## > Velocity information

Hello, I have a question about data. As the information on the website, the dataset format we get is Range-Angle heatmap. But I wonder, where is the velocity information that you mentioned as a useful feature in RODNet paper? Thanks in advance!

Posted by: Haanhsoso @ Jan. 22, 2021, 2:46 a.m.

The speed/motion information can potentially be extracted from multiple frames and multiple chirps. In the RODNet (WACV2021), we use 3D convolutions to extract this temporal information across multiple frames. You can also try to come up with better ideas to extract speed/motion information from the RF images. Please let us know if you have further questions!

Posted by: ywang26 @ Jan. 22, 2021, 7:28 a.m.

I try to extract the velocity information directly from the Range-Angle heatmap. I stack chirps of the same sequence together (get a 3D matrix) and perform FFT on the phase along the chirps axis. But I fail to get the velocity information. Instead, the output 3D complex matrix only has noisy points scattered in magnitude. I wonder if this process is not able to retrieve the velocity. If it is the case, is there a way to retrieve the velocity directly? (I think phase information contains the velocity information)

Part of my codes are as follow, where chirps_seq is a (N, w, h, 2) matrix containing N chirps in total.

chirps_fft = np.fft.fft(phase_4D(chirps_seq), axis=0).astype('complex64')

chirps_fft_float = chirps_fft.T.view(np.float32).T.reshape(chirps_fft.shape + (2,))

Posted by: zangwei @ Feb. 1, 2021, 4:22 p.m.

Yes, the velocity information is included in the multi-chirp data. However, there are two things you need to pay attention to: 1) the provided RF images are **normalized**, not the raw signal values; 2) the number of chirps is **downsampled** from 255 to 4 in the ROD2021 dataset. We didn't implement this Doppler FFT on the ROD2021 dataset so that we cannot provide you with more detailed information.

Posted by: ywang26 @ Feb. 2, 2021, 1:33 a.m.

Thanks! Can you further explain what is the "normalize" process? In the dataset description, the data is going through FFT - LPF - FFT, is there another normalizing process?

Posted by: zangwei @ Feb. 2, 2021, 3:11 a.m.

The normalization step is after the pre-processing and before sent into the network. It normalizes all the magnitude of complex numbers between 0 and 1.

Posted by: ywang26 @ Feb. 2, 2021, 3:24 a.m.