DeepLearning.ai深度学习课程笔记
  • Introduction
  • 第一门课 神经网络和深度学习(Neural-Networks-and-Deep-Learning)
    • 第一周:深度学习引言(Introduction to Deep Learning)
      • 1.1 神经网络的监督学习(Supervised Learning with Neural Networks)
      • 1.2 为什么神经网络会流行?(Why is Deep Learning taking off?)
    • 第二周:神经网络的编程基础(Basics of Neural Network programming)
      • 2.1 二分类(Binary Classification)
      • 2.2 逻辑回归(Logistic Regression)
      • 2.3 逻辑回归的代价函数(Logistic Regression Cost Function)
      • 2.4 逻辑回归的梯度下降(Logistic Regression Gradient Descent)
      • 2.5 梯度下降的例子(Gradient Descent on m Examples)
      • 2.6 向量化 logistic 回归的梯度输出(Vectorizing Logistic Regression’s Gradient Output)
      • 2.7 (选修)logistic 损失函数的解释(Explanation of logistic regression cost function )
      • Logistic Regression with a Neural Network mindset 代码
      • lr_utils.py
    • 第三周:浅层神经网络(Shallow neural networks)
      • 3.1 神经网络概述(Neural Network Overview)
      • 3.2 神经网络的表示(Neural Network Representation )
      • 3.3 计算一个神经网络的输出(Computing a Neural Network's output )
      • 3.4 多样本向量化(Vectorizing across multiple examples )
      • 3.5 激活函数(Activation functions)
      • 3.6 为什么需要( 非线性激活函数?(why need a nonlinear activation function?)
      • 3.7 激活函数的导数(Derivatives of activation functions )
      • 3.8 神经网络的梯度下降(Gradient descent for neural networks)
      • 3.9 (选修)直观理解反向传播(Backpropagation intuition )
      • 3.10 随机初始化(Random+Initialization)
      • Planar data classification with one hidden layer
      • planar_utils.py
      • testCases.py
    • 第四周:深层神经网络(Deep Neural Networks)
      • 4.1 深层神经网络(Deep L-layer neural network)
      • 4.2 前向传播和反向传播(Forward and backward propagation)
      • 4.3 深层网络中的前向传播(Forward propagation in a Deep Network )
      • 4.4 为什么使用深层表示?(Why deep representations?)
      • 4.5 搭建神经网络块(Building blocks of deep neural networks)
      • 4.6 参数 VS 超参数(Parameters vs Hyperparameters)
      • Building your Deep Neural Network Step by Step
      • dnn_utils.py
      • testCases.py
      • Deep Neural Network Application
      • dnn_app_utils.py
  • 第二门课 改善深层神经网络:超参数调试、 正 则 化 以 及 优 化 (Improving Deep Neural Networks:Hyperparameter tuning, Regulariza
    • 第二门课 改善深层神经网络:超参数调试、正则化以及优化(Improving Deep Neural Networks:Hyperparameter tuning, Regularization and
      • 第一周:深度学习的实用层面(Practical aspects of Deep Learning)
        • 1.1 训练,验证,测试集(Train / Dev / Test sets)
        • 1.2 偏差,方差(Bias /Variance)
        • 1.3 机器学习基础(Basic Recipe for Machine Learning)
        • 1.4 正则化(Regularization)
        • 1.5 为什么正则化有利于预防过拟合呢?(Why regularization reduces overfitting?)
        • 1.6 dropout 正则化(Dropout Regularization)
        • 1.7 理解 dropout(Understanding Dropout)
        • 1.8 其他正则化方法(Other regularization methods)
        • 1.9 归一化输入(Normalizing inputs)
        • 1.10 梯度消失/梯度爆炸(Vanishing / Exploding gradients)
        • 1.11 神经网络的权重初始化(Weight Initialization for Deep Networks)
        • 1.12 梯度的数值逼近(Numerical approximation of gradients)
        • 1.13 梯度检验(Gradient checking)
        • 1.14 梯度检验应用的注意事项(Gradient Checking Implementation Notes)
        • Initialization
        • Gradient Checking
        • Regularization
        • reg_utils.py
        • testCases.py
      • 第二周:优化算法 (Optimization algorithms)
        • 2.1 Mini-batch 梯度下降(Mini-batch gradient descent)
        • 2.2 理解 mini-batch 梯度下降法(Understanding mini-batch gradient descent)
        • 2.3 指数加权平均数(Exponentially weighted averages)
        • 2.4 理解指数加权平均数(Understanding exponentially weighted averages )
        • 2.5 指 数 加 权 平 均 的 偏 差 修 正 ( Bias correction in exponentially weighted averages )
        • 2.6 动量梯度下降法(Gradient descent with Momentum )
        • 2.7 RMSprop( root mean square prop)
        • 2.8 Adam 优化算法(Adam optimization algorithm)
        • 2.9 学习率衰减(Learning rate decay)
        • 2.10 局部最优的问题(The problem of local optima)
        • Optimization
        • opt_utils.py
        • testCases.py
      • 第 三 周 超 参 数 调 试 、 Batch 正 则 化 和 程 序 框 架 (Hyperparameter tuning)
        • 3.1 调试处理(Tuning process)
        • 3.2 为超参数选择合适的范围(Using an appropriate scale to pick hyperparameters)
        • 3.3 超参数训练的实践: Pandas VS Caviar(Hyperparameters tuning in practice: Pandas vs. Caviar)
        • 3.4 归一化网络的激活函数( Normalizing activations in a network)
        • 3.5 将 Batch Norm 拟合进神经网络(Fitting Batch Norm into a neural network)
        • 3.6 Batch Norm 为什么奏效?(Why does Batch Norm work?)
        • 3.7 测试时的 Batch Norm(Batch Norm at test time)
        • 3.8 Softmax 回归(Softmax regression)
        • 3.9 训练一个 Softmax 分类器(Training a Softmax classifier)
        • tensorflow tutorial
        • improv_utils.py
        • tf_utils.py
  • 第三门课 结构化机器学习项目(Structuring Machine Learning Projects)
    • 第三门课 结构化机器学习项目(Structuring Machine Learning Projects)
      • 第一周 机器学习(ML)策略(1)(ML strategy(1))
        • 1.1 为什么是 ML 策略?(Why ML Strategy?)
        • 1.2 正交化(Orthogonalization)
        • 1.3 单一数字评估指标(Single number evaluation metric)
        • 1.4 满足和优化指标(Satisficing and optimizing metrics)
        • 1.5 训练/开发/测试集划分(Train/dev/test distributions)
        • 1.6 开发集和测试集的大小(Size of dev and test sets)
        • 1.7 什么时候该改变开发/测试集和指标?(When to change dev/test sets and metrics)
        • 1.8 为什么是人的表现?( Why human-level performance?)
        • 1.9 可避免偏差(Avoidable bias)
        • 1.10 理解人的表现(Understanding human-level performance)
        • 1.11 超过人的表现(Surpassing human- level performance)
        • 1.12 改善你的模型的表现(Improving your model performance)
      • 第二周:机器学习策略(2)(ML Strategy (2))
        • 2.1 进行误差分析(Carrying out error analysis)
        • 2.2 清楚标注错误的数据(Cleaning up Incorrectly labeled data)
        • 2.3 快速搭建你的第一个系统,并进行迭代(Build your first system quickly, then iterate)
        • 2.4 在不同的划分上进行训练并测试(Training and testing on different distributions)
