => G= 40 and here one would take G=20 next. This leads to the corner cutting.

I think this issue occurs when calculating the values for each adjacent cell. Maybe I could avoid this if I set some restrictions when adding adjacent cells to the current cell?

public List<Cell> GetAdjacent(Cell _currentCell, List<Cell> _closedList, List<Cell> _gridList) 
    {
        List<Cell> adjacentList = new List<Cell>();
        List<Cell> gridList = _gridList;
        List<Cell> closedList = _closedList;
        Cell currentCell = _currentCell;

        foreach (Cell cell in gridList) 
        {
            bool containedInClosedList = closedList.Any(c => c.id == cell.id);

            if (!cell.blocked && !containedInClosedList && 
                ((cell.positionCR.X == currentCell.positionCR.X - 1 && cell.positionCR.Y == currentCell.positionCR.Y) ||
                (cell.positionCR.X == currentCell.positionCR.X + 1 && cell.positionCR.Y == currentCell.positionCR.Y) ||
                (cell.positionCR.X == currentCell.positionCR.X && cell.positionCR.Y == currentCell.positionCR.Y - 1) ||
                (cell.positionCR.X == currentCell.positionCR.X && cell.positionCR.Y == currentCell.positionCR.Y + 1) ||
                (cell.positionCR.X == currentCell.positionCR.X - 1 && cell.positionCR.Y == currentCell.positionCR.Y - 1) ||
                (cell.positionCR.X == currentCell.positionCR.X - 1 && cell.positionCR.Y == currentCell.positionCR.Y + 1) ||
                (cell.positionCR.X == currentCell.positionCR.X + 1 && cell.positionCR.Y == currentCell.positionCR.Y - 1) ||
                (cell.positionCR.X == currentCell.positionCR.X + 1 && cell.positionCR.Y == currentCell.positionCR.Y + 1)))
            {
                adjacentList.Add(cell);
            }

        }



        return adjacentList;
    }

Could it also be a problem with the custom costs I defined? I took a G=10 for straight and G=14 for diagonal cells.

I think this is the last thing which stops me from finishing the algorithm, so I am looking forward for any help or constructive input.

Thanks in advance!

Answer 1

Just as Eric mentioned in the comments above, you could produce a list with with the eight possible neighbours and check if they are valid. Valid here does not only mean to check if they are blocked, you also should check for corner cutting here.