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Train Config Generation App.

The pyodi train-config generation app can be used to automatically generate a mmdetection anchor configuration to train your model.

The design of anchors is critical for the performance of one-stage detectors. Usually, published models such Faster R-CNN or RetinaNet include default anchors which has been designed to work with general object detection purpose as COCO dataset. Nevertheless, you might be envolved in different problems which data contains only a few different classes that share similar properties, as the object sizes or shapes, this would be the case for a drone detection dataset such Drone vs Bird. You can exploit this knowledge by designing anchors that specially fit the distribution of your data, optimizing the probability of matching ground truth bounding boxes with generated anchors, which can result in an increase in the performance of your model. At the same time, you can reduce the number of anchors you use to boost inference and training time.

Procedure

The input size parameter determines the model input size and automatically reshapes images and annotations sizes to it. Ground truth boxes are assigned to the anchor base size that has highest Intersection over Union (IoU) score with them. This step, allow us to locate each ground truth bounding box in a feature level of the FPN pyramid.

Once this is done, the ratio between the scales of ground truth boxes and the scales of their associated anchors is computed. A log transform is applied to it and they are clustered using kmeans algorithm, where the number of obtained clusters depends on n_scales input parameter.

After this step, a similar procedure is followed to obtain the reference scale ratios of the dataset, computing log scales ratios of each box and clustering them with number of clusters equal to n_ratios.

Example usage:

pyodi train-config generation \\
$TINY_COCO_ANIMAL/annotations/train.json \\
--input-size [1280,720] \\
--n-ratios 3 --n-scales 3

The app shows two different plots:

Anchor clustering plot

Log Relative Scale vs Log Ratio

In this graphic you can distinguish how your bounding boxes scales and ratios are distributed. The x axis represent the log scale of the ratio between the bounding box scales and the scale of their matched anchor base size. The y axis contains the bounding box log ratios. Centroids are the result of combinating the obtained scales and ratios obtained with kmeans. We can see how clusters appear in those areas where box distribution is more dense.

We could increase the value of n_ratios from three to four, having into account that the number of anchors is goint to increase, which will influence training computational cost.

pyodi train-config generation annotations/train.json --input-size [1280,720] --n-ratios 4 --n-scales 3

In plot below we can observe the result for n_ratios equal to four.

Anchor clustering plot 4 ratios

Bounding Box Distribution

This plot is very useful to observe how the generated anchors fit you bounding box distribution. The number of anchors depends on:

  • The length of base_sizes which determines the number of FPN pyramid levels.
  • A total of n_ratios x n_scales anchors is generated per level

We can now increase the number of n_scales and observe the effect on the bounding box distribution plot.

Anchor clustering plot 4 scales

Proposed anchors are also attached in a Json file that follows mmdetection anchors format:

anchor_generator=dict(
    type='AnchorGenerator',
    scales=[1.12, 3.13, 8.0],
    ratios=[0.33, 0.67, 1.4],
    strides=[4, 8, 16, 32, 64],
    base_sizes=[4, 8, 16, 32, 64],
)

By default, pyodi train-config evaluation is used after the generation of anchors in order to compare which generated anchor config suits better your data. You can disable this evaluation by setting to False the evaluate argument, but it is strongly advised to use the anchor evaluation module.


API REFERENCE

train_config_generation(ground_truth_file, input_size=(1280, 720), n_ratios=3, n_scales=3, strides=None, base_sizes=None, show=True, output=None, output_size=(1600, 900), keep_ratio=False, evaluate=True)

Computes optimal anchors for a given COCO dataset based on iou clustering.

Parameters:

Name Type Description Default
ground_truth_file str

Path to COCO ground truth file.

required
input_size Tuple[int, int]

Model image input size. Defaults to (1280, 720).

(1280, 720)
n_ratios int

Number of ratios. Defaults to 3.

3
n_scales int

Number of scales. Defaults to 3.

3
strides Optional[List[int]]

List of strides. Defatults to [4, 8, 16, 32, 64].

None
base_sizes Optional[List[int]]

The basic sizes of anchors in multiple levels. If None is given, strides will be used as base_sizes.

None
show bool

Show results or not. Defaults to True.

True
output Optional[str]

Output directory where results going to be saved. Defaults to None.

None
output_size Tuple[int, int]

Size of saved images. Defaults to (1600, 900).

