Fix GPU S2N validation: shape mismatch crash and inverted threshold logic

[Copilot]

gpu_piv(..., validation_method="s2n") crashes with a PyCUDA shape mismatch, and when it doesn't crash (combined validation), it invalidates the wrong vectors — high-S2N instead of low-S2N.

Changes

• process.py — reshape s2n_ratio to u.shape before passing to validation. self._corr_gpu.s2n_ratio is 1D (flattened); this caused val_locations to be flat, mismatching the 2D velocity field in replace_vectors().

  # Before
  s2n_ratio = self._corr_gpu.s2n_ratio
  # After
  s2n_ratio = self._corr_gpu.s2n_ratio.reshape(u.shape)

• validation.py — fix inverted threshold in _s2n_validation. The old formula s2n_ratio / s2n_tol > 1 flagged high-S2N vectors as invalid. Replaced with s2n_tol - s2n_ratio > 0, which correctly flags vectors where s2n_ratio < s2n_tol, consistent with the CPU implementation.

  # Before — invalidates HIGH s2n (wrong)
  sig2noise_tol = s2n_ratio / DTYPE_f(s2n_tol)
  self.val_locations = _local_validation(sig2noise_tol, 1, self.val_locations)
  
  # After — invalidates LOW s2n (correct)
  # Mark invalid where s2n_ratio < s2n_tol, i.e. (s2n_tol - s2n_ratio) > 0.
  self.val_locations = _local_validation(
      DTYPE_f(s2n_tol) - s2n_ratio, 0, self.val_locations
  )

• test_validation.py — update test_validation_s2n_validation to assert the corrected logic.

[Copilot]

Pull request overview

This PR fixes GPU signal-to-noise validation by aligning the S2N array shape with velocity fields and correcting the threshold comparison so low-S2N vectors are marked invalid.

Changes:

• Reshapes GPU S2N ratios before validation in the PIV pipeline.
• Reverses S2N validation logic to flag values below the configured threshold.
• Updates the S2N validation unit test expectation.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
openpiv/gpu/process.py	Reshapes flattened correlation S2N output to match velocity field shape before validation.
openpiv/gpu/validation.py	Changes S2N validation to invalidate low-S2N vectors instead of high-S2N vectors.
openpiv/test/gpu/test_validation.py	Updates the S2N validation test to match the corrected threshold direction.

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