official/legacy/detection/ops/nms.py
# Copyright 2024 The TensorFlow Authors. All Rights Reserved.
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# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
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# http://www.apache.org/licenses/LICENSE-2.0
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# distributed under the License is distributed on an "AS IS" BASIS,
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"""Tensorflow implementation of non max suppression."""
from __future__ import absolute_import
from __future__ import division
from __future__ import print_function
import tensorflow as tf, tf_keras
from official.legacy.detection.utils import box_utils
NMS_TILE_SIZE = 512
def _self_suppression(iou, _, iou_sum):
batch_size = tf.shape(iou)[0]
can_suppress_others = tf.cast(
tf.reshape(tf.reduce_max(iou, 1) <= 0.5, [batch_size, -1, 1]), iou.dtype)
iou_suppressed = tf.reshape(
tf.cast(tf.reduce_max(can_suppress_others * iou, 1) <= 0.5, iou.dtype),
[batch_size, -1, 1]) * iou
iou_sum_new = tf.reduce_sum(iou_suppressed, [1, 2])
return [
iou_suppressed,
tf.reduce_any(iou_sum - iou_sum_new > 0.5), iou_sum_new
]
def _cross_suppression(boxes, box_slice, iou_threshold, inner_idx):
batch_size = tf.shape(boxes)[0]
new_slice = tf.slice(boxes, [0, inner_idx * NMS_TILE_SIZE, 0],
[batch_size, NMS_TILE_SIZE, 4])
iou = box_utils.bbox_overlap(new_slice, box_slice)
ret_slice = tf.expand_dims(
tf.cast(tf.reduce_all(iou < iou_threshold, [1]), box_slice.dtype),
2) * box_slice
return boxes, ret_slice, iou_threshold, inner_idx + 1
def _suppression_loop_body(boxes, iou_threshold, output_size, idx):
"""Process boxes in the range [idx*NMS_TILE_SIZE, (idx+1)*NMS_TILE_SIZE).
Args:
boxes: a tensor with a shape of [batch_size, anchors, 4].
iou_threshold: a float representing the threshold for deciding whether boxes
overlap too much with respect to IOU.
output_size: an int32 tensor of size [batch_size]. Representing the number
of selected boxes for each batch.
idx: an integer scalar representing induction variable.
Returns:
boxes: updated boxes.
iou_threshold: pass down iou_threshold to the next iteration.
output_size: the updated output_size.
idx: the updated induction variable.
"""
boxes_shape = tf.shape(boxes)
num_tiles = boxes_shape[1] // NMS_TILE_SIZE
batch_size = boxes_shape[0]
# Iterates over tiles that can possibly suppress the current tile.
box_slice = tf.slice(boxes, [0, idx * NMS_TILE_SIZE, 0],
[batch_size, NMS_TILE_SIZE, 4])
_, box_slice, _, _ = tf.while_loop(
lambda _boxes, _box_slice, _threshold, inner_idx: inner_idx < idx,
_cross_suppression, [boxes, box_slice, iou_threshold,
tf.constant(0)])
# Iterates over the current tile to compute self-suppression.
iou = box_utils.bbox_overlap(box_slice, box_slice)
mask = tf.expand_dims(
tf.reshape(tf.range(NMS_TILE_SIZE), [1, -1]) > tf.reshape(
tf.range(NMS_TILE_SIZE), [-1, 1]), 0)
iou *= tf.cast(tf.logical_and(mask, iou >= iou_threshold), iou.dtype)
suppressed_iou, _, _ = tf.while_loop(
lambda _iou, loop_condition, _iou_sum: loop_condition, _self_suppression,
[iou, tf.constant(True),
tf.reduce_sum(iou, [1, 2])])
suppressed_box = tf.reduce_sum(suppressed_iou, 1) > 0
box_slice *= tf.expand_dims(1.0 - tf.cast(suppressed_box, box_slice.dtype), 2)
# Uses box_slice to update the input boxes.
mask = tf.reshape(
tf.cast(tf.equal(tf.range(num_tiles), idx), boxes.dtype), [1, -1, 1, 1])
boxes = tf.tile(tf.expand_dims(
box_slice, [1]), [1, num_tiles, 1, 1]) * mask + tf.reshape(
boxes, [batch_size, num_tiles, NMS_TILE_SIZE, 4]) * (1 - mask)
boxes = tf.reshape(boxes, boxes_shape)
# Updates output_size.
output_size += tf.reduce_sum(
tf.cast(tf.reduce_any(box_slice > 0, [2]), tf.int32), [1])
return boxes, iou_threshold, output_size, idx + 1
def sorted_non_max_suppression_padded(scores, boxes, max_output_size,
iou_threshold):
"""A wrapper that handles non-maximum suppression.
