예제 #1
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    def __getitem__(self, item):
        # img = self.dataset[0][item]
        # label = self.dataset[1][item]
        # img = np.transpose(img, (2,0,1))
        # return img.astype(np.float32), label.astype(np.float32)

        path = self.img_paths[item]
        info = self.labels[item]

        img_height, img_weight = info[0:2].astype(np.int32).tolist()
        label = info[2:].astype(np.float32)

        image = Image.open(path).convert('RGB')

        # x, y 坐标位置调换
        # [ i-((i+1)%2<<1) for i in range(1,25)] = [1,0,3,2,5,4,7,6,9,8,11,10,13,12,15,14,17,16,19,18,21,20,23,22]
        label = label[[i - ((i + 1) % 2 << 1) for i in range(1, 25)]]

        target = label  #
        target = target.reshape(-1, 4)
        # BoxList 数据结构
        target = BoxList(target, image.size, mode="xyxy")
        # 每个box的类别
        classes = torch.tensor([1, 2, 3, 4, 5, 6])
        target.add_field('labels', classes)
        target = target.clip_to_image(remove_empty=True)

        if self.transform:
            img, target = self.transform(image, target)
        else:
            raise RuntimeError('transform not implement')

        return img, target, item, image
예제 #2
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    def __getitem__(self, idx):
        img, anno = super(COCODataset, self).__getitem__(idx)

        # filter crowd annotations
        # TODO might be better to add an extra field
        anno = [obj for obj in anno if obj["iscrowd"] == 0]

        boxes = [obj["bbox"] for obj in anno]
        boxes = torch.as_tensor(boxes).reshape(-1, 4)  # guard against no boxes
        target = BoxList(boxes, img.size, mode="xywh").convert("xyxy")

        classes = [obj["category_id"] for obj in anno]
        classes = [self.json_category_id_to_contiguous_id[c] for c in classes]
        classes = torch.tensor(classes)
        target.add_field("labels", classes)

        masks = [obj["segmentation"] for obj in anno]
        masks = SegmentationMask(masks, img.size, mode='poly')
        target.add_field("masks", masks)

        if anno and "keypoints" in anno[0]:
            keypoints = [obj["keypoints"] for obj in anno]
            keypoints = PersonKeypoints(keypoints, img.size)
            target.add_field("keypoints", keypoints)

        target = target.clip_to_image(remove_empty=True)

        if self.transforms is not None:
            img, target = self.transforms(img, target)

        return img, target, idx
예제 #3
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    def forward_for_single_feature_map(self, anchors, objectness,
                                       box_regression):
        """
        Arguments:
            anchors: list[BoxList]
            objectness: tensor of size N, A, H, W
            box_regression: tensor of size N, A * 4, H, W
        """
        device = objectness.device
        N, A, H, W = objectness.shape

        # put in the same format as anchors
        objectness = permute_and_flatten(objectness, N, A, 1, H, W).view(N, -1)
        objectness = objectness.sigmoid()

        box_regression = permute_and_flatten(box_regression, N, A, 4, H, W)

        num_anchors = A * H * W

        pre_nms_top_n = min(self.pre_nms_top_n, num_anchors)
        objectness, topk_idx = objectness.topk(pre_nms_top_n,
                                               dim=1,
                                               sorted=True)

        batch_idx = torch.arange(N, device=device)[:, None]
        box_regression = box_regression[batch_idx, topk_idx]

        image_shapes = [box.size for box in anchors]
        concat_anchors = torch.cat([a.bbox for a in anchors], dim=0)
        concat_anchors = concat_anchors.reshape(N, -1, 4)[batch_idx, topk_idx]

        proposals = self.box_coder.decode(box_regression.view(-1, 4),
                                          concat_anchors.view(-1, 4))

        proposals = proposals.view(N, -1, 4)

