安装BoW库
训练字典需要安装BoW库（DBoW3）https://github.com/rmsalinas/DBoW3
注：cmake工程，老方法编译安装即可，
记住sudo make install
编译cmake工程mkdir buildcmake ..make -j8运行此程序，将看到如下字典信息输出：
分支数量k为10，深度L为5，单词书为4970，没有达到最大容量；Weighting表示权重，Scoring表示得分。
相似度计算
./build/loop_closure data
程序演示了两种对比：图像之间的对比，以及图像与数据库之间的对比。
hf@hf:~/code/slambook-master/ch12$ ./build/loop_closure data
reading database
reading images...
detecting ORB features ...
[ INFO:0] Initialize OpenCL runtime...
comparing images with images
image 0 vs image 0 : 1
image 0 vs image 1 : 0.0238961
image 0 vs image 2 : 0.0198893
image 0 vs image 3 : 0.0339413
image 0 vs image 4 : 0.0160477
image 0 vs image 5 : 0.0300543
image 0 vs image 6 : 0.0222911
image 0 vs image 7 : 0.0197256
image 0 vs image 8 : 0.0228385
image 0 vs image 9 : 0.0583173
image 1 vs image 1 : 1
image 1 vs image 2 : 0.0422068
image 1 vs image 3 : 0.0282324
image 1 vs image 4 : 0.0225322
image 1 vs image 5 : 0.025146
image 1 vs image 6 : 0.0161122
image 1 vs image 7 : 0.0166048
image 1 vs image 8 : 0.0350047
image 1 vs image 9 : 0.0326933
image 2 vs image 2 : 1
image 2 vs image 3 : 0.0361374
image 2 vs image 4 : 0.0330401
image 2 vs image 5 : 0.0189786
image 2 vs image 6 : 0.0182909
image 2 vs image 7 : 0.0137075
image 2 vs image 8 : 0.0270761
image 2 vs image 9 : 0.0219518
image 3 vs image 3 : 1
image 3 vs image 4 : 0.0304209
image 3 vs image 5 : 0.03611
image 3 vs image 6 : 0.0205856
image 3 vs image 7 : 0.0208058
image 3 vs image 8 : 0.0312382
image 3 vs image 9 : 0.0329747
image 4 vs image 4 : 1
image 4 vs image 5 : 0.0498645
image 4 vs image 6 : 0.0345081
image 4 vs image 7 : 0.0227451
image 4 vs image 8 : 0.0208472
image 4 vs image 9 : 0.0266803
image 5 vs image 5 : 1
image 5 vs image 6 : 0.0198106
image 5 vs image 7 : 0.0162269
image 5 vs image 8 : 0.0259153
image 5 vs image 9 : 0.0220667
image 6 vs image 6 : 1
image 6 vs image 7 : 0.0212959
image 6 vs image 8 : 0.0188494
image 6 vs image 9 : 0.0208606
image 7 vs image 7 : 1
image 7 vs image 8 : 0.012798
image 7 vs image 9 : 0.0205016
image 8 vs image 8 : 1
image 8 vs image 9 : 0.0228186
image 9 vs image 9 : 1
comparing images with database
database info: Database: Entries = 10, Using direct index = no. Vocabulary: k = 10, L = 5, Weighting = tf-idf, Scoring = L1-norm, Number of words = 4970
searching for image 0 returns 4 results:
<EntryId: 0, Score: 1>
<EntryId: 9, Score: 0.0583173>
<EntryId: 3, Score: 0.0339413>
<EntryId: 5, Score: 0.0300543>
searching for image 1 returns 4 results:
<EntryId: 1, Score: 1>
<EntryId: 2, Score: 0.0422068>
<EntryId: 8, Score: 0.0350047>
<EntryId: 9, Score: 0.0326933>
searching for image 2 returns 4 results:
<EntryId: 2, Score: 1>
<EntryId: 1, Score: 0.0422068>
<EntryId: 3, Score: 0.0361374>
<EntryId: 4, Score: 0.0330401>
searching for image 3 returns 4 results:
<EntryId: 3, Score: 1>
<EntryId: 2, Score: 0.0361374>
<EntryId: 5, Score: 0.03611>
<EntryId: 0, Score: 0.0339413>
searching for image 4 returns 4 results:
<EntryId: 4, Score: 1>
<EntryId: 5, Score: 0.0498645>
<EntryId: 6, Score: 0.0345081>
<EntryId: 2, Score: 0.0330401>
searching for image 5 returns 4 results:
<EntryId: 5, Score: 1>
<EntryId: 4, Score: 0.0498645>
<EntryId: 3, Score: 0.03611>
<EntryId: 0, Score: 0.0300543>
searching for image 6 returns 4 results:
<EntryId: 6, Score: 1>
<EntryId: 4, Score: 0.0345081>
<EntryId: 0, Score: 0.0222911>
<EntryId: 7, Score: 0.0212959>
searching for image 7 returns 4 results:
<EntryId: 7, Score: 1>
<EntryId: 4, Score: 0.0227451>
<EntryId: 6, Score: 0.0212959>
<EntryId: 3, Score: 0.0208058>
searching for image 8 returns 4 results:
<EntryId: 8, Score: 1>
<EntryId: 1, Score: 0.0350047>
<EntryId: 3, Score: 0.0312382>
<EntryId: 2, Score: 0.0270761>
searching for image 9 returns 4 results:
<EntryId: 9, Score: 1>
<EntryId: 0, Score: 0.0583173>
<EntryId: 3, Score: 0.0329747>
<EntryId: 1, Score: 0.0326933>
done.
for image 9 returns 4 results:
<EntryId: 9, Score: 1>
<EntryId: 0, Score: 0.0583173>
<EntryId: 3, Score: 0.0329747>
<EntryId: 1, Score: 0.0326933>
done.