MCAP Protobuf writer has "no messages"

[aeon0]

I am trying to get started with the mcap protobuf writer to create foxglove recordings in .mcap format. I have a very simplified file just writing a few random boxes in a scene update. But when opening the generated mcap file with foxglove studio, it shows "no messages".

import random
from foxglove_schemas_protobuf.SceneUpdate_pb2 import SceneUpdate
from mcap_protobuf.writer import Writer


def main():
    f = open("test.mcap", "wb")
    mcap_writer = Writer(f)
    for i in range(0, 10):
        ts = int(i * 1e9)

        pc_scene = SceneUpdate()
        entity = pc_scene.entities.add()
        entity.timestamp.FromNanoseconds(ts)
        entity.frame_id = "base_link"
        for i in range(4):
            r = random.randint(0, 255)  # uint8
            g = random.randint(0, 255)  # uint8
            b = random.randint(0, 255)  # uint8

            cube = entity.cubes.add()
            cube.size.x = 2
            cube.size.y = 2
            cube.size.z = 2
            cube.pose.position.x = random.uniform(-20, 20)  # float32
            cube.pose.position.y = random.uniform(-20, 20)  # float32
            cube.pose.position.z = random.uniform(-2, 2)  # float32
            cube.color.r = r / 255
            cube.color.g = g / 255
            cube.color.b = b / 255
            cube.color.a = 1

        mcap_writer.write_message(
            topic="random_boxes",
            message=pc_scene,
            log_time=ts,
            publish_time=ts,
        )


if __name__ == "__main__":
    main()

[achim-k]

I believe you need an mcap_writer.finish() at the end of your main() function.

You can also use mcap doctor <mcap_file> or mcap info <mcap_file> to see if something is wrong.

[aeon0]

Thats it. Thanks!

[achim-k]

I would recommend to use the writer with a with block, then finish() is called automatically when the block goes out of scope.

Example: https://github.com/foxglove/mcap/blob/main/python/examples/protobuf/writer.py