(1600, 900)
keep_ratio bool

Whether to keep the aspect ratio or not. Defaults to False.

False
evaluate bool

Whether to evaluate or not the anchors. Check pyodi train-config evaluation for more information.

True

Returns:

Type Description
None

Anchor generator instance.

Source code in pyodi/apps/train_config/train_config_generation.py
@logger.catch
def train_config_generation(
    ground_truth_file: str,
    input_size: Tuple[int, int] = (1280, 720),
    n_ratios: int = 3,
    n_scales: int = 3,
    strides: Optional[List[int]] = None,
    base_sizes: Optional[List[int]] = None,
    show: bool = True,
    output: Optional[str] = None,
    output_size: Tuple[int, int] = (1600, 900),
    keep_ratio: bool = False,
    evaluate: bool = True,
) -> None:
    """Computes optimal anchors for a given COCO dataset based on iou clustering.

    Args:
        ground_truth_file: Path to COCO ground truth file.
        input_size: Model image input size. Defaults to (1280, 720).
        n_ratios: Number of ratios. Defaults to 3.
        n_scales: Number of scales. Defaults to 3.
        strides: List of strides. Defatults to [4, 8, 16, 32, 64].
        base_sizes: The basic sizes of anchors in multiple levels.
            If None is given, strides will be used as base_sizes.
        show: Show results or not. Defaults to True.
        output: Output directory where results going to be saved. Defaults to None.
        output_size: Size of saved images. Defaults to (1600, 900).
        keep_ratio: Whether to keep the aspect ratio or not. Defaults to False.
        evaluate: Whether to evaluate or not the anchors. Check
            [`pyodi train-config evaluation`][pyodi.apps.train_config.train_config_evaluation.train_config_evaluation]
            for more information.

    Returns:
        Anchor generator instance.
    """
    if output is not None:
        Path(output).mkdir(parents=True, exist_ok=True)

    df_annotations = coco_ground_truth_to_df(ground_truth_file)

    df_annotations = filter_zero_area_bboxes(df_annotations)

    df_annotations = scale_bbox_dimensions(
        df_annotations, input_size=input_size, keep_ratio=keep_ratio
    )

    df_annotations = get_scale_and_ratio(df_annotations, prefix="scaled")

    if strides is None:
        strides = [4, 8, 16, 32, 64]
    if base_sizes is None:
        base_sizes = strides

    # Assign fpn level
    df_annotations["fpn_level"] = find_pyramid_level(
        get_bbox_array(df_annotations, prefix="scaled")[:, 2:], base_sizes
    )

    df_annotations["fpn_level_scale"] = df_annotations["fpn_level"].replace(
        {i: scale for i, scale in enumerate(base_sizes)}
    )

    df_annotations["level_scale"] = (
        df_annotations["scaled_scale"] / df_annotations["fpn_level_scale"]
    )

    # Normalize to log scale
    df_annotations["log_ratio"] = np.log(df_annotations["scaled_ratio"])
    df_annotations["log_level_scale"] = np.log(df_annotations["level_scale"])

    # Cluster bboxes by scale and ratio independently
    clustering_results = [
        kmeans_euclidean(df_annotations[value].to_numpy(), n_clusters=n_clusters)
        for i, (value, n_clusters) in enumerate(
            zip(["log_level_scale", "log_ratio"], [n_scales, n_ratios])
        )
    ]

    # Bring back from log scale
    scales = np.e ** clustering_results[0]["centroids"]
    ratios = np.e ** clustering_results[1]["centroids"]

    anchor_generator = AnchorGenerator(
        strides=strides, ratios=ratios, scales=scales, base_sizes=base_sizes,
    )
    logger.info(f"Anchor configuration: \n{anchor_generator.to_string()}")

    plot_clustering_results(
        df_annotations,
        anchor_generator,
        show=show,
        output=output,
        output_size=output_size,
        title="COCO_anchor_generation",
    )

    if evaluate:
        anchor_config = dict(anchor_generator=anchor_generator.to_dict())
        train_config_evaluation(
            ground_truth_file=df_annotations,
            anchor_config=anchor_config,  # type: ignore
            input_size=input_size,
            show=show,
            output=output,
            output_size=output_size,
        )

    if output:
        output_file = Path(output) / "anchor_config.py"
        with open(output_file, "w") as f:
            f.write(anchor_generator.to_string())