        result = []
        for proposal, score, im_shape in zip(proposals, objectness,
                                             image_shapes):
            boxlist = BoxList(proposal, im_shape, mode="xyxy")
            boxlist.add_field("objectness", score)
            boxlist = boxlist.clip_to_image(remove_empty=False)
            boxlist = remove_small_boxes(boxlist, self.min_size)
            boxlist = boxlist_nms(
                boxlist,
                self.nms_thresh,
                max_proposals=self.post_nms_top_n,
                score_field="objectness",
            )
            result.append(boxlist)
        return result
예제 #4
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 def prepare_boxlist(self, boxes, scores, image_shape):
     """
     Returns BoxList from `boxes` and adds probability scores information
     as an extra field
     `boxes` has shape (#detections, 4 * #classes), where each row represents
     a list of predicted bounding boxes for each of the object classes in the
     dataset (including the background class). The detections in each row
     originate from the same object proposal.
     `scores` has shape (#detection, #classes), where each row represents a list
     of object detection confidence scores for each of the object classes in the
     dataset (including the background class). `scores[i, j]`` corresponds to the
     box at `boxes[i, j * 4:(j + 1) * 4]`.
     """
     boxes = boxes.reshape(-1, 4)
     scores = scores.reshape(-1)
     boxlist = BoxList(boxes, image_shape, mode="xyxy")
     boxlist.add_field("scores", scores)
     return boxlist
예제 #5
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    def __getitem__(self, item):
        img = Image.open(self.image_lists[item]).convert("RGB")

        # dummy target
        w, h = img.size
        target = BoxList([[0, 0, w, h]], img.size, mode="xyxy")

        if self.transforms is not None:
            img, target = self.transforms(img, target)

        return img, target
예제 #6
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    def forward(self, x, boxes):
        """
        Arguments:
            x (Tensor): the mask logits
            boxes (list[BoxList]): bounding boxes that are used as
                reference, one for ech image

        Returns:
            results (list[BoxList]): one BoxList for each image, containing
                the extra field mask
        """
        mask_prob = x.sigmoid()

        # select masks coresponding to the predicted classes
        num_masks = x.shape[0]
        labels = [bbox.get_field("labels") for bbox in boxes]
        labels = torch.cat(labels)
        index = torch.arange(num_masks, device=labels.device)
        mask_prob = mask_prob[index, labels][:, None]

        boxes_per_image = [len(box) for box in boxes]
        mask_prob = mask_prob.split(boxes_per_image, dim=0)

        if self.masker:
            mask_prob = self.masker(mask_prob, boxes)

        results = []
        for prob, box in zip(mask_prob, boxes):
            bbox = BoxList(box.bbox, box.size, mode="xyxy")
            for field in box.fields():
                bbox.add_field(field, box.get_field(field))
            bbox.add_field("mask", prob)
            results.append(bbox)

        return results
예제 #7
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 def forward(self, image_list, feature_maps):
     grid_sizes = [feature_map.shape[-2:] for feature_map in feature_maps]
     anchors_over_all_feature_maps = self.grid_anchors(grid_sizes)
     anchors = []
     for i, (image_height,
             image_width) in enumerate(image_list.image_sizes):
         anchors_in_image = []
         for anchors_per_feature_map in anchors_over_all_feature_maps:
             boxlist = BoxList(anchors_per_feature_map,
                               (image_width, image_height),
                               mode="xyxy")
             self.add_visibility_to(boxlist)
             anchors_in_image.append(boxlist)
         anchors.append(anchors_in_image)
     return anchors
예제 #8
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    def get_groundtruth(self, index):
        img_id = self.ids[index]
        anno = ET.parse(self._annopath % img_id).getroot()
        anno = self._preprocess_annotation(anno)

        height, width = anno["im_info"]
        target = BoxList(anno["boxes"], (width, height), mode="xyxy")
        target.add_field("labels", anno["labels"])
        target.add_field("difficult", anno["difficult"])
        return target
예제 #9
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    def select_over_all_levels(self, boxlists):
        num_images = len(boxlists)
        results = []
        for i in range(num_images):
            scores = boxlists[i].get_field("scores")
            labels = boxlists[i].get_field("labels")
            boxes = boxlists[i].bbox
            boxlist = boxlists[i]
            result = []
            # skip the background
            for j in range(1, self.num_classes):
                inds = (labels == j).nonzero().view(-1)

