Prediction-level metadata in sv.Detections

[PawelPeczek-Roboflow]

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• I have searched the Supervision issues and found no similar feature requests.

Description

sv.Detections supports data aligned detection-major (one element for one detection), guarded by

def validate_data(data: Dict[str, Any], n: int) -> None:
    for key, value in data.items():
        if isinstance(value, list):
            if len(value) != n:
                raise ValueError(f"Length of list for key '{key}' must be {n}")
        elif isinstance(value, np.ndarray):
            if value.ndim == 1 and value.shape[0] != n:
                raise ValueError(f"Shape of np.ndarray for key '{key}' must be ({n},)")
            elif value.ndim > 1 and value.shape[0] != n:
                raise ValueError(
                    f"First dimension of np.ndarray for key '{key}' must have size {n}"
                )
        else:
            raise ValueError(f"Value for key '{key}' must be a list or np.ndarray")

Putting prediction in broader context sometimes requires information about the whole prediction. Instances when those would be needed:

• one makes prediction on image previously cropped and wants to retain coordinates system conversion to transform bboxes and masks. To do it one only needs parent dimensions and shift in parent coordinates system. Currently if we wanted to put that into sv.Detections, we would need to broadcast that information into all detections - and (in absence of detections) this may not be possible (yet, would let more graceful sv.Detections processing downstream)

The solution that probably does not introduce breaking change would be:

@dataclass
class Detections:
    xyxy: np.ndarray
    mask: Optional[np.ndarray] = None
    confidence: Optional[np.ndarray] = None
    class_id: Optional[np.ndarray] = None
    tracker_id: Optional[np.ndarray] = None
    data: Dict[str, Union[np.ndarray, List]] = field(default_factory=dict)
    prediction_data: Dict[str, Any] = field(default_factory=dict)

# not including `prediction_data` into `__getitem__(...)` and iterations,
# but assuming direct access `detections.prediction_data[...]`

Use case

No response

Additional

No response

Are you willing to submit a PR?

• Yes I'd like to help by submitting a PR!

[LinasKo]

What we'd need to figure out too, is how two Detections with different prediction_data should interact.

Right now the interactions happens in:

1. Detections.merge - aimed to stack contents of 2x Detections and produce 1x Detections.
2. Detections._merge_inner_detections_objects in Add Non-Maximum Merging (NMM) to Detections #500, aimed to merge everything inside - e.g. boxes of N Detections into one Detections with one large box.

For each respective case, we can follow similar implementations we have for other variables:

1. raise when different keys are defined, merge successfully when some are missing.
   • How to do the merge itself? I don't know.
2. Keep the prediction_data of the 'winning' detection (highest confidence detection, or if both have None - first detection)

[PawelPeczek-Roboflow]

👍 yeah, haven't thought about that
I guess that keys collisions may occur, in that particular case I would just create list of values, that should be quite predictable and easy to explain default behaviour. We may also expose additional parameter - on_prediction_data_merge which would be callable given list of metadata supposed to return merged dict. That way we let people decide

[LinasKo]

Suppose it's metadata about source image. Would merging and keeping a list of [image_1.png, image_2.png] be useful? Also, do you have some specifics in mind on what you'd use it for?

[PawelPeczek-Roboflow]

I would see this issue useful in broader context, not even specifically regarding image, just regarding anything providing context information about the prediction.
Merging makes sense (for instance if u keep track of "parent_ids" - then list of them is useful after merge).

Specific use-cases:
https://github.com/roboflow/inference/blob/d6d38f18d7a1209f9e5d176b1362650fa4999cd1/inference/core/workflows/core_steps/common/utils.py

[SkalskiP]

Hi @LinasKo and @PawelPeczek-Roboflow 👋🏻

• do we need a separate field?

data: Dict[str, Union[np.ndarray, List]] = field(default_factory=dict)
prediction_data: Dict[str, Any] = field(default_factory=dict)

• mechanisms that this change may impact:
  • sv.Detections.__getitem__
  • sv.Detections.__setitem__
  • sv.Detections.merge - I honestly don't like the idea of creating List[T] while we merge two sv.Detections with T extra data. The end user may expect that field to be of T type. Especially since there are a lot of API functions that internally run merge and users don't even know about it.
  • upcoming Add Non-Maximum Merging (NMM) to Detections #500 - what do we do when we literally merge two sv.Detections objects? @LinasKo
  • sv.CSVSink and sv.JSONSink. will those new values be saved into a CSV or JSON file?

[PawelPeczek-Roboflow]

prediction_data could serve the role of container for data describing all detections, when data will remain detection-level.

Initially I thought we should not touch sv.Detections.__getitem__ and sv.Detections.__setitem__, as that would be very natural to operate on prediction_data directly, and that would probably not have a chance to introduce ambiguity in operations.

sv.Detections.merge - ok, so let's require resolver function whenever prediction_data is not empty

regarding sinks - as I believe they are supposed to dump detections one-by-one - we may let people decide if they want to broadcast metadata, conflicts with data keys may be resolved by adding prefix. This is the problem (just the opposite-way-around) that we are facing while having prediction-level metadata and broadcasting them into detection-level

[LinasKo]

Re: literal merging & data: we previously spoke that users may add anything they wish into data, and that we should drop non-winning data. E.g. we can't easily merge two distinct class names into one. Same approach would apply here - we'd keep prediction_data of winning Detections.

[PawelPeczek-Roboflow]

happy for that to be the case, as long as user can inject callable changing that behaviour

[SkalskiP]

sv.DetectionsSmoother.update_with_detections is example of call that will silently convert T type metadata into List[T]

[PawelPeczek-Roboflow]

for this case more reasonable thing to do is probably override, or letting people decide what to do

[SkalskiP]

I think in places like sv.DetectionsSmoother.update_with_detections, we just need to come up with reasonable behavior. I don't want to introduce more parameters into that API.

[PawelPeczek-Roboflow]

defaults should be there of-course, but I feel like not providing a way to alter the default behaviour as limiter

[SkalskiP]

If someone has a sv.Detections object and passes it through DetectionsSmoother.update_with_detections, they don't want that data to be mutated. They want their boxes to stop flickering.

[SkalskiP]

@PawelPeczek-Roboflow, how urgent is it? Do you want us to implement is? Or you or Grzegorz will take care of creating PR?

[PawelPeczek-Roboflow]

well, yes but we cannot predict what the use-cases would be for metadata - hence advocating for exposing a knob for the user
I am not particularly concern about this particular method, we can add knob later if that's needed there.
This is not urgent, we've solved this problem in some way that is fine for the time being - would just be good to have this supported in sv some time in the future.
We can take that, letting it to decide who would implement changes later, just want to reach conclusion if that change is ok and have high-level plan of implementation

[SkalskiP]

It is 100% okay. Most of my concerns come down to implementation details.

[PawelPeczek-Roboflow]

ok, so lets resolve as we go

[LinasKo]

Hey @PawelPeczek-Roboflow, what was the outcome? Are you using Detections.data and storing a copy of the metadata for each detected object? Or do you no longer need it?

[PawelPeczek-Roboflow]

not sure why it got closed tbh

[LinasKo]

Implemented via #1589

@PawelPeczek-Roboflow, you still need it, you may use it like so:
detections.metadata["camera_id"] = 42.

Caveats:

• Merging 2x empty detections with metadata does not preserve it.
• metadata is not yet saved in CSVSink and JSONSink

[PawelPeczek-Roboflow]

cool thanks