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Polygons Utils

filter_polygons_by_area

supervision.detection.utils.polygons.filter_polygons_by_area(polygons: list[npt.NDArray[np.number]], min_area: float | None = None, max_area: float | None = None) -> list[npt.NDArray[np.number]]

Filters a list of polygons based on their area.

Parameters:

Name Type Description Default

polygons

list[NDArray[number]]

A list of polygons, where each polygon is represented by a NumPy array of shape (N, 2), containing the x, y coordinates of the points.

 required

min_area

float | None

The minimum area threshold. Only polygons with an area greater than or equal to this value will be included in the output. If set to None, no minimum area constraint will be applied.

 None

max_area

float | None

The maximum area threshold. Only polygons with an area less than or equal to this value will be included in the output. If set to None, no maximum area constraint will be applied.

 None

Returns:

Type Description
list[NDArray[number]]

A new list of polygons containing only those with areas within the specified thresholds.
Source code in src/supervision/detection/utils/polygons.py 
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def filter_polygons_by_area(
    polygons: list[npt.NDArray[np.number]],
    min_area: float | None = None,
    max_area: float | None = None,
) -> list[npt.NDArray[np.number]]:
    """
    Filters a list of polygons based on their area.

    Args:
        polygons: A list of polygons, where each polygon is
            represented by a NumPy array of shape `(N, 2)`,
            containing the `x`, `y` coordinates of the points.
        min_area: The minimum area threshold.
            Only polygons with an area greater than or equal to this value
            will be included in the output. If set to None,
            no minimum area constraint will be applied.
        max_area: The maximum area threshold.
            Only polygons with an area less than or equal to this value
            will be included in the output. If set to None,
            no maximum area constraint will be applied.

    Returns:
        A new list of polygons containing only those with
            areas within the specified thresholds.
    """
    if min_area is None and max_area is None:
        return polygons
    ares = [cv2.contourArea(polygon) for polygon in polygons]
    return [
        polygon
        for polygon, area in zip(polygons, ares)
        if (min_area is None or area >= min_area)
        and (max_area is None or area <= max_area)
    ]

approximate_polygon

supervision.detection.utils.polygons.approximate_polygon(polygon: npt.NDArray[np.number], percentage: float, epsilon_step: float = 0.05) -> npt.NDArray[np.number]

Approximates a given polygon by reducing a certain percentage of points.

This function uses the Ramer-Douglas-Peucker algorithm to simplify the input polygon by reducing the number of points while preserving the general shape.

Parameters:

Name Type Description Default

polygon

NDArray[number]

A 2D NumPy array of shape (N, 2) containing the x, y coordinates of the input polygon's points.

 required

percentage

float

The percentage of points to be removed from the input polygon, in the range [0, 1).

 required

epsilon_step

float

Approximation accuracy step, must be positive. Epsilon is the maximum distance between the original curve and its approximation.

 0.05

Returns:

Type Description
NDArray[number]

A new 2D NumPy array of shape (M, 2) containing the x, y coordinates of the approximated polygon's points. M is at most floor(N * (1 - percentage)) (minimum 3). Because epsilon advances in discrete epsilon_step increments, M may be noticeably smaller than the budget when a single step crosses the target band. At least 3 points are always kept; when further simplification would collapse the polygon below 3 points the last valid approximation is returned and M may exceed the budget.

Raises:

Type Description
ValueError

If percentage is outside the range [0, 1).
ValueError

If epsilon_step is not positive.

Examples:

Reduce a polygon to at most half its original point count:

>>> import numpy as np
>>> polygon = np.array([[0, 0], [10, 0], [10, 10], [0, 10],
...                     [5, 10], [5, 5], [3, 7], [1, 9]])
>>> result = approximate_polygon(polygon, percentage=0.5)
>>> result.shape[1]
2
>>> len(result) <= max(int(len(polygon) * 0.5), 3)
True

Polygon already at or below target — returned unchanged:

>>> tiny = np.array([[0, 0], [5, 0], [2, 4]])
>>> approximate_polygon(tiny, percentage=0.5) is tiny
True
Source code in src/supervision/detection/utils/polygons.py 
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def approximate_polygon(
    polygon: npt.NDArray[np.number], percentage: float, epsilon_step: float = 0.05
) -> npt.NDArray[np.number]:
    """
    Approximates a given polygon by reducing a certain percentage of points.

    This function uses the Ramer-Douglas-Peucker algorithm to simplify the input
    polygon by reducing the number of points while preserving the general shape.

    Args:
        polygon: A 2D NumPy array of shape `(N, 2)` containing
            the `x`, `y` coordinates of the input polygon's points.
        percentage: The percentage of points to be removed from the
            input polygon, in the range `[0, 1)`.
        epsilon_step: Approximation accuracy step, must be positive.
            Epsilon is the maximum distance between the original curve
            and its approximation.

    Returns:
        A new 2D NumPy array of shape `(M, 2)` containing the `x`, `y`
            coordinates of the approximated polygon's points. `M` is at most
            `floor(N * (1 - percentage))` (minimum 3). Because epsilon
            advances in discrete `epsilon_step` increments, `M` may be
            noticeably smaller than the budget when a single step crosses
            the target band. At least 3 points are always kept; when further
            simplification would collapse the polygon below 3 points the last
            valid approximation is returned and `M` may exceed the budget.

    Raises:
        ValueError: If `percentage` is outside the range `[0, 1)`.
        ValueError: If `epsilon_step` is not positive.

    Examples:
        Reduce a polygon to at most half its original point count:

        >>> import numpy as np
        >>> polygon = np.array([[0, 0], [10, 0], [10, 10], [0, 10],
        ...                     [5, 10], [5, 5], [3, 7], [1, 9]])
        >>> result = approximate_polygon(polygon, percentage=0.5)
        >>> result.shape[1]
        2
        >>> len(result) <= max(int(len(polygon) * 0.5), 3)
        True

        Polygon already at or below target — returned unchanged:

        >>> tiny = np.array([[0, 0], [5, 0], [2, 4]])
        >>> approximate_polygon(tiny, percentage=0.5) is tiny
        True
    """

    if percentage < 0 or percentage >= 1:
        raise ValueError("Percentage must be in the range [0, 1).")
    if epsilon_step <= 0:
        raise ValueError("epsilon_step must be positive.")

    target_points = max(int(len(polygon) * (1 - percentage)), 3)

    if len(polygon) <= target_points:
        return polygon

    epsilon: float = 0
    approximated_points = polygon
    while len(approximated_points) > target_points:
        epsilon += epsilon_step
        candidate = np.squeeze(cv2.approxPolyDP(polygon, epsilon, closed=True), axis=1)
        # Stop before the approximation collapses below a valid polygon; keep the
        # last result with at least three points.
        if len(candidate) < 3:
            break
        approximated_points = candidate

    return approximated_points

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