py-faster-rcnn详解（2）——pascal _voc.py接口说明

imdb对象是一个pascol _voc的类实例，该类继承自imdb，用于数据交互。

初始化函数

在初始化自身的同时，先调用了父类的初始化方法，将imdb _name传入，例如(‘voc_2007_trainval’)

  class pascal _voc(imdb):  def __init__(self, image_set, year, devkit_path=None):      imdb.__init__(self, 'voc_' + year + '_' + image_set)      self._year = year      self._image_set = image_set      self._devkit_path = self._get_default_path() if devkit_path is None                            else devkit_path      self._data_path = os.path.join(self._devkit_path, 'VOC' + self._year)      self._classes = ('__background__', # always index 0                       'aeroplane', 'bicycle', 'bird', 'boat',                       'bottle', 'bus', 'car', 'cat', 'chair',                       'cow', 'diningtable', 'dog', 'horse',                       'motorbike', 'person', 'pottedplant',                       'sheep', 'sofa', 'train', 'tvmonitor')      self._class_to_ind = dict(zip(self.classes, xrange(self.num_classes)))      self._image_ext = '.jpg'      self._image_index = self._load_image_set_index()      # Default to roidb handler      self._roidb_handler = self.selective_search_roidb      self._salt = str(uuid.uuid4())      self._comp_id = 'comp4'      # PASCAL specific config options      self.config = {'cleanup'     : True,                     'use_salt'    : True,                     'use_diff'    : False,                     'matlab_eval' : False,                     'rpn_file'    : None,                     'min_size'    : 2}      assert os.path.exists(self._devkit_path),                'VOCdevkit path does not exist: {}'.format(self._devkit_path)      assert os.path.exists(self._data_path),                'Path does not exist: {}'.format(self._data_path)
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image _path _from _index

根据图片的索引，比如‘000001’获取在JPEGImages下对应的图片路径

  def image_path_at(self, i):      """      Return the absolute path to image i in the image sequence.      """      return self.image_path_from_index(self._image_index[i])  def image_path_from_index(self, index):      """      Construct an image path from the image's "index" identifier.      """      image_path = os.path.join(self._data_path, 'JPEGImages',                                index + self._image_ext)      assert os.path.exists(image_path),                'Path does not exist: {}'.format(image_path)      return image_path# load _image _set _index# 该函数根据/VOCdevkit2007/VOC2007/ImageSets/Main/ <image _set >.txt加载图像的索引  def _load_image_set_index(self):      """      Load the indexes listed in this dataset's image set file.      """      # Example path to image set file:      # self._devkit_path + /VOCdevkit2007/VOC2007/ImageSets/Main/val.txt      image_set_file = os.path.join(self._data_path, 'ImageSets', 'Main',                                    self._image_set + '.txt')      assert os.path.exists(image_set_file),                'Path does not exist: {}'.format(image_set_file)      with open(image_set_file) as f:          image_index = [x.strip() for x in f.readlines()]      return image_index
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_get _default _path

返回默认的数据源路径，这里是放在data下的VOCDevkit2007

 def _get_default_path(self):      """      Return the default path where PASCAL VOC is expected to be installed.      """      return os.path.join(cfg.DATA_DIR, 'VOCdevkit' + self._year)
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gt _roidb

这个函数返回roidb数据对象。首先它会在cache路径下找到以扩展名’.pkl’结尾的缓存，这个文件是通过cPickle工具将roidb序列化存储的。如果该文件存在，那么它会先读取这里的内容，以提高效率(所以如果你换数据集的时候，要先把cache文件给删除，否则会造成错误)。接着，它将调用 _load _pascal _annotation这个私有函数加载roidb中的数据，并将其保存在缓存文件中，返回roidb。

  def gt_roidb(self):      """      Return the database of ground-truth regions of interest.      This function loads/saves from/to a cache file to speed up future calls.      """      cache_file = os.path.join(self.cache_path, self.name + '_gt_roidb.pkl')      if os.path.exists(cache_file):          with open(cache_file, 'rb') as fid:              roidb = cPickle.load(fid)          print '{} gt roidb loaded from {}'.format(self.name, cache_file)          return roidb      gt_roidb = [self._load_pascal_annotation(index)                  for index in self.image_index]      with open(cache_file, 'wb') as fid:          cPickle.dump(gt_roidb, fid, cPickle.HIGHEST_PROTOCOL)      print 'wrote gt roidb to {}'.format(cache_file)      return gt_roidb
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_load _pascal _annotation

  根据每个图像的索引，到Annotations这个文件夹下去找相应的xml标注数据，然后加载所有的bounding box对象。最后将这些成员变量组装成roidb返回

def _load_pascal_annotation(self, index):      """      Load image and bounding boxes info from XML file in the PASCAL VOC      format.      """      filename = os.path.join(self._data_path, 'Annotations', index + '.xml')      tree = ET.parse(filename)      objs = tree.findall('object')      if not self.config['use_diff']:          # Exclude the samples labeled as difficult          non_diff_objs = [              obj for obj in objs if int(obj.find('difficult').text) == 0]          # if len(non_diff_objs) != len(objs):          #     print 'Removed {} difficult objects'.format(          #         len(objs) - len(non_diff_objs))          objs = non_diff_objs      num_objs = len(objs)      boxes = np.zeros((num_objs, 4), dtype=np.uint16)      gt_classes = np.zeros((num_objs), dtype=np.int32)      overlaps = np.zeros((num_objs, self.num_classes), dtype=np.float32)      # "Seg" area for pascal is just the box area      seg_areas = np.zeros((num_objs), dtype=np.float32)      # Load object bounding boxes into a data frame.      for ix, obj in enumerate(objs):          bbox = obj.find('bndbox')          # Make pixel indexes 0-based          x1 = float(bbox.find('xmin').text) - 1          y1 = float(bbox.find('ymin').text) - 1          x2 = float(bbox.find('xmax').text) - 1          y2 = float(bbox.find('ymax').text) - 1          cls = self._class_to_ind[obj.find('name').text.lower().strip()]          boxes[ix, :] = [x1, y1, x2, y2]          gt_classes[ix] = cls          overlaps[ix, cls] = 1.0          seg_areas[ix] = (x2 - x1 + 1) * (y2 - y1 + 1)      overlaps = scipy.sparse.csr_matrix(overlaps)      return {'boxes' : boxes,              'gt_classes': gt_classes,              'gt_overlaps' : overlaps,              'flipped' : False,              'seg_areas' : seg_areas}
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rpn _roidb

  经过RPN网络产生了proposal以后，这个函数作用是将这些proposal 的 roi与groudtruth结合起来，送入网络训练。最后用merge _roidbs将gr _roidb与rpn _roidb合并输出。

def rpn_roidb(self):          if int(self._year) == 2007 or self._image_set != 'test':              gt_roidb = self.gt_roidb()              rpn_roidb = self._load_rpn_roidb(gt_roidb)              roidb = imdb.merge_roidbs(gt_roidb, rpn_roidb)          else:              roidb = self._load_rpn_roidb(None)          return roidb  def _load_rpn_roidb(self, gt_roidb):          filename = self.config['rpn_file']          print 'loading {}'.format(filename)          assert os.path.exists(filename),                   'rpn data not found at: {}'.format(filename)          with open(filename, 'rb') as f:              box_list = cPickle.load(f)          return self.create_roidb_from_box_list(box_list, gt_roidb) 
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