Car Detection With YOLOv2
Today our task is car detection. We use the technique 'yo...

10
2018/07

# Car Detection With YOLOv2

Today our task is car detection. We use the technique 'you look only once' to detect the cars and localize them.

import argparse
import os
import matplotlib.pyplot as plt
from matplotlib.pyplot import imshow
import scipy.io
import scipy.misc
import numpy as np
import pandas as pd
import PIL
import tensorflow as tf
from keras import backend as K
from keras.layers import Input, Lambda, Conv2D

%matplotlib inline

def yolo_filter_boxes(box_confidence, boxes, box_class_probs, threshold = .6):
"""Filters YOLO boxes by thresholding on object and class confidence.

Arguments:
box_confidence -- tensor of shape (19, 19, 5, 1)
boxes -- tensor of shape (19, 19, 5, 4)
box_class_probs -- tensor of shape (19, 19, 5, 80)
threshold -- real value, if [ highest class probability score < threshold], then get rid of the corresponding box

Returns：
scores -- tensor of shape (None,), containing the class probability score for selected boxes
boxes -- tensor of shape (None, 4), containing (b_x, b_y, b_h, b_w) coordinates of selected boxes
classes -- tensor of shape (None,), containing the index of thse class detected by the selected boxes

Note: "None" is here because you don't know the exact number of selected boxes, as it depends on the threshold.
For example, the actual output size of scores would be (10,) if there are 10 boxes.
"""

# Step 1: Compute box scores
### START CODE HERE ### (≈ 1 line)
box_scores = box_confidence*box_class_probs
### END CODE HERE ###

# Step 2: Find the box_classes thanks to the max box_scores, keep track of the corresponding score
### START CODE HERE ### (≈ 2 lines)
box_classes = K.argmax(box_scores,axis = -1)
#print(box_classes.shape)
#不能用keepdims = True
box_class_scores = K.max(box_scores,axis = -1)
#print(box_class_scores.shape)
### END CODE HERE ###

# Step 3: Create a filtering mask based on "box_class_scores" by using "threshold". The mask should have the
# same dimension as box_class_scores, and be True for the boxes you want to keep (with probability >= threshold)
### START CODE HERE ### (≈ 1 line)
### END CODE HERE ###

# Step 4: Apply the mask to scores, boxes and classes
### START CODE HERE ### (≈ 3 lines)
### END CODE HERE ###

return scores, boxes, classes

#Use the non-max-suppression to filter out the boxes that detect the same car
def iou(box1, box2):
"""Implement the intersection over union (IoU) between box1 and box2

Arguments:
box1 -- first box, list object with coordinates (x1, y1, x2, y2)
box2 -- second box, list object with coordinates (x1, y1, x2, y2)
"""

# Calculate the (y1, x1, y2, x2) coordinates of the intersection of box1 and box2. Calculate its Area.
### START CODE HERE ### (≈ 5 lines)
xi1 = max(box1[0],box2[0])
yi1 = max(box1[1],box2[1])
xi2 = min(box1[2],box2[2])
yi2 = min(box1[3],box2[3])
inter_area = max(xi2-xi1,0)*max(yi2-yi1,0)
### END CODE HERE ###

# Calculate the Union area by using Formula: Union(A,B) = A + B - Inter(A,B)
### START CODE HERE ### (≈ 3 lines)
box1_area = max(box1[2]-box1[0],0)*max(box1[3]-box1[1],0)
box2_area = max(box2[2]-box2[0],0)*max(box2[3]-box2[1],0)
union_area = box1_area + box2_area - inter_area
### END CODE HERE ###

# compute the IoU
### START CODE HERE ### (≈ 1 line)
iou = inter_area/union_area
### END CODE HERE ###

return iou

def yolo_non_max_suppression(scores, boxes, classes, max_boxes = 10, iou_threshold = 0.5):
"""
Applies Non-max suppression (NMS) to set of boxes

Arguments:
scores -- tensor of shape (None,), output of yolo_filter_boxes()
boxes -- tensor of shape (None, 4), output of yolo_filter_boxes() that have been scaled to the image size (see later)
classes -- tensor of shape (None,), output of yolo_filter_boxes()
max_boxes -- integer, maximum number of predicted boxes you'd like
iou_threshold -- real value, "intersection over union" threshold used for NMS filtering

Returns:
scores -- tensor of shape (, None), predicted score for each box
boxes -- tensor of shape (4, None), predicted box coordinates
classes -- tensor of shape (, None), predicted class for each box

Note: The "None" dimension of the output tensors has obviously to be less than max_boxes. Note also that this
function will transpose the shapes of scores, boxes, classes. This is made for convenience.
"""

max_boxes_tensor = K.variable(max_boxes, dtype='int32')     # tensor to be used in tf.image.non_max_suppression()
K.get_session().run(tf.variables_initializer([max_boxes_tensor])) # initialize variable max_boxes_tensor