        • 2.5 不匹配数据划分的偏差和方差(Bias and Variance with mismatched data distributions)
        • 2.6 定位数据不匹配(Addressing data mismatch)
        • 2.7 迁移学习(Transfer learning)
        • 2.8 多任务学习(Multi-task learning)
        • 2.9 什么是端到端的深度学习?(What is end-to-end deep learning?)
        • 2.10 是否要使用端到端的深度学习?(Whether to use end-to-end learning?)
  • 第四门课 卷积神经网络(Convolutional Neural Networks)
    • 第四门课 卷积神经网络(Convolutional Neural Networks)
      • 第一周 卷积神经网络(Foundations of Convolutional Neural Networks)
        • 1.1 计算机视觉(Computer vision)
        • 1.2 边缘检测示例(Edge detection example)
        • 1.3 更多边缘检测内容(More edge detection)
        • 1.4 Padding
        • 1.5 卷积步长(Strided convolutions)
        • 1.6 三维卷积(Convolutions over volumes)
        • 1.7 单层卷积网络(One layer of a convolutional network)
        • 1.8 简单卷积网络示例(A simple convolution network example)
        • 1.9 池化层(Pooling layers)
        • 1.10 卷积神经网络示例(Convolutional neural network example)
        • 1.11 为什么使用卷积?(Why convolutions?)
        • Convolution model Step by Step
        • Convolutional Neural Networks: Application
        • cnn_utils
      • 第二周 深度卷积网络:实例探究(Deep convolutional models: case studies)
        • 2.1 经典网络(Classic networks)
        • 2.2 残差网络(Residual Networks (ResNets))
        • 2.3 残差网络为什么有用?(Why ResNets work?)
        • 2.4 网络中的网络以及 1×1 卷积(Network in Network and 1×1 convolutions)
        • 2.5 谷歌 Inception 网络简介(Inception network motivation)
        • 2.6 Inception 网络(Inception network)
        • 2.7 迁移学习(Transfer Learning)
        • 2.8 数据扩充(Data augmentation)
        • 2.9 计算机视觉现状(The state of computer vision)
        • Residual Networks
        • Keras tutorial - the Happy House
        • kt_utils.py
      • 第三周 目标检测(Object detection)
        • 3.1 目标定位(Object localization)
        • 3.2 特征点检测(Landmark detection)
        • 3.3 目标检测(Object detection)
        • 3.4 卷积的滑动窗口实现(Convolutional implementation of sliding windows)
        • 3.5 Bounding Box预测(Bounding box predictions)
        • 3.6 交并比(Intersection over union)
        • 3.7 非极大值抑制(Non-max suppression)
        • 3.8 Anchor Boxes
        • 3.9 YOLO 算法(Putting it together: YOLO algorithm)
        • 3.10 候选区域(选修)(Region proposals (Optional))
        • Autonomous driving application - Car detection
        • yolo_utils.py
      • 第四周 特殊应用:人脸识别和神经风格转换(Special applications: Face recognition &Neural style transfer)
        • 4.1 什么是人脸识别?(What is face recognition?)
        • 4.2 One-Shot学习(One-shot learning)
        • 4.3 Siamese 网络(Siamese network)
        • 4.4 Triplet 损失(Triplet 损失)
        • 4.5 面部验证与二分类(Face verification and binary classification)
        • 4.6 什么是深度卷积网络?(What are deep ConvNets learning?)
        • 4.7 代价函数(Cost function)
        • 4.8 内容代价函数(Content cost function)
        • 4.9 风格代价函数(Style cost function)
        • 4.10 一维到三维推广(1D and 3D generalizations of models)
        • Art Generation with Neural Style Transfer
        • nst_utils.py
        • Face Recognition for the Happy House
        • fr_utils.py
        • inception_blocks.py
  • 第五门课 序列模型(Sequence Models)
    • 第五门课 序列模型(Sequence Models)
      • 第一周 循环序列模型(Recurrent Neural Networks)
        • 1.1 为什么选择序列模型?(Why Sequence Models?)
        • 1.2 数学符号(Notation)
        • 1.3 循环神经网络模型(Recurrent Neural Network Model)
        • 1.4 通过时间的反向传播(Backpropagation through time)
        • 1.5 不同类型的循环神经网络(Different types of RNNs)
        • 1.6 语言模型和序列生成(Language model and sequence generation)
        • 1.7 对新序列采样(Sampling novel sequences)
        • 1.8 循环神经网络的梯度消失(Vanishing gradients with RNNs)
        • 1.9 GRU单元(Gated Recurrent Unit(GRU))
        • 1.10 长短期记忆(LSTM(long short term memory)unit)
        • 1.11 双向循环神经网络(Bidirectional RNN)
        • 1.12 深层循环神经网络(Deep RNNs)
        • Building your Recurrent Neural Network
        • rnn_utils.py
        • Dinosaurus Island -- Character level language model final
        • utils.py
        • shakespeare_utils.py
        • Improvise a Jazz Solo with an LSTM Network
      • 第二周 自然语言处理与词嵌入(Natural Language Processing and Word Embeddings)
        • 2.1 词汇表征(Word Representation)
        • 2.2 使用词嵌入(Using Word Embeddings)
        • 2.3 词嵌入的特性(Properties of Word Embeddings)
        • 2.4 嵌入矩阵(Embedding Matrix)
        • 2.5 学习词嵌入(Learning Word Embeddings)
        • 2.6 Word2Vec
        • 2.7 负采样(Negative Sampling)
        • 2.8 GloVe 词向量(GloVe Word Vectors)
        • 2.9 情感分类(Sentiment Classification)
        • 2.10 词嵌入除偏(Debiasing Word Embeddings)
        • Operations on word vectors
        • w2v_utils.py
        • Emojify
        • emo_utils.py
      • 第三周 序列模型和注意力机制(Sequence models & Attention mechanism)
        • 3.1 基础模型(Basic Models)
        • 3.2 选择最可能的句子(Picking the most likely sentence)
        • 3.3 集束搜索(Beam Search)
        • 3.4 改进集束搜索(Refinements to Beam Search)
        • 3.5 集束搜索的误差分析(Error analysis in beam search)
        • 3.6 Bleu 得分(选修)(Bleu Score (optional))
        • 3.7 注意力模型直观理解(Attention Model Intuition)
        • 3.8注意力模型(Attention Model)
        • 3.9语音识别(Speech recognition)
        • 3.10触发字检测(Trigger Word Detection)
        • Neural machine translation with attention
        • nmt_utils.py
        • Trigger word detection
        • td_utils.py
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On this page
  • 1 - Problem Statement
  • 2 - YOLO
  • 2.1 - Model details
  • 2.2 - Filtering with a threshold on class scores
  • 2.3 - Non-max suppression
  • 2.4 Wrapping up the filtering
  • 3 - Test YOLO pretrained model on images
  • 3.1 - Defining classes, anchors and image shape.
  • 3.2 - Loading a pretrained model
  • 3.3 - Convert output of the model to usable bounding box tensors
  • 3.4 - Filtering boxes
  • 3.5 - Run the graph on an image

