# Copyright (c) OpenMMLab. All rights reserved.
import torch
import torch.nn as nn
import torch.nn.functional as F
from mmcv.runner import load_checkpoint

from mmpose.core.camera import SimpleCameraTorch
from mmpose.core.post_processing.post_transforms import (
    affine_transform_torch, get_affine_transform)
from .. import builder
from ..builder import POSENETS
from .base import BasePose


class ProjectLayer(nn.Module):

    def __init__(self, image_size, heatmap_size):
        """Project layer to get voxel feature. Adapted from
        https://github.com/microsoft/voxelpose-
        pytorch/blob/main/lib/models/project_layer.py.

        Args:
            image_size (int or list): input size of the 2D model
            heatmap_size (int or list): output size of the 2D model
        """
        super(ProjectLayer, self).__init__()
        self.image_size = image_size
        self.heatmap_size = heatmap_size
        if isinstance(self.image_size, int):
            self.image_size = [self.image_size, self.image_size]
        if isinstance(self.heatmap_size, int):
            self.heatmap_size = [self.heatmap_size, self.heatmap_size]

    def compute_grid(self, box_size, box_center, num_bins, device=None):
        if isinstance(box_size, int) or isinstance(box_size, float):
            box_size = [box_size, box_size, box_size]
        if isinstance(num_bins, int):
            num_bins = [num_bins, num_bins, num_bins]

        grid_1D_x = torch.linspace(
            -box_size[0] / 2, box_size[0] / 2, num_bins[0], device=device)
        grid_1D_y = torch.linspace(
            -box_size[1] / 2, box_size[1] / 2, num_bins[1], device=device)
        grid_1D_z = torch.linspace(
            -box_size[2] / 2, box_size[2] / 2, num_bins[2], device=device)
        grid_x, grid_y, grid_z = torch.meshgrid(
            grid_1D_x + box_center[0],
            grid_1D_y + box_center[1],
            grid_1D_z + box_center[2],
        )
        grid_x = grid_x.contiguous().view(-1, 1)
        grid_y = grid_y.contiguous().view(-1, 1)
        grid_z = grid_z.contiguous().view(-1, 1)
        grid = torch.cat([grid_x, grid_y, grid_z], dim=1)

        return grid

    def get_voxel(self, feature_maps, meta, grid_size, grid_center, cube_size):
        device = feature_maps[0].device
        batch_size = feature_maps[0].shape[0]
        num_channels = feature_maps[0].shape[1]
        num_bins = cube_size[0] * cube_size[1] * cube_size[2]
        n = len(feature_maps)
        cubes = torch.zeros(
            batch_size, num_channels, 1, num_bins, n, device=device)
        w, h = self.heatmap_size
        grids = torch.zeros(batch_size, num_bins, 3, device=device)
        bounding = torch.zeros(batch_size, 1, 1, num_bins, n, device=device)
        for i in range(batch_size):
            if len(grid_center[0]) == 3 or grid_center[i][3] >= 0:
                if len(grid_center) == 1:
                    grid = self.compute_grid(
                        grid_size, grid_center[0], cube_size, device=device)
                else:
                    grid = self.compute_grid(
                        grid_size, grid_center[i], cube_size, device=device)
                grids[i:i + 1] = grid
                for c in range(n):
                    center = meta[i]['center'][c]
                    scale = meta[i]['scale'][c]

                    width, height = center * 2
                    trans = torch.as_tensor(
                        get_affine_transform(center, scale / 200.0, 0,
                                             self.image_size),
                        dtype=torch.float,
                        device=device)

                    cam_param = meta[i]['camera'][c].copy()

                    single_view_camera = SimpleCameraTorch(
                        param=cam_param, device=device)
                    xy = single_view_camera.world_to_pixel(grid)

                    bounding[i, 0, 0, :, c] = (xy[:, 0] >= 0) & (
                        xy[:, 1] >= 0) & (xy[:, 0] < width) & (
                            xy[:, 1] < height)
                    xy = torch.clamp(xy, -1.0, max(width, height))
                    xy = affine_transform_torch(xy, trans)
                    xy = xy * torch.tensor(
                        [w, h], dtype=torch.float,
                        device=device) / torch.tensor(
                            self.image_size, dtype=torch.float, device=device)
                    sample_grid = xy / torch.tensor([w - 1, h - 1],
                                                    dtype=torch.float,
                                                    device=device) * 2.0 - 1.0
                    sample_grid = torch.clamp(
                        sample_grid.view(1, 1, num_bins, 2), -1.1, 1.1)

