Passing data between hooks and stages

Passing custom data (per frame) between hooks and stages

This example contains boilerplate code that you can use to learn how to associate custom data to a frame. That custom data can be obtained back from other hooks of the same stage and even from hooks of subsequent stages.

Note this mechanism allows you to associate data to a single frame. If you need data that will be shared by all frames check the KV store documentation.


This example contains boilerplate code that you can find here (opens in a new tab).

To assign custom data to a frame such as bounding boxes results, masks, or anything you like, you just need to set frame_data['user_data']. It accepts any type as value. The only restriction is that, if you set a dictionary as value, al the keys must be strings. Some examples:

  • Integers:
    frame_data['user_data'] = 100
  • Floats:
    frame_data['user_data'] = 100.5
  • Strings:
    frame_data['user_data'] = "Hello!"
  • Heterogeneus arrays:
    frame_data['user_data'] = ["string", 13, 34.6]
  • Heterogeneus Dictionaries (IMPORTANT: all keys must be strings):
    frame_data['user_data'] = {
        "key1": 0,
        "key2": [1, "3"],
        "key3": { "inner1": "hola" }

With that, in later hooks (either from the same stage or other) you can obtain back the data just accessing that field:

my_data = frame_data['user_data']

In order to connect two or more stages, simply pass the list of stage names to the add stream command under the frame-path option. For example:

pipeless add stream --input-uri "file:///home/user/my/path.mp4" --output-uri "screen" --frame-path "stage1,stage2"

Using the above command, if you added data to the frame user_data field in a hook of stage1 you will be able to use it in the hooks of stage2.