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  1. 第四门课 卷积神经网络(Convolutional Neural Networks)
  2. 第四门课 卷积神经网络(Convolutional Neural Networks)
  3. 第三周 目标检测(Object detection)

Autonomous driving application - Car detection

Previous3.10 候选区域(选修)(Region proposals (Optional))Nextyolo_utils.py

Last updated 6 years ago

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Welcome to your week 3 programming assignment. You will learn about object detection using the very powerful YOLO model. Many of the ideas in this notebook are described in the two YOLO papers: and

You will learn to:

  • Use object detection on a car detection dataset

  • Deal with bounding boxes

Run the following cell to load the packages and dependencies that are going to be useful for your journey!

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
from keras.models import load_model, Model
from yolo_utils import read_classes, read_anchors, generate_colors, preprocess_image, draw_boxes, scale_boxes
from yad2k.models.keras_yolo import yolo_head, yolo_boxes_to_corners, preprocess_true_boxes, yolo_loss, yolo_body


%matplotlib inline

Using TensorFlow backend.

Important Note: As you can see, we import Keras's backend as K. This means that to use a Keras function in this notebook, you will need to write: K.function(...).

1 - Problem Statement

You are working on a self-driving car. As a critical component of this project, you'd like to first build a car detection system. To collect data, you've mounted a camera to the hood (meaning the front) of the car, which takes pictures of the road ahead every few seconds while you drive around.

You've gathered all these images into a folder and have labelled them by drawing bounding boxes around every car you found. Here's an example of what your bounding boxes look like.

Figure 1: Definition of a box

In this exercise, you will learn how YOLO works, then apply it to car detection. Because the YOLO model is very computationally expensive to train, we will load pre-trained weights for you to use.

2 - YOLO

YOLO ("you only look once") is a popular algoritm because it achieves high accuracy while also being able to run in real-time. This algorithm "only looks once" at the image in the sense that it requires only one forward propagation pass through the network to make predictions. After non-max suppression, it then outputs recognized objects together with the bounding boxes.

2.1 - Model details

First things to know:

  • The input is a batch of images of shape (m, 608, 608, 3)

We will use 5 anchor boxes. So you can think of the YOLO architecture as the following: IMAGE (m, 608, 608, 3) -> DEEP CNN -> ENCODING (m, 19, 19, 5, 85).

Lets look in greater detail at what this encoding represents.

Figure 2 : Encoding architecture for YOLO

If the center/midpoint of an object falls into a grid cell, that grid cell is responsible for detecting that object.

Since we are using 5 anchor boxes, each of the 19 x19 cells thus encodes information about 5 boxes. Anchor boxes are defined only by their width and height.

For simplicity, we will flatten the last two last dimensions of the shape (19, 19, 5, 85) encoding. So the output of the Deep CNN is (19, 19, 425).

Figure 3 : Flattening the last two last dimensions

Now, for each box (of each cell) we will compute the following elementwise product and extract a probability that the box contains a certain class.

Figure 4 : Find the class detected by each box

Here's one way to visualize what YOLO is predicting on an image:

  • For each of the 19x19 grid cells, find the maximum of the probability scores (taking a max across both the 5 anchor boxes and across different classes).

  • Color that grid cell according to what object that grid cell considers the most likely.

Doing this results in this picture:

Figure 5 : Each of the 19x19 grid cells colored according to which class has the largest predicted probability in that cell

Note that this visualization isn't a core part of the YOLO algorithm itself for making predictions; it's just a nice way of visualizing an intermediate result of the algorithm.

Another way to visualize YOLO's output is to plot the bounding boxes that it outputs. Doing that results in a visualization like this:

Figure 6 : Each cell gives you 5 boxes. In total, the model predicts: 19x19x5 = 1805 boxes just by looking once at the image (one forward pass through the network)! Different colors denote different classes.

In the figure above, we plotted only boxes that the model had assigned a high probability to, but this is still too many boxes. You'd like to filter the algorithm's output down to a much smaller number of detected objects. To do so, you'll use non-max suppression. Specifically, you'll carry out these steps:

  • Get rid of boxes with a low score (meaning, the box is not very confident about detecting a class)

  • Select only one box when several boxes overlap with each other and detect the same object.

2.2 - Filtering with a threshold on class scores

You are going to apply a first filter by thresholding. You would like to get rid of any box for which the class "score" is less than a chosen threshold.

