multiple_grid_anchor_generator.py 文件源码

def create_ssd_anchors(num_layers=6,
                       min_scale=0.2,
                       max_scale=0.95,
                       aspect_ratios=(1.0, 2.0, 3.0, 1.0/2, 1.0/3),
                       base_anchor_size=None,
                       reduce_boxes_in_lowest_layer=True):
  """Creates MultipleGridAnchorGenerator for SSD anchors.

  This function instantiates a MultipleGridAnchorGenerator that reproduces
  ``default box`` construction proposed by Liu et al in the SSD paper.
  See Section 2.2 for details. Grid sizes are assumed to be passed in
  at generation time from finest resolution to coarsest resolution --- this is
  used to (linearly) interpolate scales of anchor boxes corresponding to the
  intermediate grid sizes.

  Anchors that are returned by calling the `generate` method on the returned
  MultipleGridAnchorGenerator object are always in normalized coordinates
  and clipped to the unit square: (i.e. all coordinates lie in [0, 1]x[0, 1]).

  Args:
    num_layers: integer number of grid layers to create anchors for (actual
      grid sizes passed in at generation time)
    min_scale: scale of anchors corresponding to finest resolution (float)
    max_scale: scale of anchors corresponding to coarsest resolution (float)
    aspect_ratios: list or tuple of (float) aspect ratios to place on each
      grid point.
    base_anchor_size: base anchor size as [height, width].
    reduce_boxes_in_lowest_layer: a boolean to indicate whether the fixed 3
      boxes per location is used in the lowest layer.

  Returns:
    a MultipleGridAnchorGenerator
  """
  if base_anchor_size is None:
    base_anchor_size = [1.0, 1.0]
  base_anchor_size = tf.constant(base_anchor_size, dtype=tf.float32)
  box_specs_list = []
  scales = [min_scale + (max_scale - min_scale) * i / (num_layers - 1)
            for i in range(num_layers)] + [1.0]
  for layer, scale, scale_next in zip(
      range(num_layers), scales[:-1], scales[1:]):
    layer_box_specs = []
    if layer == 0 and reduce_boxes_in_lowest_layer:
      layer_box_specs = [(0.1, 1.0), (scale, 2.0), (scale, 0.5)]
    else:
      for aspect_ratio in aspect_ratios:
        layer_box_specs.append((scale, aspect_ratio))
        if aspect_ratio == 1.0:
          layer_box_specs.append((np.sqrt(scale*scale_next), 1.0))
    box_specs_list.append(layer_box_specs)
  return MultipleGridAnchorGenerator(box_specs_list, base_anchor_size)