perf(cv2): simplify fallback operations

[Borda]

This pull request focuses on removing unused cv2 compatibility symbols and streamlining the cv2-free fallback implementation, while also improving performance and precision in key image processing operations. The main changes include removing unused API surface from the _cv2 module, adding a minimal find_contours helper, vectorizing connected-component statistics, and updating color conversion to exactly match OpenCV for uint8 images.

API surface reduction and fallback simplification:

• Removed unused raw compatibility symbols from src/supervision/_cv2/__init__.py, including distanceTransform, getRotationMatrix2D, and warpAffine, as well as related constants and their exports. These are no longer needed by production consumers, which now use more targeted operations. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

• Added a new find_contours function to src/supervision/_cv2/__init__.py that provides only the mask-to-polygon conversion geometry required by production, further narrowing the fallback API. [1] [2]

Performance and precision improvements:

• Updated the cv2-free fallback for connected-component statistics in src/supervision/_cv2/_components.py to use vectorized NumPy operations and scipy.ndimage.find_objects, significantly reducing overhead and improving performance.

• Changed the BGR-to-gray conversion in src/supervision/_cv2/_color.py to use OpenCV's fixed-point uint8 arithmetic, ensuring exact matches for all possible input colors.

Changelog and documentation:

• Updated the changelog to reflect the removal of unused compatibility symbols, the new precise BGR-to-gray conversion, and the vectorized connected-component statistics. [1] [2] [3]

Minor refactoring:

• Cleaned up imports and type usage in src/supervision/_cv2/_contours.py.

[codecov]

Codecov Report

❌ Patch coverage is 95.30516% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 87%. Comparing base (a4b9c4e) to head (0ef6fb1).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

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##           develop   #2441   +/-   ##
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  Coverage       86%     87%           
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  Files           84      84           
  Lines        11786   11785    -1     
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+ Hits         10170   10196   +26     
+ Misses        1616    1589   -27     

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[Copilot]

Pull request overview

This PR streamlines Supervision’s private _cv2 compatibility layer by removing unused fallback surface area and tightening the cv2-free implementations, while improving fidelity/performance for several core image-processing primitives (contours, connected components, color conversion, resize/text rendering). It also updates library consumers and tests to validate the new, narrower contracts.

Changes:

• Reduced _cv2 API surface (removed unused transform/distance APIs) and introduced a minimal find_contours() geometry helper used by mask-to-polygon conversion.
• Improved numeric fidelity/performance in cv2-free paths: fixed-point BGR2GRAY for uint8, vectorized connected-component stats, refined approxPolyDP, and optimized Pillow-based text/resize behavior.
• Updated docs/changelog and expanded regression tests around contours, resize, text clipping, and edge-distance behavior.

Reviewed changes

Copilot reviewed 24 out of 24 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/detection/test_line_counter.py	Adds regression test ensuring rotated line-zone labels use the Pillow rotation path and preserve alpha behavior.
tests/cv2/test_transform.py	Removes tests for fallback APIs that were deleted from the _cv2 surface (affine + distance transform), keeping blur coverage.
tests/cv2/test_text.py	Adds coverage for clipping text when the baseline origin is partially off-canvas.
tests/cv2/test_image.py	Adds tighter resize regression tests for uint8 linear interpolation and float interpolation threshold behavior.
tests/cv2/test_geometry.py	Adds randomized regression tests for approxPolyDP and float32 drift bounds for polygon intersection.
tests/cv2/test_cv2.py	Updates expected _cv2 facade symbol set (replaces findContours with find_contours, removes unused symbols).
tests/cv2/test_contours.py	Updates contour expectations to geometry-only contract, adds facade test for find_contours, and validates chamfer distances/edge-distance behavior.
tests/cv2/test_constants.py	Updates constants expectations to reflect removed compatibility exports.
tests/cv2/test_components.py	Adds high-component-image regression test for vectorized connected-component stats matching OpenCV.
tests/cv2/test_color.py	Adds regression test for uint8 fixed-point BGR2GRAY rounding matching OpenCV.
src/supervision/detection/utils/masks.py	Introduces fixed-point 3×3 L2 chamfer transform and updates edge-distance filtering to avoid distanceTransform.
src/supervision/detection/utils/converters.py	Switches mask→polygon extraction to use the new cv2.find_contours() helper.
src/supervision/detection/line_zone.py	Replaces cv2 affine rotation with Pillow-based rotation for label images.
src/supervision/_cv2/constants.py	Removes unused _DIST_L2 constant from the private constants set.
src/supervision/_cv2/_video.py	Uses PyAV’s stream-template API to replace the removed local stream-copy helper.
src/supervision/_cv2/_transform.py	Removes unused affine and distance-transform fallbacks, leaving _blur.
src/supervision/_cv2/_text.py	Reworks Pillow rasterization to build a glyph-sized mask and clip safely at image boundaries.
src/supervision/_cv2/_image.py	Delegates uint8 bilinear resize to Pillow and keeps numeric half-pixel sampling for non-uint8 paths.
src/supervision/_cv2/_geometry.py	Replaces approxPolyDP simplification with an OpenCV-matching stack traversal + cleanup step.
src/supervision/_cv2/_contours.py	Simplifies contour extraction to return geometry only (no hierarchy construction).
src/supervision/_cv2/_components.py	Vectorizes centroid/statistics computation and uses ndimage.find_objects for component bounds.
src/supervision/_cv2/_color.py	Implements exact OpenCV-style fixed-point BGR2GRAY for uint8 images.
src/supervision/_cv2/init.py	Removes unused cv2-compat symbols and adds find_contours() as the minimal geometry facade function.
docs/changelog.md	Documents the fallback API reduction and the new precision/performance behaviors.