The model gives you a total of 19x19x5x85 numbers, with each box described by 85 numbers. It'll be convenient to rearrange the (19,19,5,85) (or (19,19,425)) dimensional tensor into the following variables:

Exercise: Implement yolo_filter_boxes(). 1. Compute box scores by doing the elementwise product as described in Figure 4. The following code may help you choose the right operator:

a = np.random.randn(19*19, 5, 1)
b = np.random.randn(19*19, 5, 80)
c = a * b # shape of c will be (19*19, 5, 80)
  1. For each box, find:

  2. Create a mask by using a threshold. As a reminder: ([0.9, 0.3, 0.4, 0.5, 0.1] < 0.4) returns: [False, True, False, False, True]. The mask should be True for the boxes you want to keep.

Reminder: to call a Keras function, you should use K.function(...).

# GRADED FUNCTION: yolo_filter_boxes

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 the 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  #(19, 19, 5, 80)
    ### 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)    #(19, 19, 5)
    box_class_scores = K.max(box_scores, axis = -1)  #(19, 19, 5)
    ### 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)
    filtering_mask = box_class_scores > threshold #(19, 19, 5)
    ### END CODE HERE ###

    # Step 4: Apply the mask to scores, boxes and classes
    ### START CODE HERE ### (≈ 3 lines)
    scores = tf.boolean_mask(box_class_scores, filtering_mask)
    boxes = tf.boolean_mask(boxes, filtering_mask)
    classes = tf.boolean_mask(box_classes, filtering_mask)
    ### END CODE HERE ###

    return scores, boxes, classes
with tf.Session() as test_a:
    box_confidence = tf.random_normal([19, 19, 5, 1], mean = 1, stddev = 4, seed = 1)
    boxes = tf.random_normal([19, 19, 5, 4], mean = 1, stddev = 4, seed = 1)
    boxes_classes = tf.random_normal([19, 19, 5, 80], mean = 1, stddev = 4, seed = 1)
    scores, boxes, classes = yolo_filter_boxes(box_confidence, boxes, boxes_classes, threshold = 0.5)
    print("scores[2] = " + str(scores[2].eval()))
    print("boxes[2] = " + str(boxes[2].eval()))
    print("classes[2] = " + str(classes[2].eval()))

    s, b, c = sess.run([scores, boxes, classes])
    print("scores.shape = " + str(s.shape))
    print("boxes.shape = " + str(b.shape))
    print("classes.shape = " + str(c.shape))
scores[2] = 10.750582
boxes[2] = [ 8.426533   3.2713668 -0.5313436 -4.9413733]
classes[2] = 7
scores.shape = (1783,)
boxes.shape = (1783, 4)
classes.shape = (1783,)

If you add .eval() to multiple print statements to try to see the actual dimensions in the test example, then tensorflow will execute the computation graph for each .eval() and call tf.random_normal as needed to find the result, producing new random input values. Thus, the output first dimension changes (depending on how many box classes pass the threshold for that input).

such as :

print(score.eval().shape)
print(boxes.eval().shape)
print(classes.eval().shape)

then output :

scores.shape = (1785,)
boxes.shape = (1781, 4)
classes.shape = (1776,)

Instead, commenting out all print statements and running the graph just once like so:

# Actual shapes 
s, b, c = test_a.run([scores, boxes, classes])
print('scores.shape  =', s.shape)
print('boxes.shape   =', b.shape)
print('classes.shape =', c.shape)

produces output dimensions you would expect:

scores.shape  = (1787,)
boxes.shape   = (1787, 4)
classes.shape = (1787,)

2.3 - Non-max suppression

Even after filtering by thresholding over the classes scores, you still end up a lot of overlapping boxes. A second filter for selecting the right boxes is called non-maximum suppression (NMS).

下面的这个车被多次检测到,存在多个边界框,保留概率最大的那个,去除掉与这个概率最大的边界框的IoU大于一个阙值的其余边界框。这个过程就称为非最大值抑制NMS

Figure 7: In this example, the model has predicted 3 cars, but it's actually 3 predictions of the same car. Running non-max suppression (NMS) will select only the most accurate (highest probabiliy) one of the 3 boxes.

Figure 8: Definition of "Intersection over Union".

Exercise: Implement iou(). Some hints:

  • In this exercise only, we define a box using its two corners (upper left and lower right): (x1, y1, x2, y2) rather than the midpoint and height/width.

  • To calculate the area of a rectangle you need to multiply its height (y2 - y1) by its width (x2 - x1).

  • You'll also need to find the coordinates (xi1, yi1, xi2, yi2) of the intersection of two boxes. Remember that:

  • xi1 = maximum of the x1 coordinates of the two boxes

  • yi1 = maximum of the y1 coordinates of the two boxes

  • xi2 = minimum of the x2 coordinates of the two boxes

  • yi2 = minimum of the y2 coordinates of the two boxes

  • In order to compute the intersection area, you need to make sure the height and width of the intersection are positive, otherwise the intersection area should be zero. Use max(height, 0) and max(width, 0).

In this code, we use the convention that (0,0) is the top-left corner of an image, (1,0) is the upper-right corner, and (1,1) the lower-right corner.

# GRADED FUNCTION: iou

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 = np.maximum(box1[0], box2[0])
    yi1 = np.maximum(box1[1], box2[1])
    xi2 = np.minimum(box1[2], box2[2])
    yi2 = np.minimum(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 = (box1[2] - box1[0])*(box1[3] - box1[1])
    box2_area = (box2[2 ] - box2[0])*(box2[3] - box2[1])
    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
box1 = (2, 1, 4, 3)
box2 = (1, 2, 3, 4)
print("iou = " + str(iou(box1, box2)))

"""
iou = 0.142857142857
"""

You are now ready to implement non-max suppression. The key steps are: 1. Select the box that has the highest score. 2. Compute its overlap with all other boxes, and remove boxes that overlap it more than iou_threshold. 3. Go back to step 1 and iterate until there's no more boxes with a lower score than the current selected box.

This will remove all boxes that have a large overlap with the selected boxes. Only the "best" boxes remain.