                    cubes[i:i + 1, :, :, :, c] += F.grid_sample(
                        feature_maps[c][i:i + 1, :, :, :],
                        sample_grid,
                        align_corners=True)

        cubes = torch.sum(
            torch.mul(cubes, bounding), dim=-1) / (
                torch.sum(bounding, dim=-1) + 1e-6)
        cubes[cubes != cubes] = 0.0
        cubes = cubes.clamp(0.0, 1.0)

        cubes = cubes.view(batch_size, num_channels, cube_size[0],
                           cube_size[1], cube_size[2])
        return cubes, grids

    def forward(self, feature_maps, meta, grid_size, grid_center, cube_size):
        cubes, grids = self.get_voxel(feature_maps, meta, grid_size,
                                      grid_center, cube_size)
        return cubes, grids


@POSENETS.register_module()
class DetectAndRegress(BasePose):
    """DetectAndRegress approach for multiview human pose detection.

    Args:
        backbone (ConfigDict): Dictionary to construct the 2D pose detector
        human_detector (ConfigDict): dictionary to construct human detector
        pose_regressor (ConfigDict): dictionary to construct pose regressor
        train_cfg (ConfigDict): Config for training. Default: None.
        test_cfg (ConfigDict): Config for testing. Default: None.
        pretrained (str): Path to the pretrained 2D model. Default: None.
        freeze_2d (bool): Whether to freeze the 2D model in training.
            Default: True.
    """

    def __init__(self,
                 backbone,
                 human_detector,
                 pose_regressor,
                 train_cfg=None,
                 test_cfg=None,
                 pretrained=None,
                 freeze_2d=True):
        super(DetectAndRegress, self).__init__()
        if backbone is not None:
            self.backbone = builder.build_posenet(backbone)
            if self.training and pretrained is not None:
                load_checkpoint(self.backbone, pretrained)
        else:
            self.backbone = None

        self.freeze_2d = freeze_2d
        self.human_detector = builder.MODELS.build(human_detector)
        self.pose_regressor = builder.MODELS.build(pose_regressor)

        self.train_cfg = train_cfg
        self.test_cfg = test_cfg

    @staticmethod
    def _freeze(model):
        """Freeze parameters."""
        model.eval()
        for param in model.parameters():
            param.requires_grad = False

    def train(self, mode=True):
        """Sets the module in training mode.
        Args:
            mode (bool): whether to set training mode (``True``)
                or evaluation mode (``False``). Default: ``True``.

        Returns:
            Module: self
        """
        super().train(mode)
        if mode and self.freeze_2d and self.backbone is not None:
            self._freeze(self.backbone)

        return self

    def forward(self,
                img=None,
                img_metas=None,
                return_loss=True,
                targets=None,
                masks=None,
                targets_3d=None,
                input_heatmaps=None,
                **kwargs):
        """
        Note:
            batch_size: N
            num_keypoints: K
            num_img_channel: C
            img_width: imgW
            img_height: imgH
            feature_maps width: W
            feature_maps height: H
            volume_length: cubeL
            volume_width: cubeW
            volume_height: cubeH

        Args:
            img (list(torch.Tensor[NxCximgHximgW])):
                Multi-camera input images to the 2D model.
            img_metas (list(dict)):
                Information about image, 3D groundtruth and camera parameters.
            return_loss: Option to `return loss`. `return loss=True`
                for training, `return loss=False` for validation & test.
            targets (list(torch.Tensor[NxKxHxW])):
                Multi-camera target feature_maps of the 2D model.
            masks (list(torch.Tensor[NxHxW])):
                Multi-camera masks of the input to the 2D model.
            targets_3d (torch.Tensor[NxcubeLxcubeWxcubeH]):
                Ground-truth 3D heatmap of human centers.
            input_heatmaps (list(torch.Tensor[NxKxHxW])):
                Multi-camera feature_maps when the 2D model is not available.
                 Default: None.
            **kwargs:

        Returns:
            dict: if 'return_loss' is true, then return losses.
              Otherwise, return predicted poses, human centers and sample_id

        """
        if return_loss:
            return self.forward_train(img, img_metas, targets, masks,
                                      targets_3d, input_heatmaps)
        else:
            return self.forward_test(img, img_metas, input_heatmaps)

    def train_step(self, data_batch, optimizer, **kwargs):
        """The iteration step during training.

