models/matcher.py

"""
Modules to compute the matching cost and solve the corresponding LSAP.
"""
import torch
from scipy.optimize import linear_sum_assignment
from torch import nn

class HungarianMatcher_Line(nn.Module):
    """This class computes an assignment between the targets and the predictions of the network

    For efficiency reasons, the targets don't include the no_object. Because of this, in general,
    there are more predictions than targets. In this case, we do a 1-to-1 matching of the best predictions,
    while the others are un-matched (and thus treated as non-objects).
    """

    def __init__(self, cost_class: float = 1, cost_line: float = 1):
        """Creates the matcher

        Params:
            cost_class: This is the relative weight of the classification error in the matching cost
            cost_line: This is the relative weight of the L1 error of the bounding box coordinates in the matching cost
        """
        super().__init__()
        self.cost_class = cost_class
        self.cost_line = cost_line
        assert cost_class != 0 or cost_line != 0, "all costs cant be 0"

    @torch.no_grad()
    def forward(self, outputs, targets):
        """ Performs the matching

        Params:
            outputs: This is a dict that contains at least these entries:
                 "pred_logits": Tensor of dim [batch_size, num_queries, num_classes] with the classification logits
                 "pred_lines": Tensor of dim [batch_size, num_queries, 4] with the predicted box coordinates

            targets: This is a list of targets (len(targets) = batch_size), where each target is a dict containing:
                 "labels": Tensor of dim [num_target_lines] (where num_target_lines is the number of ground-truth
                           objects in the target) containing the class labels
                 "lines": Tensor of dim [num_target_lines, 4] containing the target box coordinates

        Returns:
            A list of size batch_size, containing tuples of (index_i, index_j) where:
                - index_i is the indices of the selected predictions (in order)
                - index_j is the indices of the corresponding selected targets (in order)
            For each batch element, it holds:
                len(index_i) = len(index_j) = min(num_queries, num_target_lines)
        """
        bs, num_queries = outputs["pred_logits"].shape[:2]

        # We flatten to compute the cost matrices in a batch
        out_prob = outputs["pred_logits"].flatten(0, 1).softmax(-1)  # [batch_size * num_queries, num_classes]

        out_line = outputs["pred_lines"].flatten(0, 1)  # [batch_size * num_queries, 4]
        tgt_line = torch.cat([v["lines"] for v in targets])

                
        # Also concat the target labels and lines
        tgt_ids = torch.cat([v["labels"] for v in targets])

        # Compute the classification cost. Contrary to the loss, we don't use the NLL,
        # but approximate it in 1 - proba[target class].
        # The 1 is a constant that doesn't change the matching, it can be ommitted.
        cost_class = -out_prob[:, tgt_ids]

        # Compute the L1 cost between lines
        cost_line = torch.cdist(out_line, tgt_line, p=1)

        # Final cost matrix
        C = self.cost_line * cost_line  + self.cost_class * cost_class
        C = C.view(bs, num_queries, -1).cpu()

        sizes = [len(v["lines"]) for v in targets]
        indices = [linear_sum_assignment(c[i]) for i, c in enumerate(C.split(sizes, -1))]

        return [(torch.as_tensor(i, dtype=torch.int64), torch.as_tensor(j, dtype=torch.int64)) for i, j in indices]



def build_matcher(args, type=None):
    return HungarianMatcher_Line(cost_class=args.set_cost_class, cost_line=args.set_cost_line)