Exercise: Implement yolo_non_max_suppression() using TensorFlow. TensorFlow has two built-in functions that are used to implement non-max suppression (so you don't actually need to use your iou() implementation):

    • tf.image.non_max_suppression( boxes, scores, max_output_size, iou_threshold=0.5, score_threshold=float('-inf'), name=None )

      • boxes:形状为[num_boxes, 4]的二维浮点型Tensor

      • scores:形状为[num_boxes]的一维浮点型Tensor,表示与每个框(每行框)对应的单个分数

      • max_output_size:一个标量整数Tensor,表示通过非最大抑制选择的框的最大数量

      • iou_threshold:一种浮点数,表示判断框是否相对于IOU重叠太多的阈值

      • name:操作的名称(可选)

      返回:

      • selected_indices:形状为[M]的一维整数Tensor,表示从box张量中选择的指数,其中M <= max_output_size

    • K.gather(reference, indices) : 在给定的张量中搜索给定下标的向量

      • reference表示被搜寻的向量

      • indices表示整数张量,要查询的元素的下标

      • 返回值:一个与参数reference数据类型一致的张量

Example :

from keras import backend as K
A = [[1, 2],
     [3, 4],
     [5, 6],
     [7, 8]]
B = [0, 1, 1, 2, 3]

with tf.Session() as sess:
     print(sess.run(K.gather(A, B))

###
[[1, 2],
 [3, 4],
 [3, 4],
 [5, 6],
 [7, 8]]
# GRADED FUNCTION: yolo_non_max_suppression

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_tensor, 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
with tf.Session() as test_b:
    scores = tf.random_normal([54,], mean=1, stddev=4, seed = 1)
    boxes = tf.random_normal([54, 4], mean=1, stddev=4, seed = 1)
    classes = tf.random_normal([54,], mean=1, stddev=4, seed = 1)
    scores, boxes, classes = yolo_non_max_suppression(scores, boxes, classes)
    print("scores[2] = " + str(scores[2].eval()))
    print("boxes[2] = " + str(boxes[2].eval()))
    print("classes[2] = " + str(classes[2].eval()))
    print("scores.shape = " + str(scores.eval().shape))
    print("boxes.shape = " + str(boxes.eval().shape))
    print("classes.shape = " + str(classes.eval().shape))
scores[2] = 6.9384
boxes[2] = [-5.299932 3.13798141 4.45036697 0.95942086]
classes[2] = -2.24527
scores.shape = (10,)
boxes.shape = (10, 4)
classes.shape = (10,)

2.4 Wrapping up the filtering

It's time to implement a function taking the output of the deep CNN (the 19x19x5x85 dimensional encoding) and filtering through all the boxes using the functions you've just implemented.

Exercise: Implement yolo_eval() which takes the output of the YOLO encoding and filters the boxes using score threshold and NMS. There's just one last implementational detail you have to know. There're a few ways of representing boxes, such as via their corners or via their midpoint and height/width. YOLO converts between a few such formats at different times, using the following functions (which we have provided):

boxes = yolo_boxes_to_corners(box_xy, box_wh)

which converts the yolo box coordinates (x,y,w,h) to box corners' coordinates (x1, y1, x2, y2) to fit the input of yolo_filter_boxes

boxes = scale_boxes(boxes, image_shape)

YOLO's network was trained to run on 608x608 images. If you are testing this data on a different size image--for example, the car detection dataset had 720x1280 images--this step rescales the boxes so that they can be plotted on top of the original 720x1280 image.

Don't worry about these two functions; we'll show you where they need to be called.

# GRADED FUNCTION: yolo_eval

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)

    # 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
with tf.Session() as test_b:
    yolo_outputs = (tf.random_normal([19, 19, 5, 1], mean=1, stddev=4, seed = 1),
                    tf.random_normal([19, 19, 5, 2], mean=1, stddev=4, seed = 1),
                    tf.random_normal([19, 19, 5, 2], mean=1, stddev=4, seed = 1),
                    tf.random_normal([19, 19, 5, 80], mean=1, stddev=4, seed = 1))
    scores, boxes, classes = yolo_eval(yolo_outputs)
    print("scores[2] = " + str(scores[2].eval()))
    print("boxes[2] = " + str(boxes[2].eval()))
    print("classes[2] = " + str(classes[2].eval()))
    print("scores.shape = " + str(scores.eval().shape))
    print("boxes.shape = " + str(boxes.eval().shape))
    print("classes.shape = " + str(classes.eval().shape))
scores[2] = 138.791
boxes[2] = [ 1292.32971191 -278.52166748 3876.98925781 -835.56494141]
classes[2] = 54
scores.shape = (10,)
boxes.shape = (10, 4)
classes.shape = (10,)

Summary for YOLO:

  • Input image (608, 608, 3)

  • The input image goes through a CNN, resulting in a (19,19,5,85) dimensional output.

  • After flattening the last two dimensions, the output is a volume of shape (19, 19, 425):

  • Each cell in a 19x19 grid over the input image gives 425 numbers.

  • 425 = 5 x 85 because each cell contains predictions for 5 boxes, corresponding to 5 anchor boxes, as seen in lecture.

  • You then select only few boxes based on:

  • Score-thresholding: throw away boxes that have detected a class with a score less than the threshold

  • Non-max suppression: Compute the Intersection over Union and avoid selecting overlapping boxes

  • This gives you YOLO's final output.

3 - Test YOLO pretrained model on images

In this part, you are going to use a pretrained model and test it on the car detection dataset. As usual, you start by creating a session to start your graph. Run the following cell.

sess = K.get_session()

3.1 - Defining classes, anchors and image shape.

Recall that we are trying to detect 80 classes, and are using 5 anchor boxes. We have gathered the information about the 80 classes and 5 boxes in two files "coco_classes.txt" and "yolo_anchors.txt". Let's load these quantities into the model by running the next cell.

The car detection dataset has 720x1280 images, which we've pre-processed into 608x608 images.

class_names = read_classes("model_data/coco_classes.txt")
anchors = read_anchors("model_data/yolo_anchors.txt")
image_shape = (720., 1280.)

