> My understanding is that the proposal from @Zongpei_Jiang is to map uint8[] to builtin.bytes as several ROS 1 messages use uint8[] to send binary data around

I agree with that . Just map all the `uint8[]` to `bytes[]` instead of `list[]` may cause other problems since some algorithms are designed using the `msg.data` as `list` in python .

So that's why i want to create a type like `imguint8` that i can also create a `BytesImage.msg` which use the type to deliver image messages like what 'Image.msg' does . And i have changed some files in `rosidl` package and `sensor_msg` package and succeeded in ament building . 

I have changed following files to add the `imguint8`:  

    rosidl/rosidl_generator_py/resource/_msg_support.c.em
    rosidl/rosidl_generator_py/resource/_msg.py.em
    rosidl/rosidl_generator_py/msg/Primitives.msg
    rosidl/rosidl_generator_py/rosidl_generator_py/generate_py_impl.py
    rosidl/rosidl_parser/rosidl_parser/__init__.py
    common_interfaces/sensor_msgs/msg/BytesImage.msg
    common_interfaces/sensor_msgs/CMakeLists.txt
    rosidl/rosidl_generator_c/include/rosidl_generator_c/primitives_array.h
    rosidl/rosidl_generator_c/include/rosidl_generator_c/primitives_array_functions.h
    rosidl/rosidl_generator_c/msg/ImgUint8.msg

    rosidl/rosidl_generator_c/msg/Primitive*.msg

    rosidl/rosidl_generator_c/resource/msg__struct.h.em

    rosidl/rosidl_generator_c/rosidl_generator_c/__init__.py
    rosidl/rosidl_generator_c/src/primitives_array_functions.c


    rosidl/rosidl_typesupport_introspection_c/include/rosidl_typesupport_introspection_c/field_types.h

    rosidl/rosidl_generator_cpp/msg/Primitive*.msg

    rosidl/rosidl_generator_cpp/resource/msg__struct.hpp.em
    rosidl/rosidl_generator_cpp/rosidl_generator_cpp/__init__.py

    rosidl/rosidl_typesupport_introspection_cpp/include/rosidl_typesupport_introspection_cpp/field_types.hpp



But i failed using the new type to publish the `BytesImage.msg` type with reporting :  
`Assertion `convert_from_py != NULL && "unable to retrieve convert_from_py function, type_support mustn't have been imported"' failed.`

So i want to know whether i can add the `imguint8` type and if i can , what i have missed to make the `BytesImage.msg` can work .

Thanks





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