                scores_j = scores[inds]
                boxes_j = boxes[inds, :].view(-1, 4)
                boxlist_for_class = BoxList(boxes_j, boxlist.size, mode="xyxy")
                boxlist_for_class.add_field("scores", scores_j)
                boxlist_for_class = boxlist_nms(
                    boxlist_for_class, self.nms_thresh,
                    score_field="scores"
                )
                num_labels = len(boxlist_for_class)
                boxlist_for_class.add_field(
                    "labels", torch.full((num_labels,), j,
                                         dtype=torch.int64,
                                         device=scores.device)
                )
                result.append(boxlist_for_class)

            result = cat_boxlist(result)
            number_of_detections = len(result)

            # Limit to max_per_image detections **over all classes**
            if number_of_detections > self.fpn_post_nms_top_n > 0:
                cls_scores = result.get_field("scores")
                image_thresh, _ = torch.kthvalue(
                    cls_scores.cpu(),
                    number_of_detections - self.fpn_post_nms_top_n + 1
                )
                keep = cls_scores >= image_thresh.item()
                keep = torch.nonzero(keep).squeeze(1)
                result = result[keep]
            results.append(result)
        return results
예제 #10
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    def filter_results(self, boxlist, num_classes):
        """Returns bounding-box detection results by thresholding on scores and
        applying non-maximum suppression (NMS).
        """
        # unwrap the boxlist to avoid additional overhead.
        # if we had multi-class NMS, we could perform this directly on the boxlist
        boxes = boxlist.bbox.reshape(-1, num_classes * 4)
        scores = boxlist.get_field("scores").reshape(-1, num_classes)

        device = scores.device
        result = []
        # Apply threshold on detection probabilities and apply NMS
        # Skip j = 0, because it's the background class
        inds_all = scores > self.score_thresh
        for j in range(1, num_classes):
            inds = inds_all[:, j].nonzero().squeeze(1)
            scores_j = scores[inds, j]
            boxes_j = boxes[inds, j * 4:(j + 1) * 4]
            boxlist_for_class = BoxList(boxes_j, boxlist.size, mode="xyxy")
            boxlist_for_class.add_field("scores", scores_j)
            boxlist_for_class = boxlist_nms(boxlist_for_class, self.nms)
            num_labels = len(boxlist_for_class)
            boxlist_for_class.add_field(
                "labels",
                torch.full((num_labels, ), j, dtype=torch.int64,
                           device=device))
            result.append(boxlist_for_class)

        result = cat_boxlist(result)
        number_of_detections = len(result)

        # Limit to max_per_image detections **over all classes**
        if number_of_detections > self.detections_per_img > 0:
            cls_scores = result.get_field("scores")
            image_thresh, _ = torch.kthvalue(
                cls_scores.cpu(),
                number_of_detections - self.detections_per_img + 1)
            keep = cls_scores >= image_thresh.item()
            keep = torch.nonzero(keep).squeeze(1)
            result = result[keep]
        return result
예제 #11
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    def forward(self, x, boxes):
        mask_prob = x

        scores = None
        if self.keypointer:
            mask_prob, scores = self.keypointer(x, boxes)

        assert len(boxes) == 1, "Only non-batched inference supported for now"
        boxes_per_image = [box.bbox.size(0) for box in boxes]
        mask_prob = mask_prob.split(boxes_per_image, dim=0)
        scores = scores.split(boxes_per_image, dim=0)

        results = []
        for prob, box, score in zip(mask_prob, boxes, scores):
            bbox = BoxList(box.bbox, box.size, mode="xyxy")
            for field in box.fields():
                bbox.add_field(field, box.get_field(field))
            prob = PersonKeypoints(prob, box.size)
            prob.add_field("logits", score)
            bbox.add_field("keypoints", prob)
            results.append(bbox)