3.2 - Loading a pretrained model

Training a YOLO model takes a very long time and requires a fairly large dataset of labelled bounding boxes for a large range of target classes. You are going to load an existing pretrained Keras YOLO model stored in "yolo.h5". (These weights come from the official YOLO website, and were converted using a function written by Allan Zelener. References are at the end of this notebook. Technically, these are the parameters from the "YOLOv2" model, but we will more simply refer to it as "YOLO" in this notebook.) Run the cell below to load the model from this file.

yolo_model = load_model("model_data/yolo.h5")

This loads the weights of a trained YOLO model. Here's a summary of the layers your model contains.

yolo_model.summary()
____________________________________________________________________________________________________
Layer (type) Output Shape Param # Connected to
====================================================================================================
input_1 (InputLayer) (None, 608, 608, 3) 0
____________________________________________________________________________________________________
conv2d_1 (Conv2D) (None, 608, 608, 32) 864 input_1[0][0]
____________________________________________________________________________________________________
batch_normalization_1 (BatchNorm (None, 608, 608, 32) 128 conv2d_1[0][0]
____________________________________________________________________________________________________
leaky_re_lu_1 (LeakyReLU) (None, 608, 608, 32) 0 batch_normalization_1[0][0]
____________________________________________________________________________________________________
max_pooling2d_1 (MaxPooling2D) (None, 304, 304, 32) 0 leaky_re_lu_1[0][0]
____________________________________________________________________________________________________
conv2d_2 (Conv2D) (None, 304, 304, 64) 18432 max_pooling2d_1[0][0]
____________________________________________________________________________________________________
batch_normalization_2 (BatchNorm (None, 304, 304, 64) 256 conv2d_2[0][0]
____________________________________________________________________________________________________
leaky_re_lu_2 (LeakyReLU) (None, 304, 304, 64) 0 batch_normalization_2[0][0]
____________________________________________________________________________________________________
max_pooling2d_2 (MaxPooling2D) (None, 152, 152, 64) 0 leaky_re_lu_2[0][0]
____________________________________________________________________________________________________
conv2d_3 (Conv2D) (None, 152, 152, 128) 73728 max_pooling2d_2[0][0]
____________________________________________________________________________________________________
batch_normalization_3 (BatchNorm (None, 152, 152, 128) 512 conv2d_3[0][0]
____________________________________________________________________________________________________
leaky_re_lu_3 (LeakyReLU) (None, 152, 152, 128) 0 batch_normalization_3[0][0]
____________________________________________________________________________________________________
conv2d_4 (Conv2D) (None, 152, 152, 64) 8192 leaky_re_lu_3[0][0]
____________________________________________________________________________________________________
batch_normalization_4 (BatchNorm (None, 152, 152, 64) 256 conv2d_4[0][0]
____________________________________________________________________________________________________
leaky_re_lu_4 (LeakyReLU) (None, 152, 152, 64) 0 batch_normalization_4[0][0]
____________________________________________________________________________________________________
conv2d_5 (Conv2D) (None, 152, 152, 128) 73728 leaky_re_lu_4[0][0]
____________________________________________________________________________________________________
batch_normalization_5 (BatchNorm (None, 152, 152, 128) 512 conv2d_5[0][0]
____________________________________________________________________________________________________
leaky_re_lu_5 (LeakyReLU) (None, 152, 152, 128) 0 batch_normalization_5[0][0]
____________________________________________________________________________________________________
max_pooling2d_3 (MaxPooling2D) (None, 76, 76, 128) 0 leaky_re_lu_5[0][0]
____________________________________________________________________________________________________
conv2d_6 (Conv2D) (None, 76, 76, 256) 294912 max_pooling2d_3[0][0]
____________________________________________________________________________________________________
batch_normalization_6 (BatchNorm (None, 76, 76, 256) 1024 conv2d_6[0][0]
____________________________________________________________________________________________________
leaky_re_lu_6 (LeakyReLU) (None, 76, 76, 256) 0 batch_normalization_6[0][0]
____________________________________________________________________________________________________
conv2d_7 (Conv2D) (None, 76, 76, 128) 32768 leaky_re_lu_6[0][0]
____________________________________________________________________________________________________
batch_normalization_7 (BatchNorm (None, 76, 76, 128) 512 conv2d_7[0][0]
____________________________________________________________________________________________________
leaky_re_lu_7 (LeakyReLU) (None, 76, 76, 128) 0 batch_normalization_7[0][0]
____________________________________________________________________________________________________
conv2d_8 (Conv2D) (None, 76, 76, 256) 294912 leaky_re_lu_7[0][0]
____________________________________________________________________________________________________
batch_normalization_8 (BatchNorm (None, 76, 76, 256) 1024 conv2d_8[0][0]
____________________________________________________________________________________________________
leaky_re_lu_8 (LeakyReLU) (None, 76, 76, 256) 0 batch_normalization_8[0][0]
____________________________________________________________________________________________________
max_pooling2d_4 (MaxPooling2D) (None, 38, 38, 256) 0 leaky_re_lu_8[0][0]
____________________________________________________________________________________________________
conv2d_9 (Conv2D) (None, 38, 38, 512) 1179648 max_pooling2d_4[0][0]
____________________________________________________________________________________________________
batch_normalization_9 (BatchNorm (None, 38, 38, 512) 2048 conv2d_9[0][0]
____________________________________________________________________________________________________
leaky_re_lu_9 (LeakyReLU) (None, 38, 38, 512) 0 batch_normalization_9[0][0]
____________________________________________________________________________________________________
conv2d_10 (Conv2D) (None, 38, 38, 256) 131072 leaky_re_lu_9[0][0]
____________________________________________________________________________________________________
batch_normalization_10 (BatchNor (None, 38, 38, 256) 1024 conv2d_10[0][0]
____________________________________________________________________________________________________
leaky_re_lu_10 (LeakyReLU) (None, 38, 38, 256) 0 batch_normalization_10[0][0]
____________________________________________________________________________________________________
conv2d_11 (Conv2D) (None, 38, 38, 512) 1179648 leaky_re_lu_10[0][0]
____________________________________________________________________________________________________
batch_normalization_11 (BatchNor (None, 38, 38, 512) 2048 conv2d_11[0][0]
____________________________________________________________________________________________________
leaky_re_lu_11 (LeakyReLU) (None, 38, 38, 512) 0 batch_normalization_11[0][0]
____________________________________________________________________________________________________
conv2d_12 (Conv2D) (None, 38, 38, 256) 131072 leaky_re_lu_11[0][0]
____________________________________________________________________________________________________
batch_normalization_12 (BatchNor (None, 38, 38, 256) 1024 conv2d_12[0][0]
____________________________________________________________________________________________________
leaky_re_lu_12 (LeakyReLU) (None, 38, 38, 256) 0 batch_normalization_12[0][0]
____________________________________________________________________________________________________
conv2d_13 (Conv2D) (None, 38, 38, 512) 1179648 leaky_re_lu_12[0][0]
____________________________________________________________________________________________________
batch_normalization_13 (BatchNor (None, 38, 38, 512) 2048 conv2d_13[0][0]
____________________________________________________________________________________________________
leaky_re_lu_13 (LeakyReLU) (None, 38, 38, 512) 0 batch_normalization_13[0][0]
____________________________________________________________________________________________________
max_pooling2d_5 (MaxPooling2D) (None, 19, 19, 512) 0 leaky_re_lu_13[0][0]
____________________________________________________________________________________________________
conv2d_14 (Conv2D) (None, 19, 19, 1024) 4718592 max_pooling2d_5[0][0]
____________________________________________________________________________________________________
batch_normalization_14 (BatchNor (None, 19, 19, 1024) 4096 conv2d_14[0][0]
____________________________________________________________________________________________________
leaky_re_lu_14 (LeakyReLU) (None, 19, 19, 1024) 0 batch_normalization_14[0][0]
____________________________________________________________________________________________________
conv2d_15 (Conv2D) (None, 19, 19, 512) 524288 leaky_re_lu_14[0][0]
____________________________________________________________________________________________________
batch_normalization_15 (BatchNor (None, 19, 19, 512) 2048 conv2d_15[0][0]
____________________________________________________________________________________________________
leaky_re_lu_15 (LeakyReLU) (None, 19, 19, 512) 0 batch_normalization_15[0][0]
____________________________________________________________________________________________________
conv2d_16 (Conv2D) (None, 19, 19, 1024) 4718592 leaky_re_lu_15[0][0]
____________________________________________________________________________________________________
batch_normalization_16 (BatchNor (None, 19, 19, 1024) 4096 conv2d_16[0][0]
____________________________________________________________________________________________________
leaky_re_lu_16 (LeakyReLU) (None, 19, 19, 1024) 0 batch_normalization_16[0][0]
____________________________________________________________________________________________________
conv2d_17 (Conv2D) (None, 19, 19, 512) 524288 leaky_re_lu_16[0][0]
____________________________________________________________________________________________________
batch_normalization_17 (BatchNor (None, 19, 19, 512) 2048 conv2d_17[0][0]
____________________________________________________________________________________________________
leaky_re_lu_17 (LeakyReLU) (None, 19, 19, 512) 0 batch_normalization_17[0][0]
____________________________________________________________________________________________________
conv2d_18 (Conv2D) (None, 19, 19, 1024) 4718592 leaky_re_lu_17[0][0]
____________________________________________________________________________________________________
batch_normalization_18 (BatchNor (None, 19, 19, 1024) 4096 conv2d_18[0][0]
____________________________________________________________________________________________________
leaky_re_lu_18 (LeakyReLU) (None, 19, 19, 1024) 0 batch_normalization_18[0][0]
____________________________________________________________________________________________________
conv2d_19 (Conv2D) (None, 19, 19, 1024) 9437184 leaky_re_lu_18[0][0]
____________________________________________________________________________________________________
batch_normalization_19 (BatchNor (None, 19, 19, 1024) 4096 conv2d_19[0][0]
____________________________________________________________________________________________________
conv2d_21 (Conv2D) (None, 38, 38, 64) 32768 leaky_re_lu_13[0][0]
____________________________________________________________________________________________________
leaky_re_lu_19 (LeakyReLU) (None, 19, 19, 1024) 0 batch_normalization_19[0][0]
____________________________________________________________________________________________________
batch_normalization_21 (BatchNor (None, 38, 38, 64) 256 conv2d_21[0][0]
____________________________________________________________________________________________________
conv2d_20 (Conv2D) (None, 19, 19, 1024) 9437184 leaky_re_lu_19[0][0]
____________________________________________________________________________________________________
leaky_re_lu_21 (LeakyReLU) (None, 38, 38, 64) 0 batch_normalization_21[0][0]
____________________________________________________________________________________________________
batch_normalization_20 (BatchNor (None, 19, 19, 1024) 4096 conv2d_20[0][0]
____________________________________________________________________________________________________
space_to_depth_x2 (Lambda) (None, 19, 19, 256) 0 leaky_re_lu_21[0][0]
____________________________________________________________________________________________________
leaky_re_lu_20 (LeakyReLU) (None, 19, 19, 1024) 0 batch_normalization_20[0][0]
____________________________________________________________________________________________________
concatenate_1 (Concatenate) (None, 19, 19, 1280) 0 space_to_depth_x2[0][0]
leaky_re_lu_20[0][0]
____________________________________________________________________________________________________
conv2d_22 (Conv2D) (None, 19, 19, 1024) 11796480 concatenate_1[0][0]
____________________________________________________________________________________________________
batch_normalization_22 (BatchNor (None, 19, 19, 1024) 4096 conv2d_22[0][0]
____________________________________________________________________________________________________
leaky_re_lu_22 (LeakyReLU) (None, 19, 19, 1024) 0 batch_normalization_22[0][0]
____________________________________________________________________________________________________
conv2d_23 (Conv2D) (None, 19, 19, 425) 435625 leaky_re_lu_22[0][0]
====================================================================================================
Total params: 50,983,561
Trainable params: 50,962,889
Non-trainable params: 20,672
____________________________________________________________________________________________________

