open-mmlab/mmdetection · error · ValueError

Invalid flipping direction '{flip_direction}'

Error message

Invalid flipping direction '{flip_direction}'

What it means

merge_aug_masks reverts test-time-augmentation flips on predicted masks and only understands 'horizontal', 'vertical', and 'diagonal' flip directions. When the augmentation config produced another direction string, it raises ValueError.

Source

Thrown at mmdet/models/test_time_augs/merge_augs.py:212

    recovered_masks = []
    for i, mask in enumerate(aug_masks):
        if weights is not None:
            assert len(weights) == len(aug_masks)
            weight = weights[i]
        else:
            weight = 1
        flip = img_metas.get('flip', False)
        if flip:
            flip_direction = img_metas['flip_direction']
            if flip_direction == 'horizontal':
                mask = mask[:, :, :, ::-1]
            elif flip_direction == 'vertical':
                mask = mask[:, :, ::-1, :]
            elif flip_direction == 'diagonal':
                mask = mask[:, :, :, ::-1]
                mask = mask[:, :, ::-1, :]
            else:
                raise ValueError(
                    f"Invalid flipping direction '{flip_direction}'")
        recovered_masks.append(mask[None, :] * weight)

    merged_masks = torch.cat(recovered_masks, 0).mean(dim=0)
    if weights is not None:
        merged_masks = merged_masks * len(weights) / sum(weights)
    return merged_masks

View on GitHub (pinned to cfd5d3a985)

Solutions

1. Use only 'horizontal', 'vertical', or 'diagonal' in flip_direction in the test-time aug pipeline
2. Check custom Flip transforms to ensure they emit these exact lowercase strings

Example fix

# before
tta_pipeline=[..., dict(type='Flip', flip_direction='diag')]
# after
tta_pipeline=[..., dict(type='Flip', flip_direction='diagonal')]

assert flip_direction in ('horizontal', 'vertical', 'diagonal'), flip_direction

def is_valid_flip_direction(d: str) -> bool:
    return d in ('horizontal', 'vertical', 'diagonal')

try:
    merge_aug_masks(masks, cfg)
except ValueError as e:
    if 'Invalid flipping direction' in str(e):
        # normalize or drop unsupported flips
        ...
    raise

When it happens

Trigger: Calling merge_aug_masks (via multi-scale / flip TTA predict) where aug_masks metadata carries a flip_direction other than the three supported values, e.g. 'diag' or a custom string set in the test pipeline.

Common situations: Custom flip transforms in the test pipeline, or configs migrated from other repos (mmdet3d uses 'DIAGONAL') where direction casing/values differ.

Understand the failure class

Background: Invalid enum value errors: "Unknown type", "Invalid scope", "must be one of" — when a string is not on the library's allowed list — this error's family across 23 libraries.

Related errors

• Unsupported {type(mask)} data type
• LoadImageFromFile is not found in the test pipeline
• Visualization needs the "visualizer" termdefined in the conf
• Unsupported input type: {type(single_input)}
• panopticapi is not installed, please install it by: pip inst

AI-assisted analysis of open-mmlab/mmdetection@cfd5d3a985 (2026-08-27). Data as JSON: /api/errors/6735dbaa4c13d687. Report an issue: GitHub.