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Waymo Open Dataset
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Per-point timestamps for LiDAR data #619

Open ajyang99 opened 1 year ago

ajyang99 commented 1 year ago

Hello, for the LiDAR data in the Perception dataset, are per-point timestamps provided? I looked through the documentation and the code but was unable to find this information, so just want to double check here. Thank you!

VincentCa commented 1 year ago

Hi, thanks for your question!

We provide timestamp_micros for each frame, which gives you the time of the first LiDAR scan. Knowing the spinning direction and frequency of the top (main) LiDAR, this should allow you to get a rough time estimate for when each top LiDAR point was scanned.

You might also want to look at https://github.com/waymo-research/waymo-open-dataset/blob/9d0be7c4aeed2ef5e11f2e739537bf2d27cec65b/src/waymo_open_dataset/dataset.proto#L163-L175 In the "virtual range image" representation, the point is essentially transformed in time to a common reference point (though only accounting for ego vehicle motion, not independent motion). There are some subtleties in the ego pose alignment that you may want to consider depending on your use case.

ajyang99 commented 1 year ago

Got it, thanks for the reply! Just to further clarify:

Thanks!

VincentCa commented 1 year ago

Does this mean that with the rough estimate, each column in the range image will have the same timestamp, and from left to right of the image the estimated timestamps will be uniformly distributed from beginning time of the sweep up to end time of the sweep?

Yes, that's correct (it won't be exact, but should result in a reasonable approximation).

Wrt the virtual range image, is the virtual range image obtained by transforming each point in the raw range image with the underlying vehicle pose for that point, and then transforming to the fixed vehicle frame defined by the middle column in the range image?

Yes.

Do all LiDARs start at the same time for every sweep? If not, does this mean we can only infer LiDAR point timestamps for the top LiDAR?

Unfortunately, we can not share more details on the other LiDARs. Note that we also provide the range image pose for the top LiDAR only. As mentioned in our Waymo Open Dataset paper, our LiDARs are synchronized.

Hope this helps!

ajyang99 commented 1 year ago

This helps a lot, thanks for all the information!