Note: On some computers, you may see a warning message from Keras. Don't worry about it if you do--it is fine.

Reminder: this model converts a preprocessed batch of input images (shape: (m, 608, 608, 3)) into a tensor of shape (m, 19, 19, 5, 85) as explained in Figure (2).

3.3 - Convert output of the model to usable bounding box tensors

The output of yolo_model is a (m, 19, 19, 5, 85) tensor that needs to pass through non-trivial processing and conversion. The following cell does that for you.

def yolo_head(feats, anchors, num_classes):
    """Convert final layer features to bounding box parameters.
    Parameters
    ----------
    feats : tensor
        Final convolutional layer features.
    anchors : array-like
        Anchor box widths and heights.
    num_classes : int
        Number of target classes.
    Returns
    -------
    box_xy : tensor
        x, y box predictions adjusted by spatial location in conv layer.
    box_wh : tensor
        w, h box predictions adjusted by anchors and conv spatial resolution.
    box_conf : tensor
        Probability estimate for whether each box contains any object.
    box_class_pred : tensor
        Probability distribution estimate for each box over class labels.
    """
yolo_outputs = yolo_head(yolo_model.output, anchors, len(class_names))

You added yolo_outputs to your graph. This set of 4 tensors is ready to be used as input by your yolo_eval function.