        This method defines an iteration step during training, except for the
        back propagation and optimizer updating, which are done in an optimizer
        hook. Note that in some complicated cases or models, the whole process
        including back propagation and optimizer updating is also defined in
        this method, such as GAN.

        Args:
            data_batch (dict): The output of dataloader.
            optimizer (:obj:`torch.optim.Optimizer` | dict): The optimizer of
                runner is passed to ``train_step()``. This argument is unused
                and reserved.

        Returns:
            dict: It should contain at least 3 keys: ``loss``, ``log_vars``,
                ``num_samples``.
                ``loss`` is a tensor for back propagation, which can be a
                weighted sum of multiple losses.
                ``log_vars`` contains all the variables to be sent to the
                logger.
                ``num_samples`` indicates the batch size (when the model is
                DDP, it means the batch size on each GPU), which is used for
                averaging the logs.
        """
        losses = self.forward(**data_batch)

        loss, log_vars = self._parse_losses(losses)
        if 'img' in data_batch:
            batch_size = data_batch['img'][0].shape[0]
        else:
            assert 'input_heatmaps' in data_batch
            batch_size = data_batch['input_heatmaps'][0][0].shape[0]

        outputs = dict(loss=loss, log_vars=log_vars, num_samples=batch_size)

        return outputs

    def forward_train(self,
                      img,
                      img_metas,
                      targets=None,
                      masks=None,
                      targets_3d=None,
                      input_heatmaps=None):
        """
        Note:
            batch_size: N
            num_keypoints: K
            num_img_channel: C
            img_width: imgW
            img_height: imgH
            feature_maps width: W
            feature_maps height: H
            volume_length: cubeL
            volume_width: cubeW
            volume_height: cubeH

        Args:
            img (list(torch.Tensor[NxCximgHximgW])):
                Multi-camera input images to the 2D model.
            img_metas (list(dict)):
                Information about image, 3D groundtruth and camera parameters.
            targets (list(torch.Tensor[NxKxHxW])):
                Multi-camera target feature_maps of the 2D model.
            masks (list(torch.Tensor[NxHxW])):
                Multi-camera masks of the input to the 2D model.
            targets_3d (torch.Tensor[NxcubeLxcubeWxcubeH]):
                Ground-truth 3D heatmap of human centers.
            input_heatmaps (list(torch.Tensor[NxKxHxW])):
                Multi-camera feature_maps when the 2D model is not available.
                 Default: None.

        Returns:
            dict: losses.

        """
        if self.backbone is None:
            assert input_heatmaps is not None
            feature_maps = []
            for input_heatmap in input_heatmaps:
                feature_maps.append(input_heatmap[0])
        else:
            feature_maps = []
            assert isinstance(img, list)
            for img_ in img:
                feature_maps.append(self.backbone.forward_dummy(img_)[0])

        losses = dict()
        human_candidates, human_loss = self.human_detector.forward_train(
            None, img_metas, feature_maps, targets_3d, return_preds=True)
        losses.update(human_loss)

        pose_loss = self.pose_regressor(
            None,
            img_metas,
            return_loss=True,
            feature_maps=feature_maps,
            human_candidates=human_candidates)
        losses.update(pose_loss)

        if not self.freeze_2d:
            losses_2d = {}
            heatmaps_tensor = torch.cat(feature_maps, dim=0)
            targets_tensor = torch.cat(targets, dim=0)
            masks_tensor = torch.cat(masks, dim=0)
            losses_2d_ = self.backbone.get_loss(heatmaps_tensor,
                                                targets_tensor, masks_tensor)
            for k, v in losses_2d_.items():
                losses_2d[k + '_2d'] = v
            losses.update(losses_2d)

        return losses

    def forward_test(
        self,
        img,
        img_metas,
        input_heatmaps=None,
    ):
        """
        Note:
            batch_size: N
            num_keypoints: K
            num_img_channel: C
            img_width: imgW
            img_height: imgH
            feature_maps width: W
            feature_maps height: H
            volume_length: cubeL
            volume_width: cubeW
            volume_height: cubeH

        Args:
            img (list(torch.Tensor[NxCximgHximgW])):
                Multi-camera input images to the 2D model.
            img_metas (list(dict)):
                Information about image, 3D groundtruth and camera parameters.
            input_heatmaps (list(torch.Tensor[NxKxHxW])):
                Multi-camera feature_maps when the 2D model is not available.
                 Default: None.

