Extracating raw values of slices (I/P/B Slices)

[makisukurisu]

Hi! Is it possible to extract values of slices (not values/pixels of the video stream itself) using this library?
To avoid confusion, when I'm talking about slices here, I'm talking about I/P/B (and other) as defined by H.264

For example, I can see that there's a pict_type attribute on a frame, so this got me thinking that I can extract the values of slices (they have to extracted somewhere to reconstruct the image, right?)

I've tried using to_ndarray method, and it's counterpart - from_ndarray to alter the videostream, but even if it returns some kind of representation of the slices, I can't use it to get back the stream.
to_ndarray returns 2-dim array, but from_ndarray requires 3-dims. Also the to_ndarray method returns (in my case) different shape, then my video. For example, 19201080 returns 16201920.

Perhaps I misunderstood how these methods should be used (it looks like I can input per-channel pixel values with from_ndarray?)

For some context, I'm designing a steganography algorithm that will embed data to (alter value of) I/P/B slices (blocks) of compressed H.264 video. I guess I can implement parsing on my own, of course, but if there's a ready-made solution for decoding/encoding video files, I'd like to use that instead.

Thanks, and sorry for my ramble.