I want to compute mean value for every RGB channel through all dataset stored in a numpy array. I know it's done with `np.mean` and I know its basic usage.

```
np.mean(arr, axis=(??))

```

But as the array has 4 dimensions, I'm a bit lost in setting the correct axis. All examples I found were dealing with just 1-D or 2-D arrays. So how should the function call look like having an array e.g. `(1000, 512, 512, 3)`?