课程Github地址：https://github.com/wrk666/VSLAM-Course/tree/master
最后一次课了，加油！
- 内容
; 1. 回环检测与词袋
回顾视觉SLAM流程图
重点讲解 如何检测到回环的发生。
- 任意两个关键帧进行特征匹配，显然这样的计算量非常大。
- 基于里程计的方法：找里程计里面相临的帧
- 基于外观的方法（是主流）：核心在于如何衡量图相间的相似性，灰度，特征点或者其它方式？
基于外观的方法：有词袋和ML方法，多用词袋（现在ML方法很多性能比词袋更好，ML是以后的一个趋势）。
词袋就是把特征进行聚类形成word，如鼻子类，自行车坐类
很多word组成dictionary
图像的相似性就是word的相似性，看一张图片中的word的相似性，不管顺序。
字典实际上就是特征集，可以使用聚类算法（如k-means）对特征点进行聚类，每一个类就是一个特征集
遍历比较是O(n)的复杂度。建立kd树，将相似的类进行聚类，比较的时候先比较大类，在进入到某个大类中进行进一步比较，能够加速，时间复杂度是O(log n)的。
[TencentCloudSDKException] code:FailedOperation.ServiceIsolate message:service is stopped due to arrears, please recharge your account in Tencent Cloud requestId:e0331b58-b689-494a-a6a1-ccd84640ed0b
[En]
[TencentCloudSDKException] code:FailedOperation.ServiceIsolate message:service is stopped due to arrears, please recharge your account in Tencent Cloud requestId:c36eb3a4-e0d0-43cd-b948-52d79f68e2e7
[TencentCloudSDKException] code:FailedOperation.ServiceIsolate message:service is stopped due to arrears, please recharge your account in Tencent Cloud requestId:0b51fdc2-731d-4b88-8cdd-9c752592ba38
[En]
[TencentCloudSDKException] code:FailedOperation.ServiceIsolate message:service is stopped due to arrears, please recharge your account in Tencent Cloud requestId:285fc170-924c-484b-bfa9-acefe3ccd74b
上面的相似度使用了L 1 L_1 L 1 范数的形式。
比如n-1和n帧的相似度为0.5，那么你的阈值也差不多设置为大于0.5，要比相邻的帧的相似度大才行，这里是一些工程上的问题。
[TencentCloudSDKException] code:FailedOperation.ServiceIsolate message:service is stopped due to arrears, please recharge your account in Tencent Cloud requestId:9e1979f0-fe65-4665-9d8e-ce47eb0e72cd
[En]
[TencentCloudSDKException] code:FailedOperation.ServiceIsolate message:service is stopped due to arrears, please recharge your account in Tencent Cloud requestId:3d8839b2-ff92-4a13-b565-fd18ca692403
[TencentCloudSDKException] code:FailedOperation.ServiceIsolate message:service is stopped due to arrears, please recharge your account in Tencent Cloud requestId:a0596d2a-fccc-442a-8f4e-dbd6118c707c
[En]
[TencentCloudSDKException] code:FailedOperation.ServiceIsolate message:service is stopped due to arrears, please recharge your account in Tencent Cloud requestId:b1d67bdc-9021-469e-b6de-faaa3a77aab2
闭了之后就可以全局BA或者Pose Graph进行优化。
根据《SLAM14讲》上对于回环检测部分的讨论，此部分还存在以下一些细节性的问题：
- 相似性评分的处理：可以使用先验相似度进行归一化。
- 关键帧的处理：回环检测的关键帧通常是稀疏地选择的，否则相邻的帧总是被检测为平凡的回环，没有意义。
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检测之后的验证：由于词袋的word没有顺序，检测出来的回环容易出现假阳性。可以使用缓存机制（连续几帧回环才闭）或者空间上的一致性检测（估计与检测出来的回环的位姿，跟之前的位姿图进行对比）。
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建图（教材12讲）
[TencentCloudSDKException] code:FailedOperation.ServiceIsolate message:service is stopped due to arrears, please recharge your account in Tencent Cloud requestId:ef0604f3-b4f4-4e63-8481-138cff2a560a
[En]
[TencentCloudSDKException] code:FailedOperation.ServiceIsolate message:service is stopped due to arrears, please recharge your account in Tencent Cloud requestId:45c8f4cd-1b38-44c7-a1cd-5fffedbcf035
