def accuracy_2d(output, target, topk=(1,)):
    """
    Computes the precision@k for the specified values of k

    Considers output is : NxCxHxW and target is : NxHxW
    """
    maxk = max(topk)
    batch_size = target.size(0)
    total_nelem = batch_size*target.size(-1)*target.size(-2)

    _, pred = output.topk(maxk, 1, True, True)
    correct = target.unsqueeze(1).expand(pred.size())
    correct = pred.eq(correct)

    res = []
    for k in topk:
        correct_k = correct[:, :k].contiguous().view(-1).float().sum(0)
        res.append(correct_k.mul_(100.0 / total_nelem))
    return res