        Returns:
            dict: predicted poses, human centers and sample_id

        """
        if self.backbone is None:
            assert input_heatmaps is not None
            feature_maps = []
            for input_heatmap in input_heatmaps:
                feature_maps.append(input_heatmap[0])
        else:
            feature_maps = []
            assert isinstance(img, list)
            for img_ in img:
                feature_maps.append(self.backbone.forward_dummy(img_)[0])

        human_candidates = self.human_detector.forward_test(
            None, img_metas, feature_maps)

        human_poses = self.pose_regressor(
            None,
            img_metas,
            return_loss=False,
            feature_maps=feature_maps,
            human_candidates=human_candidates)

        result = {}
        result['pose_3d'] = human_poses.cpu().numpy()
        result['human_detection_3d'] = human_candidates.cpu().numpy()
        result['sample_id'] = [img_meta['sample_id'] for img_meta in img_metas]

        return result

    def show_result(self, **kwargs):
        """Visualize the results."""
        raise NotImplementedError

    def forward_dummy(self, img, input_heatmaps=None, num_candidates=5):
        """Used for computing network FLOPs."""
        if self.backbone is None:
            assert input_heatmaps is not None
            feature_maps = []
            for input_heatmap in input_heatmaps:
                feature_maps.append(input_heatmap[0])
        else:
            feature_maps = []
            assert isinstance(img, list)
            for img_ in img:
                feature_maps.append(self.backbone.forward_dummy(img_)[0])

        _ = self.human_detector.forward_dummy(feature_maps)

        _ = self.pose_regressor.forward_dummy(feature_maps, num_candidates)


@POSENETS.register_module()
class VoxelSinglePose(BasePose):
    """VoxelPose Please refer to the `paper <https://arxiv.org/abs/2004.06239>`
    for details.

    Args:
        image_size (list): input size of the 2D model.
        heatmap_size (list): output size of the 2D model.
        sub_space_size (list): Size of the cuboid human proposal.
        sub_cube_size (list): Size of the input volume to the pose net.
        pose_net (ConfigDict): Dictionary to construct the pose net.
        pose_head (ConfigDict): Dictionary to construct the pose head.
        train_cfg (ConfigDict): Config for training. Default: None.
        test_cfg (ConfigDict): Config for testing. Default: None.
    """

    def __init__(
        self,
        image_size,
        heatmap_size,
        sub_space_size,
        sub_cube_size,
        num_joints,
        pose_net,
        pose_head,
        train_cfg=None,
        test_cfg=None,
    ):
        super(VoxelSinglePose, self).__init__()
        self.project_layer = ProjectLayer(image_size, heatmap_size)
        self.pose_net = builder.build_backbone(pose_net)
        self.pose_head = builder.build_head(pose_head)

        self.sub_space_size = sub_space_size
        self.sub_cube_size = sub_cube_size

        self.num_joints = num_joints
        self.train_cfg = train_cfg
        self.test_cfg = test_cfg

    def forward(self,
                img,
                img_metas,
                return_loss=True,
                feature_maps=None,
                human_candidates=None,
                **kwargs):
        """
        Note:
            batch_size: N
            num_keypoints: K
            num_img_channel: C
            img_width: imgW
            img_height: imgH
            feature_maps width: W
            feature_maps height: H
            volume_length: cubeL
            volume_width: cubeW
            volume_height: cubeH

        Args:
            img (list(torch.Tensor[NxCximgHximgW])):
                Multi-camera input images to the 2D model.
            feature_maps (list(torch.Tensor[NxCxHxW])):
                Multi-camera input feature_maps.
            img_metas (list(dict)):
                Information about image, 3D groundtruth and camera parameters.
            human_candidates (torch.Tensor[NxPx5]):
                Human candidates.
            return_loss: Option to `return loss`. `return loss=True`
                for training, `return loss=False` for validation & test.

