"""Compute the matching cost and solve the corresponding LSAP."""
import torch
from scipy.optimize import linear_sum_assignment
from torch import nn
from discopat.nn_models.torch_box_ops import (
box_cxcywh_to_xyxy,
generalized_box_iou,
)
[docs]
class HungarianMatcher(nn.Module):
"""Compute 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_bbox: float = 1, cost_giou: float = 1
):
"""Create the matcher.
Params:
cost_class: This is the relative weight of the classification error in the matching cost
cost_bbox: This is the relative weight of the L1 error of the bounding box coordinates in the matching cost
cost_giou: This is the relative weight of the giou loss of the bounding box in the matching cost
"""
super().__init__()
self.cost_class = cost_class
self.cost_bbox = cost_bbox
self.cost_giou = cost_giou
assert cost_class != 0 or cost_bbox != 0 or cost_giou != 0, (
"all costs cant be 0"
)
[docs]
@torch.no_grad()
def forward(self, outputs, targets):
"""Perform 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_boxes": 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_boxes] (where num_target_boxes is the number of ground-truth
objects in the target) containing the class labels
"boxes": Tensor of dim [num_target_boxes, 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_boxes)
"""
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_bbox = outputs["pred_boxes"].flatten(
0, 1
) # [batch_size * num_queries, 4]
# Also concat the target labels and boxes
tgt_ids = torch.cat([v["labels"] for v in targets])
tgt_bbox = torch.cat([v["boxes"] 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 boxes
cost_bbox = torch.cdist(out_bbox, tgt_bbox, p=1)
# Compute the giou cost betwen boxes
cost_giou = -generalized_box_iou(
box_cxcywh_to_xyxy(out_bbox), box_cxcywh_to_xyxy(tgt_bbox)
)
# Final cost matrix
cost_matrix = (
self.cost_bbox * cost_bbox
+ self.cost_class * cost_class
+ self.cost_giou * cost_giou
)
cost_matrix = cost_matrix.view(bs, num_queries, -1).cpu()
sizes = [len(v["boxes"]) for v in targets]
indices = [
linear_sum_assignment(c[i])
for i, c in enumerate(cost_matrix.split(sizes, -1))
]
return [
(
torch.as_tensor(i, dtype=torch.int64),
torch.as_tensor(j, dtype=torch.int64),
)
for i, j in indices
]