Compression Ratios mismatch - post bag compression vs runtime compression

[karthik101200]

I ran a small experiment where I had a bag which had 3 pointcloud topics in the required mcap format which had a size of ~550mb.

• First experminet was to use the CLI tool - cloudini_rosbag_converter for compressing bag at resolution 0.001. The size for which came out to be ~105mb.

• Second experiment was to run the topic converter for each topic while playing the source bag and record the compressed topic at resolution 0.001. This size for the bag for all three topics was ~ 215mb. For reference the compression ratios for 3 pointclouds while running the converter was approximately 0.54, 0.41 and 0.26 respectively.

Just wanted to know what might be the possible reason for the mismatch and if there is a way to get the compression ratio < 0.25 during "runtime" with similar resolutions.
Thanks

[facontidavide]

Can you share the "ros2 bag info ..." output of the 3 bags:

• original one
• compressed with CLI
• created subscribing to compressed topics