        """
        if return_loss:
            return self.forward_train(img, img_metas, feature_maps,
                                      human_candidates)
        else:
            return self.forward_test(img, img_metas, feature_maps,
                                     human_candidates)

    def forward_train(self,
                      img,
                      img_metas,
                      feature_maps=None,
                      human_candidates=None,
                      return_preds=False,
                      **kwargs):
        """Defines the computation performed at training.
        Note:
            batch_size: N
            num_keypoints: K
            num_img_channel: C
            img_width: imgW
            img_height: imgH
            feature_maps width: W
            feature_maps height: H
            volume_length: cubeL
            volume_width: cubeW
            volume_height: cubeH

        Args:
            img (list(torch.Tensor[NxCximgHximgW])):
                Multi-camera input images to the 2D model.
            feature_maps (list(torch.Tensor[NxCxHxW])):
                Multi-camera input feature_maps.
            img_metas (list(dict)):
                Information about image, 3D groundtruth and camera parameters.
            human_candidates (torch.Tensor[NxPx5]):
                Human candidates.
            return_preds (bool): Whether to return prediction results

        Returns:
            dict: losses.

        """
        batch_size, num_candidates, _ = human_candidates.shape
        pred = human_candidates.new_zeros(batch_size, num_candidates,
                                          self.num_joints, 5)
        pred[:, :, :, 3:] = human_candidates[:, :, None, 3:]

        device = feature_maps[0].device
        gt_3d = torch.stack([
            torch.tensor(img_meta['joints_3d'], device=device)
            for img_meta in img_metas
        ])
        gt_3d_vis = torch.stack([
            torch.tensor(img_meta['joints_3d_visible'], device=device)
            for img_meta in img_metas
        ])
        valid_preds = []
        valid_targets = []
        valid_weights = []

        for n in range(num_candidates):
            index = pred[:, n, 0, 3] >= 0
            num_valid = index.sum()
            if num_valid > 0:
                pose_input_cube, coordinates \
                    = self.project_layer(feature_maps,
                                         img_metas,
                                         self.sub_space_size,
                                         human_candidates[:, n, :3],
                                         self.sub_cube_size)
                pose_heatmaps_3d = self.pose_net(pose_input_cube)
                pose_3d = self.pose_head(pose_heatmaps_3d[index],
                                         coordinates[index])

                pred[index, n, :, 0:3] = pose_3d.detach()
                valid_targets.append(gt_3d[index, pred[index, n, 0, 3].long()])
                valid_weights.append(gt_3d_vis[index, pred[index, n, 0,
                                                           3].long(), :,
                                               0:1].float())
                valid_preds.append(pose_3d)

        losses = dict()
        if len(valid_preds) > 0:
            valid_targets = torch.cat(valid_targets, dim=0)
            valid_weights = torch.cat(valid_weights, dim=0)
            valid_preds = torch.cat(valid_preds, dim=0)
            losses.update(
                self.pose_head.get_loss(valid_preds, valid_targets,
                                        valid_weights))
        else:
            pose_input_cube = feature_maps[0].new_zeros(
                batch_size, self.num_joints, *self.sub_cube_size)
            coordinates = feature_maps[0].new_zeros(batch_size,
                                                    *self.sub_cube_size,
                                                    3).view(batch_size, -1, 3)
            pseudo_targets = feature_maps[0].new_zeros(batch_size,
                                                       self.num_joints, 3)
            pseudo_weights = feature_maps[0].new_zeros(batch_size,
                                                       self.num_joints, 1)
            pose_heatmaps_3d = self.pose_net(pose_input_cube)
            pose_3d = self.pose_head(pose_heatmaps_3d, coordinates)
            losses.update(
                self.pose_head.get_loss(pose_3d, pseudo_targets,
                                        pseudo_weights))
        if return_preds:
            return pred, losses
        else:
            return losses

    def forward_test(self,
                     img,
                     img_metas,
                     feature_maps=None,
                     human_candidates=None,
                     **kwargs):
        """Defines the computation performed at training.
        Note:
            batch_size: N
            num_keypoints: K
            num_img_channel: C
            img_width: imgW
            img_height: imgH
            feature_maps width: W
            feature_maps height: H
            volume_length: cubeL
            volume_width: cubeW
            volume_height: cubeH

        Args:
            img (list(torch.Tensor[NxCximgHximgW])):
                Multi-camera input images to the 2D model.
            feature_maps (list(torch.Tensor[NxCxHxW])):
                Multi-camera input feature_maps.
            img_metas (list(dict)):
                Information about image, 3D groundtruth and camera parameters.
            human_candidates (torch.Tensor[NxPx5]):
                Human candidates.

