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introlab / rtabmap

RTAB-Map library and standalone application
https://introlab.github.io/rtabmap
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Compression Algorithm for UserData #1205

Closed 360wcui closed 5 months ago

360wcui commented 5 months ago

Hello, I am trying to read user_data from rtabmap.db directly, and notice that the data is compressed: https://github.com/introlab/rtabmap/blob/b374c6cd8e9d1046277555c58ff0fe1b81e3bff1/corelib/src/SensorData.cpp#L422

Which compression algorithm do you use, what's the rationale behind using the compression algorithm for UserData. Would it be possible to disable compression? Thank you.

360wcui commented 5 months ago

The compression uses zlib library, which employs the DEFLATE compression algorithm.

matlabbe commented 5 months ago

We do compress by default as we cannot know how big the data could be (could be an image). Compressing the data also help to know which kind of matrix it is (the matrix type and dimensions are encoded in the compressed byte array saved in the database).

We indeed use zlib. https://github.com/introlab/rtabmap/blob/c27507bc772cfda34f6624d386ab2cd62d49e90d/corelib/src/Compression.cpp#L247-L259

We have also a python example to uncompress: https://github.com/introlab/rtabmap_ros/blob/master/rtabmap_python/src/rtabmap_python/compression.py