3.4 - Filtering boxes

yolo_outputs gave you all the predicted boxes of yolo_model in the correct format. You're now ready to perform filtering and select only the best boxes. Lets now call yolo_eval, which you had previously implemented, to do this.

scores, boxes, classes = yolo_eval(yolo_outputs, image_shape)

3.5 - Run the graph on an image

Let the fun begin. You have created a (sess) graph that can be summarized as follows:

  1. yolo_model.input is given to yolo_model. The model is used to compute the output yolo_model.output

  2. yolo_model.output is processed by yolo_head. It gives you yolo_outputs

  3. yolo_outputs goes through a filtering function, yolo_eval. It outputs your predictions: scores, boxes, classes

Exercise: Implement predict() which runs the graph to test YOLO on an image. You will need to run a TensorFlow session, to have it compute scores, boxes, classes.

The code below also uses the following function:

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

which outputs:

  • image: a python (PIL) representation of your image used for drawing boxes. You won't need to use it.

  • image_data: a numpy-array representing the image. This will be the input to the CNN.

Important note: when a model uses BatchNorm (as is the case in YOLO), you will need to pass an additional placeholder in the feed_dict {K.learning_phase(): 0}.

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. 
    """

    # Preprocess your image
    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
    output_image = scipy.misc.imread(os.path.join("out", image_file))
    imshow(output_image)

    return out_scores, out_boxes, out_classes

Run the following cell on the "test.jpg" image to verify that your function is correct.

out_scores, out_boxes, out_classes = predict(sess, "test.jpg")
Found 7 boxes for test.jpg
car 0.60 (925, 285) (1045, 374)
car 0.66 (706, 279) (786, 350)
bus 0.67 (5, 266) (220, 407)
car 0.70 (947, 324) (1280, 705)
car 0.74 (159, 303) (346, 440)
car 0.80 (761, 282) (942, 412)
car 0.89 (367, 300) (745, 648)

The model you've just run is actually able to detect 80 different classes listed in "coco_classes.txt". To test the model on your own images: 1. Click on "File" in the upper bar of this notebook, then click "Open" to go on your Coursera Hub. 2. Add your image to this Jupyter Notebook's directory, in the "images" folder 3. Write your image's name in the cell above code 4. Run the code and see the output of the algorithm!

If you were to run your session in a for loop over all your images. Here's what you would get:

What you should remember:

  • YOLO is a state-of-the-art object detection model that is fast and accurate

  • It runs an input image through a CNN which outputs a 19x19x5x85 dimensional volume.

  • The encoding can be seen as a grid where each of the 19x19 cells contains information about 5 boxes.

  • You filter through all the boxes using non-max suppression. Specifically:

  • Score thresholding on the probability of detecting a class to keep only accurate (high probability) boxes

  • Intersection over Union (IoU) thresholding to eliminate overlapping boxes

  • Because training a YOLO model from randomly initialized weights is non-trivial and requires a large dataset as well as lot of computation, we used previously trained model parameters in this exercise. If you wish, you can also try fine-tuning the YOLO model with your own dataset, though this would be a fairly non-trivial exercise.

References: The ideas presented in this notebook came primarily from the two YOLO papers. The implementation here also took significant inspiration and used many components from Allan Zelener's github repository. The pretrained weights used in this exercise came from the official YOLO website.

Pictures taken from a car-mounted camera while driving around Silicon Valley. We would like to especially thank for providing this dataset! Drive.ai is a company building the brains of self-driving vehicles.

If you have 80 classes that you want YOLO to recognize, you can represent the class label ccc either as an integer from 1 to 80, or as an 80-dimensional vector (with 80 numbers) one component of which is 1 and the rest of which are 0. The video lectures had used the latter representation; in this notebook, we will use both representations, depending on which is more convenient for a particular step.

The output is a list of bounding boxes along with the recognized classes. Each bounding box is represented by 6 numbers (pc,bx,by,bh,bw,c)(p_c, b_x, b_y, b_h, b_w, c)(pc​,bx​,by​,bh​,bw​,c) as explained above. If you expand ccc into an 80-dimensional vector, each bounding box is then represented by 85 numbers.

box_confidence: tensor of shape (19×19,5,1)(19 \times 19, 5, 1)(19×19,5,1) containing pcp_cpc​ (confidence probability that there's some object) for each of the 5 boxes predicted in each of the 19x19 cells.

boxes: tensor of shape (19×19,5,4)(19 \times 19, 5, 4)(19×19,5,4) containing (bx,by,bh,bw)(b_x, b_y, b_h, b_w)(bx​,by​,bh​,bw​) for each of the 5 boxes per cell.

box_class_probs: tensor of shape (19×19,5,80)(19 \times 19, 5, 80)(19×19,5,80) containing the detection probabilities (c1,c2,...c80)(c_1, c_2, ... c_{80})(c1​,c2​,...c80​) for each of the 80 classes for each of the 5 boxes per cell.

the index of the class with the maximum box score () (Be careful with what axis you choose; consider using axis=-1)

the corresponding box score () (Be careful with what axis you choose; consider using axis=-1)

Use TensorFlow to apply the mask to box_class_scores, boxes and box_classes to filter out the boxes we don't want. You should be left with just the subset of boxes you want to keep. ()

Non-max suppression uses the very important function called "Intersection over Union", or IoU.

: 按照参数scores的降序贪婪的选择边界框的子集, 删除掉那些与之前的选择的边框具有很高的IOU的边框

85 = 5 + 80 where 5 is because (pc,bx,by,bh,bw)(p_c, b_x, b_y, b_h, b_w)(pc​,bx​,by​,bh​,bw​) has 5 numbers, and 80 is the number of classes we'd like to detect

Predictions of the YOLO model on pictures taken from a camera while driving around the Silicon Valley Thanks for providing this dataset!

Joseph Redmon, Santosh Divvala, Ross Girshick, Ali Farhadi - (2015)

Joseph Redmon, Ali Farhadi - (2016)

Allan Zelener -

Car detection dataset: The Drive.ai Sample Dataset (provided by drive.ai) is licensed under a . We are especially grateful to Brody Huval, Chih Hu and Rahul Patel for collecting and providing this dataset.

Redmon et al., 2016
Redmon and Farhadi, 2016
drive.ai
Hint
Hint
Hint
tf.image.non_max_suppression()
K.gather()
drive.ai
You Only Look Once: Unified, Real-Time Object Detection
YOLO9000: Better, Faster, Stronger
YAD2K: Yet Another Darknet 2 Keras
The official YOLO website
Creative Commons Attribution 4.0 International License