        Returns:
            dict: predicted poses, human centers and sample_id

        """
        batch_size, num_candidates, _ = human_candidates.shape
        pred = human_candidates.new_zeros(batch_size, num_candidates,
                                          self.num_joints, 5)
        pred[:, :, :, 3:] = human_candidates[:, :, None, 3:]

        for n in range(num_candidates):
            index = pred[:, n, 0, 3] >= 0
            num_valid = index.sum()
            if num_valid > 0:
                pose_input_cube, coordinates \
                    = self.project_layer(feature_maps,
                                         img_metas,
                                         self.sub_space_size,
                                         human_candidates[:, n, :3],
                                         self.sub_cube_size)
                pose_heatmaps_3d = self.pose_net(pose_input_cube)
                pose_3d = self.pose_head(pose_heatmaps_3d[index],
                                         coordinates[index])

                pred[index, n, :, 0:3] = pose_3d.detach()

        return pred

    def show_result(self, **kwargs):
        """Visualize the results."""
        raise NotImplementedError

    def forward_dummy(self, feature_maps, num_candidates=5):
        """Used for computing network FLOPs."""
        batch_size, num_channels = feature_maps[0].shape
        pose_input_cube = feature_maps[0].new_zeros(batch_size, num_channels,
                                                    *self.sub_cube_size)
        for n in range(num_candidates):
            _ = self.pose_net(pose_input_cube)


@POSENETS.register_module()
class VoxelCenterDetector(BasePose):
    """Detect human center by 3D CNN on voxels.

    Please refer to the
    `paper <https://arxiv.org/abs/2004.06239>` for details.
    Args:
        image_size (list): input size of the 2D model.
        heatmap_size (list): output size of the 2D model.
        space_size (list): Size of the 3D space.
        cube_size (list): Size of the input volume to the 3D CNN.
        space_center (list): Coordinate of the center of the 3D space.
        center_net (ConfigDict): Dictionary to construct the center net.
        center_head (ConfigDict): Dictionary to construct the center head.
        train_cfg (ConfigDict): Config for training. Default: None.
        test_cfg (ConfigDict): Config for testing. Default: None.
    """

    def __init__(
        self,
        image_size,
        heatmap_size,
        space_size,
        cube_size,
        space_center,
        center_net,
        center_head,
        train_cfg=None,
        test_cfg=None,
    ):
        super(VoxelCenterDetector, self).__init__()
        self.project_layer = ProjectLayer(image_size, heatmap_size)
        self.center_net = builder.build_backbone(center_net)
        self.center_head = builder.build_head(center_head)

        self.space_size = space_size
        self.cube_size = cube_size
        self.space_center = space_center

        self.train_cfg = train_cfg
        self.test_cfg = test_cfg

    def assign2gt(self, center_candidates, gt_centers, gt_num_persons):
        """"Assign gt id to each valid human center candidate."""
        det_centers = center_candidates[..., :3]
        batch_size = center_candidates.shape[0]
        cand_num = center_candidates.shape[1]
        cand2gt = torch.zeros(batch_size, cand_num)

        for i in range(batch_size):
            cand = det_centers[i].view(cand_num, 1, -1)
            gt = gt_centers[None, i, :gt_num_persons[i]]

            dist = torch.sqrt(torch.sum((cand - gt)**2, dim=-1))
            min_dist, min_gt = torch.min(dist, dim=-1)

            cand2gt[i] = min_gt
            cand2gt[i][min_dist > self.train_cfg['dist_threshold']] = -1.0

