fix: support grayscale letterbox images

[Bortlesboat]

Summary

• guard letterbox_image's alpha-channel branch so grayscale NumPy images do not crash after resizing
• add a regression test covering grayscale padding output

Tests

• uv run pytest tests/utils/test_image.py::test_letterbox_image_for_grayscale_opencv_image -q
• uv run pytest tests/utils/test_image.py -q
• uv run pytest tests/utils -q
• uv run pre-commit run --files src/supervision/utils/image.py tests/utils/test_image.py
• uv run pytest -q

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79%. Comparing base (35006d7) to head (0d192e5).

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #2297   +/-   ##
=======================================
  Coverage       79%     79%           
=======================================
  Files           66      66           
  Lines         8640    8635    -5     
=======================================
- Hits          6863    6862    -1     
+ Misses        1777    1773    -4     

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[Copilot]

Pull request overview

This PR updates letterbox_image to better handle grayscale NumPy images (2D arrays) by guarding alpha-channel handling, and adds a regression test to ensure grayscale padding output is correct.

Changes:

• Guard the alpha-channel branch in letterbox_image to avoid crashes on grayscale NumPy images.
• Add a regression test validating letterbox padding behavior for grayscale OpenCV/NumPy images.

Quality Assessment (n/5)

• Code quality: 2/5
• Testing: 4/5
• Documentation: 3/5

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/supervision/utils/image.py	Adds a dimension guard around the alpha-channel logic in letterbox_image.
tests/utils/test_image.py	Adds a regression test for grayscale NumPy input letterboxing and padding output.

Comments suppressed due to low confidence (1)

src/supervision/utils/image.py:270

• The alpha-channel padding fix is applied to the original image instead of the padded image_with_borders, so it (a) does not affect the returned value and (b) can unexpectedly mutate the caller’s input array. The indices used (height_new - padding_bottom, width_new - padding_right) are also inconsistent with the bordered image coordinate system and can target the wrong region. Update the block to operate on image_with_borders and compute the start of the bottom/right padding using resolution_wh (or padding_top + height_new, padding_left + width_new).

    if image.ndim == 3 and image.shape[2] == 4:
        image[:padding_top, :, 3] = 0
        image[height_new - padding_bottom :, :, 3] = 0
        image[:, :padding_left, 3] = 0
        image[:, width_new - padding_right :, 3] = 0