        return results
예제 #12
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def im_detect_bbox_aug(model, images, device):
    # Collect detections computed under different transformations
    boxlists_ts = []
    for _ in range(len(images)):
        boxlists_ts.append([])

    def add_preds_t(boxlists_t):
        for i, boxlist_t in enumerate(boxlists_t):
            if len(boxlists_ts[i]) == 0:
                # The first one is identity transform, no need to resize the boxlist
                boxlists_ts[i].append(boxlist_t)
            else:
                # Resize the boxlist as the first one
                boxlists_ts[i].append(boxlist_t.resize(boxlists_ts[i][0].size))

    # Compute detections for the original image (identity transform)
    boxlists_i = im_detect_bbox(model, images, cfg.INPUT.MIN_SIZE_TEST,
                                cfg.INPUT.MAX_SIZE_TEST, device)
    add_preds_t(boxlists_i)

    # Perform detection on the horizontally flipped image
    if cfg.TEST.BBOX_AUG.H_FLIP:
        boxlists_hf = im_detect_bbox_hflip(model, images,
                                           cfg.INPUT.MIN_SIZE_TEST,
                                           cfg.INPUT.MAX_SIZE_TEST, device)
        add_preds_t(boxlists_hf)

    # Compute detections at different scales
    for scale in cfg.TEST.BBOX_AUG.SCALES:
        max_size = cfg.TEST.BBOX_AUG.MAX_SIZE
        boxlists_scl = im_detect_bbox_scale(model, images, scale, max_size,
                                            device)
        add_preds_t(boxlists_scl)

        if cfg.TEST.BBOX_AUG.SCALE_H_FLIP:
            boxlists_scl_hf = im_detect_bbox_scale(model,
                                                   images,
                                                   scale,
                                                   max_size,
                                                   device,
                                                   hflip=True)
            add_preds_t(boxlists_scl_hf)

    # Merge boxlists detected by different bbox aug params
    boxlists = []
    for i, boxlist_ts in enumerate(boxlists_ts):
        bbox = torch.cat([boxlist_t.bbox for boxlist_t in boxlist_ts])
        scores = torch.cat(
            [boxlist_t.get_field('scores') for boxlist_t in boxlist_ts])
        boxlist = BoxList(bbox, boxlist_ts[0].size, boxlist_ts[0].mode)
        boxlist.add_field('scores', scores)
        boxlists.append(boxlist)

    # Apply NMS and limit the final detections
    results = []
    post_processor = make_roi_box_post_processor(cfg)
    for boxlist in boxlists:
        results.append(
            post_processor.filter_results(boxlist,
                                          cfg.MODEL.ROI_BOX_HEAD.NUM_CLASSES))

    return results
예제 #13
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def evaluate_box_proposals(
    predictions, dataset, thresholds=None, area="all", limit=None
):
    """Evaluate detection proposal recall metrics. This function is a much
    faster alternative to the official COCO API recall evaluation code. However,
    it produces slightly different results.
    """
    # Record max overlap value for each gt box
    # Return vector of overlap values
    areas = {
        "all": 0,
        "small": 1,
        "medium": 2,
        "large": 3,
        "96-128": 4,
        "128-256": 5,
        "256-512": 6,
        "512-inf": 7,
    }
    area_ranges = [
        [0 ** 2, 1e5 ** 2],  # all
        [0 ** 2, 32 ** 2],  # small
        [32 ** 2, 96 ** 2],  # medium
        [96 ** 2, 1e5 ** 2],  # large
        [96 ** 2, 128 ** 2],  # 96-128
        [128 ** 2, 256 ** 2],  # 128-256
        [256 ** 2, 512 ** 2],  # 256-512
        [512 ** 2, 1e5 ** 2],
    ]  # 512-inf
    assert area in areas, "Unknown area range: {}".format(area)
    area_range = area_ranges[areas[area]]
    gt_overlaps = []
    num_pos = 0

    for image_id, prediction in enumerate(predictions):
        original_id = dataset.id_to_img_map[image_id]

        img_info = dataset.get_img_info(image_id)
        image_width = img_info["width"]
        image_height = img_info["height"]
        prediction = prediction.resize((image_width, image_height))

