COCO数据集深入理解

• TensorExpand/TensorExpand/Object detection/Data_interface/MSCOCO/
• 深度学习数据集介绍及相互转换

• Pascal VOC数据格式转COCO数据格式脚本（Object Detection）

    Object segmentation
    Recognition in context
    Superpixel stuff segmentation
    330K images (>200K labeled)
    1.5 million object instances
     object categories
     stuff categories
     captions per image
    , people with keypoints

1. 对stuff任务：118282（118K）训练，5k验证

2. 对instance任务：118k训练，instances_minival2014.json（5k）测试

3. 全景分割任务：40890（40k）训练，5k测试

数据格式

All annotations share the same basic data structure below:

{
"info": info,
"images": [image],
"annotations": [annotation],
"licenses": [license],
}

2. Stuff Segmentation

The stuff annotation format is identical and fully compatible to the object detection format above (except iscrowd is unnecessary and set to 0 by default). We provide annotations in both JSON and png format for easier access, as well as conversion scripts between the two formats. In the JSON format, each category present in an image is encoded with a single RLE annotation (see the Mask API for more details). The category_id represents the id of the current stuff category. For more details on stuff categories and supercategories see the stuff evaluation page. See also the stuff task.

注： instance和stuff任务都categorise没有isthing和color字段；

For a class-aware detector, if you feed it an image, it will return a set of bounding boxes, each box associated with the class of the object inside (i.e. dog, cat, car). It means that by the time the detector finished detecting, it knows what type of object was detected.

For class-agnostic detector, it detects a bunch of objects without knowing what class they belong to. To put it simply, they only detect “foreground” objects. Foreground is a broad term, but usually it is a set that contains all specific classes we want to find in an image, i.e. foreground = {cat, dog, car, airplane, …}. Since it doesn’t know the class of the object it detected, we call it class-agnostic.

Class-agnostic detectors are often used as a pre-processor: to produce a bunch of interesting bounding boxes that have a high chance of containing cat, dog, car, etc. Obviously, we need a specialized classifier after a class-agnostic detector to actually know what class each bounding box contains

COCO api： coco.py

def __init__(self, annotation_file=None):
        """
        Constructor of Microsoft COCO helper class for reading and visualizing annotations.
        :param annotation_file (str): location of annotation file
        :param image_folder (str): location to the folder that hosts images.
        :return:
        """
        # load dataset
        self.dataset,self.anns,self.cats,self.imgs = dict(),dict(),dict(),dict()
        self.imgToAnns, self.catToImgs = defaultdict(list), defaultdict(list)
        if not annotation_file == None:
            print('loading annotations into memory...')
            tic = time.time()
            dataset = json.load(open(annotation_file, 'r')) # 加载进内存
            assert type(dataset)==dict, 'annotation file format {} not supported'.format(type(dataset))
            print('Done (t={:0.2f}s)'.format(time.time()- tic))
            self.dataset = dataset
            self.createIndex()

    def createIndex(self):
        # create index
        print('creating index...')
        anns, cats, imgs = {}, {}, {} # 这几个都是根据字段一一对应，没有重复
        imgToAnns,catToImgs = defaultdict(list),defaultdict(list)
        if 'annotations' in self.dataset:
            for ann in self.dataset['annotations']:
                imgToAnns[ann['image_id']].append(ann) # 同一image_id可能有很多标注框
                anns[ann['id']] = ann

        if 'images' in self.dataset:
            for img in self.dataset['images']:
                imgs[img['id']] = img

        if 'categories' in self.dataset:
            for cat in self.dataset['categories']:
                cats[cat['id']] = cat

        if 'annotations' in self.dataset and 'categories' in self.dataset:
            for ann in self.dataset['annotations']:
                catToImgs[ann['category_id']].append(ann['image_id']) # 将有某一种类别标注-->和所有image_id对应

        print('index created!')

        # create class members
        self.anns = anns
        self.imgToAnns = imgToAnns
        self.catToImgs = catToImgs
        self.imgs = imgs
        self.cats = cats

- 主要是实例化一个cocco对象，利用json文件初始化各种对应关系：其中图像，标注，类别id都唯一建立映射；图像id->标注,类别id->图像，存在一对多映射；

- 在通过一些其他接口处理数据

# The following API functions are defined:
#  COCO       - COCO api class that loads COCO annotation file and prepare data structures.
#  decodeMask - Decode binary mask M encoded via run-length encoding.
#  encodeMask - Encode binary mask M using run-length encoding.
#  getAnnIds  - Get ann ids that satisfy given filter conditions. #annotations
#  getCatIds  - Get cat ids that satisfy given filter conditions. #category
#  getImgIds  - Get img ids that satisfy given filter conditions.
#  loadAnns   - Load anns with the specified ids.
#  loadCats   - Load cats with the specified ids.
#  loadImgs   - Load imgs with the specified ids.
#  annToMask  - Convert segmentation in an annotation to binary mask.
#  showAnns   - Display the specified annotations.
#  loadRes    - Load algorithm results and create API for accessing them.
#  download   - Download COCO images from mscoco.org server.

