[Feature-Request] RVL Depth Image Compression

[borongyuan]

RVL is a lossless depth image compression algorithm developed by Microsoft. RVL frame is supported by some Orbbec cameras. RVL codec has been added to ROS since Noetic/Jazzy. I think it will help reduce the size of the database and save bandwidth (for camera data transfer and ROS communication).

[matlabbe]

Thanks for the link, that could be indeed useful! As a comparison, we are currently using PNG:

Implementation details:

• Example RVL encoding: https://github.com/ros-perception/image_transport_plugins/blob/ba6a7632613ba19a6d9f3cb33483af053e6ae7a9/compressed_depth_image_transport/src/codec.cpp#L307-L319
• Example RVL decoding: https://github.com/ros-perception/image_transport_plugins/blob/ba6a7632613ba19a6d9f3cb33483af053e6ae7a9/compressed_depth_image_transport/src/codec.cpp#L113-L142
• With current rtabmap compression code:
  • on encoding phase, we could provide "rvl" format option
  • on decoding phase, we current rely on opencv decode function to know image size and type of the compressed data (which can be RGB, depth 16bits or depth 32bits). We may check if opencv is adding some header in compressed data when encoding, so we could create a new encoding header to know the compressed data is "rvl", and not "png" or "jpg". That should be backward compatible to still load old databases.

[borongyuan]

Hi, RVL only supports the depth image in 16 bits format. So I went to see how the depth image in 32 bits format was processed previously. According to https://github.com/introlab/rtabmap/blob/b2e26e81c14bd7a1394da10998d0264968089cbe/corelib/src/Compression.cpp#L105C1-L110C4 It looks like it will be converted to 8bits-4channel data first and then encoded using png. What I'm curious about is can this be compressed effectively? Because the resulting data after the conversion doesn't seem to be a reasonable image.

If I add another _depthCompressionFormat option, how should 32 bits images be processed when it is set to '.rvl'? From a compatibility perspective, we should keep the original approach. Or force conversion to 16 bits format and give a warning when ".rvl" is set?