单目的稠密深度估计等
[TencentCloudSDKException] code:FailedOperation.ServiceIsolate message:service is stopped due to arrears, please recharge your account in Tencent Cloud requestId:8a577d88-4506-4126-9c74-2d30a21ed2ec
[En]
[TencentCloudSDKException] code:FailedOperation.ServiceIsolate message:service is stopped due to arrears, please recharge your account in Tencent Cloud requestId:5cde3186-8763-4d05-9c34-0a79ebbd21ac
[TencentCloudSDKException] code:FailedOperation.ServiceIsolate message:service is stopped due to arrears, please recharge your account in Tencent Cloud requestId:a7b30418-6451-4a78-b2e1-c22f5b287d37
[En]
[TencentCloudSDKException] code:FailedOperation.ServiceIsolate message:service is stopped due to arrears, please recharge your account in Tencent Cloud requestId:91434f29-0a33-48c9-ae80-d0628cb5ba98
[TencentCloudSDKException] code:FailedOperation.ServiceIsolate message:service is stopped due to arrears, please recharge your account in Tencent Cloud requestId:d1c5c91c-1ccb-4a1d-a388-9d507ce73d37
[En]
[TencentCloudSDKException] code:FailedOperation.ServiceIsolate message:service is stopped due to arrears, please recharge your account in Tencent Cloud requestId:7f7ea3db-b83f-4964-9d1d-8ad580c85c94
[TencentCloudSDKException] code:FailedOperation.ServiceIsolate message:service is stopped due to arrears, please recharge your account in Tencent Cloud requestId:e110389c-89e4-433e-a343-925333957bab
[En]
[TencentCloudSDKException] code:FailedOperation.ServiceIsolate message:service is stopped due to arrears, please recharge your account in Tencent Cloud requestId:5420180f-ce72-4064-8845-91d75e313951
; 3. 展望（教材14讲）
一些2016年时的展望，如果深入了解过其中某个开源方案的话会对了解SLAM有很大帮助。
下面可能进行VIO的学习。
[TencentCloudSDKException] code:FailedOperation.ServiceIsolate message:service is stopped due to arrears, please recharge your account in Tencent Cloud requestId:eb90ecae-d7ba-48a6-a644-e168325a5be0
[En]
[TencentCloudSDKException] code:FailedOperation.ServiceIsolate message:service is stopped due to arrears, please recharge your account in Tencent Cloud requestId:b45211b7-4dab-4e82-9397-a552b412070b
- 实践环节
在使用 feature_training.cpp生成字典的时候注意data的路径，否则生成的字典可能是空的，
在调用 loop_closure.cpp时会报错 Vocabulary does not exist
[TencentCloudSDKException] code:FailedOperation.ServiceIsolate message:service is stopped due to arrears, please recharge your account in Tencent Cloud requestId:abab52be-e1f8-4f14-89d4-7e94905a5546
[En]
[TencentCloudSDKException] code:FailedOperation.ServiceIsolate message:service is stopped due to arrears, please recharge your account in Tencent Cloud requestId:688e94ae-b104-45f7-bb6f-8962e77ec73f
使用了提供的 vocab_larger.yml.gz，但没有什么效果，甚至9的分数还下降了，
更换了字典，使用了ORB-SLAM2的字典，字典地址，区分度好了一些：
; 5. 设计SLAM系统在最后的Final Project中
Original: https://blog.csdn.net/qq_37746927/article/details/123842020
Author: 读书健身敲代码
Title: 深蓝学院-视觉SLAM课程-第8讲笔记-回环检测
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