- 其中loadRes将训练结果转换为coco对象（json格式）；

网络输出：

[{"image_id": 139, "category_id": 1, "bbox": [418.3974914550781, 159.67330932617188, 47.4359130859375, 137.63726806640625], "score": 0.9947304725646973}, ...]

[{"image_id": 139, "category_id": 1, "segmentation": {"size": [426, 640], "counts": "cia53R==kCEj:a0mDFP;c0cDC[;X1N1O1O2N2N2N4L3M2N1O0110107YE`ML0o9Y3K5K0O3M10O0O2O1N1O2N4L5K5XNmEOY:CVF6R:^OWF=m9]O[F=g9_OdF7a9CURY2"}, "score": 0.9947304725646973}, ...]

MS COCO数据集目标检测评估（Detection Evaluation）

- 调用方法：cocoGt,cocoDt分别为coco对象

# running evaluation
cocoEval = COCOeval(cocoGt,cocoDt,annType)
cocoEval.evaluate()
cocoEval.accumulate()
cocoEval.summarize()

- cocoeval.py:

# per image per category establish dict with IOU(gt,pre)

- 调试理解：

class COCOeval:
    # Interface for evaluating detection on the Microsoft COCO dataset.
    #
    # The usage for CocoEval is as follows:
    #  cocoGt=..., cocoDt=...       # load dataset and results
    #  E = CocoEval(cocoGt,cocoDt); # initialize CocoEval object
    #  E.params.recThrs = ...;      # set parameters as desired
    #  E.evaluate();                # run per image evaluation
    #  E.accumulate();              # accumulate per image results
    #  E.summarize();               # display summary metrics of results
    # For example usage see evalDemo.m and http://mscoco.org/.
    #
    # The evaluation parameters are as follows (defaults in brackets):
    #  imgIds     - [all] N img ids to use for evaluation
    #  catIds     - [all] K cat ids to use for evaluation
    #  iouThrs    - [.5:.05:.95] T=10 IoU thresholds for evaluation
    #  recThrs    - [0:.01:1] R=101 recall thresholds for evaluation
    #  areaRng    - [...] A=4 object area ranges for evaluation
    #  maxDets    - [1 10 100] M=3 thresholds on max detections per image
    #  iouType    - ['segm'] set iouType to 'segm', 'bbox' or 'keypoints'
    #  iouType replaced the now DEPRECATED useSegm parameter.
    #  useCats    - [1] if true use category labels for evaluation
    # Note: if useCats=0 category labels are ignored as in proposal scoring.
    # Note: multiple areaRngs [Ax2] and maxDets [Mx1] can be specified.
    #
    # evaluate(): evaluates detections on every image and every category and
    # concats the results into the "evalImgs" with fields:
    #  dtIds      - [1xD] id for each of the D detections (dt)
    #  gtIds      - [1xG] id for each of the G ground truths (gt)
    #  dtMatches  - [TxD] matching gt id at each IoU or 0
    #  gtMatches  - [TxG] matching dt id at each IoU or 0
    #  dtScores   - [1xD] confidence of each dt
    #  gtIgnore   - [1xG] ignore flag for each gt
    #  dtIgnore   - [TxD] ignore flag for each dt at each IoU
    #
    # accumulate(): accumulates the per-image, per-category evaluation
    # results in "evalImgs" into the dictionary "eval" with fields:
    #  params     - parameters used for evaluation
    #  date       - date evaluation was performed
    #  counts     - [T,R,K,A,M] parameter dimensions (see above)
    #  precision  - [TxRxKxAxM] precision for every evaluation setting
    #  recall     - [TxKxAxM] max recall for every evaluation setting
    # Note: precision and recall==-1 for settings with no gt objects.
    #
    # See also coco, mask, pycocoDemo, pycocoEvalDemo
    #
    # Microsoft COCO Toolbox.      version 2.0
    # Data, paper, and tutorials available at:  http://mscoco.org/
    # Code written by Piotr Dollar and Tsung-Yi Lin, 2015.
    # Licensed under the Simplified BSD License [see coco/license.txt]
    def __init__(self, cocoGt=None, cocoDt=None, iouType='segm'):
        '''
        Initialize CocoEval using coco APIs for gt and dt
        :param cocoGt: coco object with ground truth annotations
        :param cocoDt: coco object with detection results
        :return: None
        '''
        if not iouType:
            print('iouType not specified. use default iouType segm')
        self.cocoGt   = cocoGt              # ground truth COCO API
        self.cocoDt   = cocoDt              # detections COCO API
        self.params   = {}                  # evaluation parameters
        self.evalImgs = defaultdict(list)   # per-image per-category evaluation results [KxAxI] elements
        self.eval     = {}                  # accumulated evaluation results
        self._gts = defaultdict(list)       # gt for evaluation
        self._dts = defaultdict(list)       # dt for evaluation
        self.params = Params(iouType=iouType) # parameters
        self._paramsEval = {}               # parameters for evaluation
        self.stats = []                     # result summarization
        self.ious = {}                      # ious between all gts and dts
        if not cocoGt is None:
            self.params.imgIds = sorted(cocoGt.getImgIds())
            self.params.catIds = sorted(cocoGt.getCatIds())