Hello Everyone,
There is a bit of a confusion about calcluating the mean and std of a dataset. Can you please answer my questions?

What is the correct way to calculate the mean and std of a dataset?
a) Calculate the mean and std for the whole dataset before splitting the dataset in to test,train and eval?
b) Split the dataset in to test,train and eval and then calculate the mean and std individually for test, train and eval
In case, if we are using datasets of different modalities. Example (RGB, Infrared, depthmaps etc). How to calculate the mean and std?
a) Individual mean and std for each modality?
b) Mix the image modalities and the calulate the mean and std for all the modalites together?

Thank you.