About coordinate and box_velocity about nuscenes-devkit HOT 4 CLOSED

@whyekit-motional Oh, I totally got it! Thanks a lot

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@Nan-yxchen here are my responses to your queries:

1. Yes, box_velocity should be in the global frame
2. Both methods are valid ways to calculate the velocity of the ego, and should produce similar values (but not necessarily equal)
3. Yes, the x-axis points forward of the ego, the y-axis points to the left of the ego, and the z-axiz points upward of the ego

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@whyekit-motional I got it, Thanks! But I still confused about some questions:

1. If I use the CAN_BUS information to calculate the velocity of ego, Is the CAN_BUS information in the global frame? And is the ego_pose.json information in the global frame too?

2. The x,y,z-axis in your response means the IMU coordinate and ego coordinate? I read other issues said that the x axis of global coordinate is going from the top left corner to the top right corner along the top edge of that rectangle, and the y axis of global coordinate is going from the top left corner to the bottom left corner along the left edge of that rectangle.

Greatly thanks in advance!!!

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@Nan-yxchen

1. If you read the velocity of the ego directly from the CAN bus, it will be in the ego frame - pls see https://github.com/nutonomy/nuscenes-devkit/blob/master/python-sdk/nuscenes/can_bus/README.md#pose
2. I was referring to both the IMU frame and ego frame in my response (you can also visualize the various frames at https://www.nuscenes.org/static/media/data.9ef46c59.png)

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