        # sort predictions in descending order
        # TODO maybe remove this and make it explicit in the documentation
        inds = prediction.get_field("objectness").sort(descending=True)[1]
        prediction = prediction[inds]

        ann_ids = dataset.coco.getAnnIds(imgIds=original_id)
        anno = dataset.coco.loadAnns(ann_ids)
        gt_boxes = [obj["bbox"] for obj in anno if obj["iscrowd"] == 0]
        gt_boxes = torch.as_tensor(gt_boxes).reshape(-1, 4)  # guard against no boxes
        gt_boxes = BoxList(gt_boxes, (image_width, image_height), mode="xywh").convert(
            "xyxy"
        )
        gt_areas = torch.as_tensor([obj["area"] for obj in anno if obj["iscrowd"] == 0])

        if len(gt_boxes) == 0:
            continue

        valid_gt_inds = (gt_areas >= area_range[0]) & (gt_areas <= area_range[1])
        gt_boxes = gt_boxes[valid_gt_inds]

        num_pos += len(gt_boxes)

        if len(gt_boxes) == 0:
            continue

        if len(prediction) == 0:
            continue

        if limit is not None and len(prediction) > limit:
            prediction = prediction[:limit]

        overlaps = boxlist_iou(prediction, gt_boxes)

        _gt_overlaps = torch.zeros(len(gt_boxes))
        for j in range(min(len(prediction), len(gt_boxes))):
            # find which proposal box maximally covers each gt box
            # and get the iou amount of coverage for each gt box
            max_overlaps, argmax_overlaps = overlaps.max(dim=0)

            # find which gt box is 'best' covered (i.e. 'best' = most iou)
            gt_ovr, gt_ind = max_overlaps.max(dim=0)
            assert gt_ovr >= 0
            # find the proposal box that covers the best covered gt box
            box_ind = argmax_overlaps[gt_ind]
            # record the iou coverage of this gt box
            _gt_overlaps[j] = overlaps[box_ind, gt_ind]
            assert _gt_overlaps[j] == gt_ovr
            # mark the proposal box and the gt box as used
            overlaps[box_ind, :] = -1
            overlaps[:, gt_ind] = -1

        # append recorded iou coverage level
        gt_overlaps.append(_gt_overlaps)
    gt_overlaps = torch.cat(gt_overlaps, dim=0)
    gt_overlaps, _ = torch.sort(gt_overlaps)

    if thresholds is None:
        step = 0.05
        thresholds = torch.arange(0.5, 0.95 + 1e-5, step, dtype=torch.float32)
    recalls = torch.zeros_like(thresholds)
    # compute recall for each iou threshold
    for i, t in enumerate(thresholds):
        recalls[i] = (gt_overlaps >= t).float().sum() / float(num_pos)
    # ar = 2 * np.trapz(recalls, thresholds)
    ar = recalls.mean()
    return {
        "ar": ar,
        "recalls": recalls,
        "thresholds": thresholds,
        "gt_overlaps": gt_overlaps,
        "num_pos": num_pos,
    }
예제 #14
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    def predict_all(self, datas):
        '''

        :param datas: list[{'image_path':'hand/0000001.jpg'},{'image_path':'hand/0000002.jpg'} ... ]
        :return: outputs: list[np(1,24), np(1,24)...]
                img_size: list[224, 224]
        '''
        img_size = [512, 512]
        device = torch.device('cuda')
        net = torch.load(os.path.join(MODEL_PATH, 'last.pkl'))

        net = net.to(device)
        net = net.eval()

        outputs = []
        for data in datas:
            image_path = data['image_path']
            image = Image.open(os.path.join(DATA_PATH, image_path)).convert('RGB')

            target_fault = BoxList(torch.zeros(1,4), img_size)