Assumption:
* The boxes are sorted by scores unless the box is a dot (all coordinates
are zero).
* Boxes with higher scores can be used to suppress boxes with lower scores.
The overal design of the algorithm is to handle boxes tile-by-tile:
boxes = boxes.pad_to_multiply_of(tile_size)
num_tiles = len(boxes) // tile_size
output_boxes = []
for i in range(num_tiles):
box_tile = boxes[i*tile_size : (i+1)*tile_size]
for j in range(i - 1):
suppressing_tile = boxes[j*tile_size : (j+1)*tile_size]
iou = bbox_overlap(box_tile, suppressing_tile)
# if the box is suppressed in iou, clear it to a dot
box_tile *= _update_boxes(iou)
# Iteratively handle the diagnal tile.
iou = _box_overlap(box_tile, box_tile)
iou_changed = True
while iou_changed:
# boxes that are not suppressed by anything else
suppressing_boxes = _get_suppressing_boxes(iou)
# boxes that are suppressed by suppressing_boxes
suppressed_boxes = _get_suppressed_boxes(iou, suppressing_boxes)
# clear iou to 0 for boxes that are suppressed, as they cannot be used
# to suppress other boxes any more
new_iou = _clear_iou(iou, suppressed_boxes)
iou_changed = (new_iou != iou)
iou = new_iou
# remaining boxes that can still suppress others, are selected boxes.
output_boxes.append(_get_suppressing_boxes(iou))
if len(output_boxes) >= max_output_size:
break
Args:
scores: a tensor with a shape of [batch_size, anchors].
boxes: a tensor with a shape of [batch_size, anchors, 4].
max_output_size: a scalar integer `Tensor` representing the maximum number
of boxes to be selected by non max suppression.
iou_threshold: a float representing the threshold for deciding whether boxes
overlap too much with respect to IOU.
Returns:
nms_scores: a tensor with a shape of [batch_size, anchors]. It has same
dtype as input scores.
nms_proposals: a tensor with a shape of [batch_size, anchors, 4]. It has
same dtype as input boxes.
"""
batch_size = tf.shape(boxes)[0]
num_boxes = tf.shape(boxes)[1]
pad = tf.cast(
tf.math.ceil(tf.cast(num_boxes, tf.float32) / NMS_TILE_SIZE),
tf.int32) * NMS_TILE_SIZE - num_boxes
boxes = tf.pad(tf.cast(boxes, tf.float32), [[0, 0], [0, pad], [0, 0]])
scores = tf.pad(
tf.cast(scores, tf.float32), [[0, 0], [0, pad]], constant_values=-1)
num_boxes += pad
def _loop_cond(unused_boxes, unused_threshold, output_size, idx):
return tf.logical_and(
tf.reduce_min(output_size) < max_output_size,
idx < num_boxes // NMS_TILE_SIZE)
selected_boxes, _, output_size, _ = tf.while_loop(
_loop_cond, _suppression_loop_body,
[boxes, iou_threshold,
tf.zeros([batch_size], tf.int32),
tf.constant(0)])
idx = num_boxes - tf.cast(
tf.nn.top_k(
tf.cast(tf.reduce_any(selected_boxes > 0, [2]), tf.int32) *
tf.expand_dims(tf.range(num_boxes, 0, -1), 0), max_output_size)[0],
tf.int32)
idx = tf.minimum(idx, num_boxes - 1)
idx = tf.reshape(idx + tf.reshape(tf.range(batch_size) * num_boxes, [-1, 1]),
[-1])
boxes = tf.reshape(
tf.gather(tf.reshape(boxes, [-1, 4]), idx),
[batch_size, max_output_size, 4])
boxes = boxes * tf.cast(
tf.reshape(tf.range(max_output_size), [1, -1, 1]) < tf.reshape(
output_size, [-1, 1, 1]), boxes.dtype)
scores = tf.reshape(
tf.gather(tf.reshape(scores, [-1, 1]), idx),
[batch_size, max_output_size])
scores = scores * tf.cast(
tf.reshape(tf.range(max_output_size), [1, -1]) < tf.reshape(
output_size, [-1, 1]), scores.dtype)
return scores, boxes