super-gradients/src/super_gradients/training/losses/ssd_loss.py

from typing import Tuple

import torch
from torch import nn
from torch.nn.modules.loss import _Loss

from super_gradients.training.utils.detection_utils import calculate_bbox_iou_matrix
from super_gradients.training.utils.ssd_utils import DefaultBoxes


class HardMiningCrossEntropyLoss(_Loss):
    """
    L_cls = [CE of all positives] + [CE of the hardest backgrounds]
    where the second term is built from [neg_pos_ratio * positive pairs] background cells with the highest CE
    (the hardest background cells)
    """

    def __init__(self, neg_pos_ratio: float):
        """
        :param neg_pos_ratio:   a ratio of negative samples to positive samples in the loss
                                (unlike positives, not all negatives will be used:
                                for each positive the [neg_pos_ratio] hardest negatives will be selected)
        """
        super().__init__()
        self.neg_pos_ratio = neg_pos_ratio
        self.ce = nn.CrossEntropyLoss(reduce=False)

    def forward(self, pred_labels, target_labels):
        mask = target_labels > 0  # not background
        pos_num = mask.sum(dim=1)

        # HARD NEGATIVE MINING
        con = self.ce(pred_labels, target_labels)

        # POSITIVE MASK WILL NOT BE SELECTED
        # set 0. loss for all positive objects, leave the loss where the object is background
        con_neg = con.clone()
        con_neg[mask] = 0
        # sort background cells by CE loss value (bigger_first)
        _, con_idx = con_neg.sort(dim=1, descending=True)
        # restore cells order, get each cell's order (rank) in CE loss sorting
        _, con_rank = con_idx.sort(dim=1)

        # NUMBER OF NEGATIVE THREE TIMES POSITIVE
        neg_num = torch.clamp(self.neg_pos_ratio * pos_num, max=mask.size(1)).unsqueeze(-1)
        # for each image into neg mask we'll take (3 * positive pairs) background objects with the highest CE
        neg_mask = con_rank < neg_num

        closs = (con * (mask.float() + neg_mask.float())).sum(dim=1)
        return closs


class SSDLoss(_Loss):
    """
        Implements the loss as the sum of the followings:
        1. Confidence Loss: All labels, with hard negative mining
        2. Localization Loss: Only on positive labels

    L = (2 - alpha) * L_l1 + alpha * L_cls, where
        * L_cls is HardMiningCrossEntropyLoss
        * L_l1 = [SmoothL1Loss for all positives]
    """

    def __init__(self, dboxes: DefaultBoxes, alpha: float = 1.0, iou_thresh: float = 0.5, neg_pos_ratio: float = 3.):
        """
        :param dboxes:          model anchors, shape [Num Grid Cells * Num anchors x 4]
        :param alpha:           a weighting factor between classification and regression loss
        :param iou_thresh:      a threshold for matching of anchors in each grid cell to GTs
                                (a match should have IoU > iou_thresh)
        :param neg_pos_ratio:   a ratio for HardMiningCrossEntropyLoss
        """
        super(SSDLoss, self).__init__()
        self.scale_xy = dboxes.scale_xy
        self.scale_wh = dboxes.scale_wh
        self.alpha = alpha
        self.dboxes = nn.Parameter(dboxes(order="xywh").transpose(0, 1).unsqueeze(dim=0), requires_grad=False)
        self.sl1_loss = nn.SmoothL1Loss(reduce=False)

        self.con_loss = HardMiningCrossEntropyLoss(neg_pos_ratio)
        self.iou_thresh = iou_thresh

    def _norm_relative_bbox(self, loc):
        """
        convert bbox locations into relative locations (relative to the dboxes)
        :param loc a tensor of shape [batch, 4, num_boxes]
        """
        gxy = ((loc[:, :2, :] - self.dboxes[:, :2, :]) / self.dboxes[:, 2:, ]) / self.scale_xy
        gwh = (loc[:, 2:, :] / self.dboxes[:, 2:, :]).log() / self.scale_wh
        return torch.cat((gxy, gwh), dim=1).contiguous()

    def match_dboxes(self, targets):
        """
        creates tensors with target boxes and labels for each dboxes, so with the same len as dboxes.

