Ridges Beyond the Window: Structure-First Completion of Contactless Fingerprints from the Capture Sweep

A contactless fingerprint captured in the wild is both degraded and partial, because a phone camera sees a blurred, noisy patch of the fingertip rather than a clean rolled impression. We present SweepFill, a single network that restores what such a capture shows and generates ridges of the correct flow where it shows nothing. Each capture is a short video of the turning finger, which we call a sweep, so a part hidden in one frame often appears in another, and no published completion method uses this. Because the exact position of ridges (their phase) beyond the visible window is not determined by the observation, while their direction (flow) and spacing (period) are, SweepFill predicts structure before pixels. A head on the bottleneck predicts ridge flow and frequency for the whole print, a phase-free spectral loss lets the decoder draw ridges without being punished for phase it cannot know, and joint attention over other frames of the same sweep turns much of the generation into registration, all learned by regression alone. We also show that inserting the mask token after the positional encoding makes every hidden token of a masked-bottleneck transformer identical, a failure not specific to fingerprints. We evaluate with two fingerprint matchers under a cross-session protocol. When 30% of the print is visible, SweepFill reduces hidden-region orientation error from 20.07 to 11.01 degrees and raises ridge contrast from 0.15 to 0.67 relative to a DiT-UNet restorer trained on more samples, and it lowers the deep matcher’s EER from 26.0% to 11.6% on held-out MMFV subjects. Completing a poor frame before fusing scores over the other frames raises fused rank-1 by +8.3 and +11.5 points when 50% and 70% of the print is visible (not significantly at 30%), and an in-domain fine-tune improves both matchers on a second device. The generated flow is within 10.5 degrees RMSE of the clean frame, but its phase is uncorrelated with it, and only 21% of true minutiae are recovered, so the completions are plausible rather than reconstructed.