Slice 5: GPU Pass 2 — coefficient reconstruction + inverse DTCWT

[joeljose]

Context

Parent: #11 | PRD: #10 | Design: docs/design/gpu-acceleration.md Section B, Pass 2

Goal

Implement the second GPU pass: re-run forward DTCWT to recover Yl and amplitudes, apply modified phases, reconstruct coefficients, and run inverse DTCWT.

Acceptance Criteria

• _gpu_inverse_pass(data, phase_arrays, nlevels, biort, qshift, device) returns reconstructed (num_frames, H, W) float32 array
• Re-runs DTCWTForward to recover Yl and amplitudes (verified deterministic — 0.00 diff)
• Reconstructs modified Yh: real = amp * cos(phase), imag = amp * sin(phase), stacked to (B, 1, 6, H, W, 2)
• DTCWTInverse((Yl, Yh_modified)) produces reconstructed frames
• Output cropped to original (H, W) if inverse output is larger (padding)
• Same batch size auto-tuning as Pass 1
• OOM caught with actionable error
• Progress reporting

Blocked by

#15 (Slice 4) — needs modified phase arrays from temporal filter.

Files likely involved

• motion_mag.py — _gpu_inverse_pass() function

Testing approach

• Unit test: identity test — extract phases in Pass 1, skip modification, reconstruct in Pass 2 → output matches input (PSNR > 120 dB)
• Unit test: output shape matches input shape
• Unit test: output values are finite and in reasonable range

[joeljose]

Completed in PR #18, released in v2.0.0