            input, target_fault = self.transform(image, target_fault)
            input = input.to(device)
            input = to_image_list(input, 32)
            box_result = net(input)
            box_result = box_result[0].resize(img_size)
            labels = box_result.get_field('labels')
            scores = box_result.get_field('scores')

            i = 1
            last_result = [torch.tensor((0,0,512,512), device=device, dtype=torch.float) for i in range(6)]
            scores_temp = 0
            class_flag = [0, 0, 0, 0, 0, 0]

            for l, s, box in list(zip(labels.tolist(), scores.tolist(), box_result.bbox)):
                if i > 6:
                    break
                if l == i:
                    i = l + 1
                    last_result[l - 1] = box
                    class_flag[l - 1] = 1
                    scores_temp = s
                elif l > i:
                    last_result[l - 1] = box
                    class_flag[l - 1] = 1
                    i = l + 1
                elif l < i:
                    if s > scores_temp:
                        last_result[l - 1] = box
                    scores_temp = s

            if sum(class_flag) < 6:
                top_left = last_result[0][0:2]
                bottom_right = last_result[1][2:]
                for idx in range(6):
                    if class_flag[idx] < 1:
                        sys.stderr.write('{}\t'.format(idx))
                        # if idx == 4:
                        #     r_4 = (last_result[3] + last_result[5]) / 2
                        #     last_result[4] = r_4
                        # if idx == 5:
                        #     y1 = (top_left[1] + bottom_right[1]) / 2 - 20
                        #     y2 = y1 + 40
                        #     x1, x2 = top_left[0], top_left[0] + 75
                        #     last_result[5] = torch.tensor((x1, x2, y1, y2), device=device)
                sys.stderr.write('\n')


            last_result = torch.cat(last_result).detach().cpu().numpy()
            outputs.append(last_result[[2*i for i in range(12)] + [2*i+1 for i in range(12)]].reshape(1,24))

            # torch.cuda.empty_cache()

        return [outputs, img_size]
예제 #15
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def calc_detection_voc_prec_rec(gt_boxlists, pred_boxlists, iou_thresh=0.5):
    """Calculate precision and recall based on evaluation code of PASCAL VOC.
    This function calculates precision and recall of
    predicted bounding boxes obtained from a dataset which has :math:`N`
    images.
    The code is based on the evaluation code used in PASCAL VOC Challenge.
   """
    n_pos = defaultdict(int)
    score = defaultdict(list)
    match = defaultdict(list)
    for gt_boxlist, pred_boxlist in zip(gt_boxlists, pred_boxlists):
        pred_bbox = pred_boxlist.bbox.numpy()
        pred_label = pred_boxlist.get_field("labels").numpy()
        pred_score = pred_boxlist.get_field("scores").numpy()
        gt_bbox = gt_boxlist.bbox.numpy()
        gt_label = gt_boxlist.get_field("labels").numpy()
        gt_difficult = gt_boxlist.get_field("difficult").numpy()

        for l in np.unique(np.concatenate((pred_label, gt_label)).astype(int)):
            pred_mask_l = pred_label == l
            pred_bbox_l = pred_bbox[pred_mask_l]
            pred_score_l = pred_score[pred_mask_l]
            # sort by score
            order = pred_score_l.argsort()[::-1]
            pred_bbox_l = pred_bbox_l[order]
            pred_score_l = pred_score_l[order]

            gt_mask_l = gt_label == l
            gt_bbox_l = gt_bbox[gt_mask_l]
            gt_difficult_l = gt_difficult[gt_mask_l]

            n_pos[l] += np.logical_not(gt_difficult_l).sum()
            score[l].extend(pred_score_l)

            if len(pred_bbox_l) == 0:
                continue
            if len(gt_bbox_l) == 0:
                match[l].extend((0, ) * pred_bbox_l.shape[0])
                continue

