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Readme file updated with instructions for the LZ4 compilation issue. #1

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Jun 28, 2022
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Readme file updated with instructions for the LZ4 compilation issue. #1

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27 changes: 26 additions & 1 deletion README.md
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Original file line number Diff line number Diff line change
Expand Up @@ -32,7 +32,7 @@ cd /home/${USER}/Documents/oxford-eval-sequences/2019-01-10-12-32-52-radar-oxfor
```

## Running
The odometry an be launched in two modes.
The odometry can be launched in two modes.
* Offline: (used in this guide) runs at the maximum possible rate.
* Online: Standard rostopic interface. radar_driver_node and odometry_keyframes. The radar_driver_node subscribes to "/Navtech/Polar"

Expand All @@ -59,10 +59,35 @@ doi={10.1109/IROS51168.2021.9636253}}


## Troubleshooting
### Malformed bagfile
```console
# Problem
user@computer:~$ terminate called after throwing an instance of 'rosbag::BagFormatException' what(): Required 'op' field missing
# Solution
rosbag reindex 2019-01-10-12-32-52-radar-oxford-10k.bag
```
### Compilation issue due to incompatible versions of LZ4 in ROS Melodic
During compilation, this error can occur on some installations of Ubuntu 18.04 with ROS Melodic:
```console
/usr/include/flann/ext/lz4.h:196:57: error: conflicting declaration ‘typedef struct LZ4_stream_t LZ4_stream_t’
typedef struct { long long table[LZ4_STREAMSIZE_U64]; } LZ4_stream_t;
...
/usr/include/lz4.h:196:57: note: previous declaration as ‘typedef struct LZ4_stream_t LZ4_stream_t’
typedef struct { long long table[LZ4_STREAMSIZE_U64]; } LZ4_stream_t;
```
and
```console
/usr/include/flann/ext/lz4.h:249:72: error: conflicting declaration ‘typedef struct LZ4_streamDecode_t LZ4_streamDecode_t’
typedef struct { unsigned long long table[LZ4_STREAMDECODESIZE_U64]; } LZ4_streamDecode_t;
...
/usr/include/lz4.h:249:72: note: previous declaration as ‘typedef struct LZ4_streamDecode_t LZ4_streamDecode_t’
typedef struct { unsigned long long table[LZ4_STREAMDECODESIZE_U64]; } LZ4_streamDecode_t;
```
This error is caused by two conflicting versions of LZ4, one used for ROS serialization and the other one by flann kdtree in PCL. As this [GitHub issue](https://github.com/ethz-asl/lidar_align/issues/16) suggests, there are a few ways to fix it, yet none of them can be really considered clean. We suggest using simlinks to point to a single version of the conficting header files, since this modification can be easily reverted:
```console
sudo mv /usr/include/flann/ext/lz4.h /usr/include/flann/ext/lz4.h.bak
sudo mv /usr/include/flann/ext/lz4hc.h /usr/include/flann/ext/lz4.h.bak

sudo ln -s /usr/include/lz4.h /usr/include/flann/ext/lz4.h
sudo ln -s /usr/include/lz4hc.h /usr/include/flann/ext/lz4hc.h
```