Data-preprocessing for kinetics-400

Hi, I would like to know how to preprocess the kinetics-400 for reproducing the results. I found that extracting tvl1 flow before rescale the rgb images leads to worse flow recognition accuracy.
So, currently, I first resampling videos at 25 fps. Then I extract rgb frames and resize with shorter side setting 256 pixels. I am using opencv3.4 version of cv::cuda::OpticalFlowDual_TVL1 for flow extraction on the resize gray-scale frames. All the pixels values are rescale as mention in the project. Are there any details i am missing in this preprossing procedure? Or, am I conducting the right way for extracting optical flow? Thanks.