        center_candidates[:, :, 3] = cand2gt

        return center_candidates

    def forward(self,
                img,
                img_metas,
                return_loss=True,
                feature_maps=None,
                targets_3d=None):
        """
        Note:
            batch_size: N
            num_keypoints: K
            num_img_channel: C
            img_width: imgW
            img_height: imgH
            heatmaps width: W
            heatmaps height: H
        Args:
            img (list(torch.Tensor[NxCximgHximgW])):
                Multi-camera input images to the 2D model.
            img_metas (list(dict)):
                Information about image, 3D groundtruth and camera parameters.
            return_loss: Option to `return loss`. `return loss=True`
                for training, `return loss=False` for validation & test.
            targets_3d (torch.Tensor[NxcubeLxcubeWxcubeH]):
                Ground-truth 3D heatmap of human centers.
            feature_maps (list(torch.Tensor[NxKxHxW])):
                Multi-camera feature_maps.
        Returns:
            dict: if 'return_loss' is true, then return losses.
                Otherwise, return predicted poses
        """
        if return_loss:
            return self.forward_train(img, img_metas, feature_maps, targets_3d)
        else:
            return self.forward_test(img, img_metas, feature_maps)

    def forward_train(self,
                      img,
                      img_metas,
                      feature_maps=None,
                      targets_3d=None,
                      return_preds=False):
        """
        Note:
            batch_size: N
            num_keypoints: K
            num_img_channel: C
            img_width: imgW
            img_height: imgH
            heatmaps width: W
            heatmaps height: H
        Args:
            img (list(torch.Tensor[NxCximgHximgW])):
                Multi-camera input images to the 2D model.
            img_metas (list(dict)):
                Information about image, 3D groundtruth and camera parameters.
            targets_3d (torch.Tensor[NxcubeLxcubeWxcubeH]):
                Ground-truth 3D heatmap of human centers.
            feature_maps (list(torch.Tensor[NxKxHxW])):
                Multi-camera feature_maps.
            return_preds (bool): Whether to return prediction results
        Returns:
            dict: if 'return_pred' is true, then return losses
                and human centers. Otherwise, return losses only
        """
        initial_cubes, _ = self.project_layer(feature_maps, img_metas,
                                              self.space_size,
                                              [self.space_center],
                                              self.cube_size)
        center_heatmaps_3d = self.center_net(initial_cubes)
        center_heatmaps_3d = center_heatmaps_3d.squeeze(1)
        center_candidates = self.center_head(center_heatmaps_3d)

        device = center_candidates.device

        gt_centers = torch.stack([
            torch.tensor(img_meta['roots_3d'], device=device)
            for img_meta in img_metas
        ])
        gt_num_persons = torch.stack([
            torch.tensor(img_meta['num_persons'], device=device)
            for img_meta in img_metas
        ])
        center_candidates = self.assign2gt(center_candidates, gt_centers,
                                           gt_num_persons)

        losses = dict()
        losses.update(
            self.center_head.get_loss(center_heatmaps_3d, targets_3d))

        if return_preds:
            return center_candidates, losses
        else:
            return losses

    def forward_test(self, img, img_metas, feature_maps=None):
        """
        Note:
            batch_size: N
            num_keypoints: K
            num_img_channel: C
            img_width: imgW
            img_height: imgH
            heatmaps width: W
            heatmaps height: H
        Args:
            img (list(torch.Tensor[NxCximgHximgW])):
                Multi-camera input images to the 2D model.
            img_metas (list(dict)):
                Information about image, 3D groundtruth and camera parameters.
            feature_maps (list(torch.Tensor[NxKxHxW])):
                Multi-camera feature_maps.
        Returns:
            human centers
        """
        initial_cubes, _ = self.project_layer(feature_maps, img_metas,
                                              self.space_size,
                                              [self.space_center],
                                              self.cube_size)
        center_heatmaps_3d = self.center_net(initial_cubes)
        center_heatmaps_3d = center_heatmaps_3d.squeeze(1)
        center_candidates = self.center_head(center_heatmaps_3d)
        center_candidates[..., 3] = \
            (center_candidates[..., 4] >
             self.test_cfg['center_threshold']).float() - 1.0

        return center_candidates

    def show_result(self, **kwargs):
        """Visualize the results."""
        raise NotImplementedError

    def forward_dummy(self, feature_maps):
        """Used for computing network FLOPs."""
        batch_size, num_channels, _, _ = feature_maps[0].shape
        initial_cubes = feature_maps[0].new_zeros(batch_size, num_channels,
                                                  *self.cube_size)
        _ = self.center_net(initial_cubes)