            # VOC evaluation follows integer typed bounding boxes.
            pred_bbox_l = pred_bbox_l.copy()
            pred_bbox_l[:, 2:] += 1
            gt_bbox_l = gt_bbox_l.copy()
            gt_bbox_l[:, 2:] += 1
            iou = boxlist_iou(
                BoxList(pred_bbox_l, gt_boxlist.size),
                BoxList(gt_bbox_l, gt_boxlist.size),
            ).numpy()
            gt_index = iou.argmax(axis=1)
            # set -1 if there is no matching ground truth
            gt_index[iou.max(axis=1) < iou_thresh] = -1
            del iou

            selec = np.zeros(gt_bbox_l.shape[0], dtype=bool)
            for gt_idx in gt_index:
                if gt_idx >= 0:
                    if gt_difficult_l[gt_idx]:
                        match[l].append(-1)
                    else:
                        if not selec[gt_idx]:
                            match[l].append(1)
                        else:
                            match[l].append(0)
                    selec[gt_idx] = True
                else:
                    match[l].append(0)

    n_fg_class = max(n_pos.keys()) + 1
    prec = [None] * n_fg_class
    rec = [None] * n_fg_class

    for l in n_pos.keys():
        score_l = np.array(score[l])
        match_l = np.array(match[l], dtype=np.int8)

        order = score_l.argsort()[::-1]
        match_l = match_l[order]

        tp = np.cumsum(match_l == 1)
        fp = np.cumsum(match_l == 0)

        # If an element of fp + tp is 0,
        # the corresponding element of prec[l] is nan.
        prec[l] = tp / (fp + tp)
        # If n_pos[l] is 0, rec[l] is None.
        if n_pos[l] > 0:
            rec[l] = tp / n_pos[l]

    return prec, rec
예제 #16
0
    def forward_for_single_feature_map(
            self, anchors, box_cls, box_regression):
        """
        Arguments:
            anchors: list[BoxList]
            box_cls: tensor of size N, A * C, H, W
            box_regression: tensor of size N, A * 4, H, W
        """
        device = box_cls.device
        N, _, H, W = box_cls.shape
        A = box_regression.size(1) // 4
        C = box_cls.size(1) // A

        # put in the same format as anchors
        box_cls = permute_and_flatten(box_cls, N, A, C, H, W)
        box_cls = box_cls.sigmoid()

        box_regression = permute_and_flatten(box_regression, N, A, 4, H, W)
        box_regression = box_regression.reshape(N, -1, 4)

        num_anchors = A * H * W

        candidate_inds = box_cls > self.pre_nms_thresh

        pre_nms_top_n = candidate_inds.view(N, -1).sum(1)
        pre_nms_top_n = pre_nms_top_n.clamp(max=self.pre_nms_top_n)

        results = []
        for per_box_cls, per_box_regression, per_pre_nms_top_n, \
        per_candidate_inds, per_anchors in zip(
            box_cls,
            box_regression,
            pre_nms_top_n,
            candidate_inds,
            anchors):

            # Sort and select TopN
            # TODO most of this can be made out of the loop for
            # all images. 
            # TODO:Yang: Not easy to do. Because the numbers of detections are
            # different in each image. Therefore, this part needs to be done
            # per image. 
            per_box_cls = per_box_cls[per_candidate_inds]
 
            per_box_cls, top_k_indices = \
                    per_box_cls.topk(per_pre_nms_top_n, sorted=False)

            per_candidate_nonzeros = \
                    per_candidate_inds.nonzero()[top_k_indices, :]

            per_box_loc = per_candidate_nonzeros[:, 0]
            per_class = per_candidate_nonzeros[:, 1]
            per_class += 1

            detections = self.box_coder.decode(
                per_box_regression[per_box_loc, :].view(-1, 4),
                per_anchors.bbox[per_box_loc, :].view(-1, 4)
            )

            boxlist = BoxList(detections, per_anchors.size, mode="xyxy")
            boxlist.add_field("labels", per_class)
            boxlist.add_field("scores", per_box_cls)
            boxlist = boxlist.clip_to_image(remove_empty=False)
            boxlist = remove_small_boxes(boxlist, self.min_size)
            results.append(boxlist)

        return results