# Use tf.image.non_max_suppression() to get the list of indices corresponding to boxes you keep
### START CODE HERE ### (≈ 1 line)
nms_indices = tf.image.non_max_suppression(boxes,scores,max_boxes,iou_threshold)
### END CODE HERE ###

# Use K.gather() to select only nms_indices from scores, boxes and classes
### START CODE HERE ### (≈ 3 lines)
scores = K.gather(scores,nms_indices)
boxes = K.gather(boxes,nms_indices)
classes = K.gather(classes,nms_indices)
### END CODE HERE ###

return scores, boxes, classes

def yolo_eval(yolo_outputs, image_shape = (720., 1280.), max_boxes=10, score_threshold=.6, iou_threshold=.5):
"""
Converts the output of YOLO encoding (a lot of boxes) to your predicted boxes along with their scores, box coordinates and classes.

Arguments:
yolo_outputs -- output of the encoding model (for image_shape of (608, 608, 3)), contains 4 tensors:
box_confidence: tensor of shape (None, 19, 19, 5, 1)
box_xy: tensor of shape (None, 19, 19, 5, 2)
box_wh: tensor of shape (None, 19, 19, 5, 2)
box_class_probs: tensor of shape (None, 19, 19, 5, 80)
image_shape -- tensor of shape (2,) containing the input shape, in this notebook we use (608., 608.) (has to be float32 dtype)
max_boxes -- integer, maximum number of predicted boxes you'd like
score_threshold -- real value, if [ highest class probability score < threshold], then get rid of the corresponding box
iou_threshold -- real value, "intersection over union" threshold used for NMS filtering

Returns:
scores -- tensor of shape (None, ), predicted score for each box
boxes -- tensor of shape (None, 4), predicted box coordinates
classes -- tensor of shape (None,), predicted class for each box
"""

### START CODE HERE ###

# Retrieve outputs of the YOLO model (≈1 line)
box_confidence, box_xy, box_wh, box_class_probs = yolo_outputs

# Convert boxes to be ready for filtering functions
boxes = yolo_boxes_to_corners(box_xy, box_wh)

# Use one of the functions you've implemented to perform Score-filtering with a threshold of score_threshold (≈1 line)
scores, boxes, classes = yolo_filter_boxes(box_confidence,boxes,box_class_probs,score_threshold)

# Scale boxes back to original image shape.
boxes = scale_boxes(boxes, image_shape)

# Use one of the functions you've implemented to perform Non-max suppression with a threshold of iou_threshold (≈1 line)
scores, boxes, classes = yolo_non_max_suppression(scores,boxes,classes,max_boxes,iou_threshold)

### END CODE HERE ###

return scores, boxes, classes

sess = K.get_session()
image_shape = (720., 1280.)
yolo_model.summary()

def predict(sess, image_file):
"""
Runs the graph stored in "sess" to predict boxes for "image_file". Prints and plots the preditions.

Arguments:
sess -- your tensorflow/Keras session containing the YOLO graph
image_file -- name of an image stored in the "images" folder.

Returns:
out_scores -- tensor of shape (None, ), scores of the predicted boxes
out_boxes -- tensor of shape (None, 4), coordinates of the predicted boxes
out_classes -- tensor of shape (None, ), class index of the predicted boxes

Note: "None" actually represents the number of predicted boxes, it varies between 0 and max_boxes.
"""

image, image_data = preprocess_image("images/" + image_file, model_image_size = (608, 608))

# Run the session with the correct tensors and choose the correct placeholders in the feed_dict.
# You'll need to use feed_dict={yolo_model.input: ... , K.learning_phase(): 0})
### START CODE HERE ### (≈ 1 line)
out_scores, out_boxes, out_classes = sess.run(
[scores, boxes, classes],
feed_dict={
yolo_model.input: image_data,
K.learning_phase(): 0
})
### END CODE HERE ###

# Print predictions info
print('Found {} boxes for {}'.format(len(out_boxes), image_file))
# Generate colors for drawing bounding boxes.
colors = generate_colors(class_names)
# Draw bounding boxes on the image file
draw_boxes(image, out_scores, out_boxes, out_classes, class_names, colors)
# Save the predicted bounding box on the image
image.save(os.path.join("out", image_file), quality=90)
# Display the results in the notebook
imshow(output_image)

return out_scores, out_boxes, out_classes

• We use filters two times in this process. First we select the class with the highest possibility, then we filter out the repeated boxes. During the first step, pay attention to the K.argmax and the K.max functions.
• Understand the computation of the iou
Last modification：March 13th, 2019 at 07:06 pm
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