        * Each GT is assigned with a grid cell with the highest IoU, this creates a pair for each GT and some cells;
        * The rest of grid cells are assigned to a GT with the highest IoU, assuming it's > self.iou_thresh;
          If this condition is not met the grid cell is marked as background

        GT-wise: one to many
        Grid-cell-wise: one to one

        :param targets: a tensor containing the boxes for a single image;
                        shape [num_boxes, 6] (image_id, label, x, y, w, h)
        :return:        two tensors
                        boxes - shape of dboxes [4, num_dboxes] (x,y,w,h)
                        labels - sahpe [num_dboxes]
        """
        device = targets.device
        each_cell_target_locations = self.dboxes.data.clone().squeeze()
        each_cell_target_labels = torch.zeros((self.dboxes.data.shape[2])).to(device)

        if len(targets) > 0:
            target_boxes = targets[:, 2:]
            target_labels = targets[:, 1]
            ious = calculate_bbox_iou_matrix(target_boxes, self.dboxes.data.squeeze().T, x1y1x2y2=False)

            # one best GT for EACH cell (does not guarantee that all GTs will be used)
            best_target_per_cell, best_target_per_cell_index = ious.max(0)

            # one best grid cell (anchor in it) for EACH target
            best_cell_per_target, best_cell_per_target_index = ious.max(1)
            # make sure EACH target has a grid cell assigned
            best_target_per_cell_index[best_cell_per_target_index] = torch.arange(len(targets)).to(device)
            # 2. is higher than any IoU, so it is guaranteed to pass any IoU threshold
            # which ensures that the pairs selected for each target will be included in the mask below
            # while the threshold will only affect other grid cell anchors that aren't pre-assigned to any target
            best_target_per_cell[best_cell_per_target_index] = 2.

            mask = best_target_per_cell > self.iou_thresh
            each_cell_target_locations[:, mask] = target_boxes[best_target_per_cell_index[mask]].T
            each_cell_target_labels[mask] = target_labels[best_target_per_cell_index[mask]] + 1

        return each_cell_target_locations, each_cell_target_labels

    def forward(self, predictions: Tuple, targets):
        """
        Compute the loss
            :param predictions - predictions tensor coming from the network,
            tuple with shapes ([Batch Size, 4, num_dboxes], [Batch Size, num_classes + 1, num_dboxes])
            were predictions have logprobs for background and other classes
            :param targets - targets for the batch. [num targets, 6] (index in batch, label, x,y,w,h)
        """
        if isinstance(predictions, tuple) and isinstance(predictions[1], tuple):
            # Calculate loss in a validation mode
            predictions = predictions[1]
        batch_target_locations = []
        batch_target_labels = []
        (ploc, plabel) = predictions
        targets = targets.to(self.dboxes.device)
        for i in range(ploc.shape[0]):
            target_locations, target_labels = self.match_dboxes(targets[targets[:, 0] == i])
            batch_target_locations.append(target_locations)
            batch_target_labels.append(target_labels)
        batch_target_locations = torch.stack(batch_target_locations)
        batch_target_labels = torch.stack(batch_target_labels).type(torch.long)

        mask = batch_target_labels > 0  # not background
        pos_num = mask.sum(dim=1)

        vec_gd = self._norm_relative_bbox(batch_target_locations)

        # SUM ON FOUR COORDINATES, AND MASK
        sl1 = self.sl1_loss(ploc, vec_gd).sum(dim=1)
        sl1 = (mask.float() * sl1).sum(dim=1)

        closs = self.con_loss(plabel, batch_target_labels)

        # AVOID NO OBJECT DETECTED
        total_loss = (2 - self.alpha) * sl1 + self.alpha * closs
        num_mask = (pos_num > 0).float()  # a mask with 0 for images that have no positive pairs at all
        pos_num = pos_num.float().clamp(min=1e-6)
        ret = (total_loss * num_mask / pos_num).mean(dim=0)  # normalize by the number of positive pairs

        return ret, torch.cat((sl1.mean().unsqueeze(0), closs.mean().unsqueeze(0), ret.unsqueeze(0))).detach()