Add PSF-guided hashgrid auto-configuration for Nerfstudio models

[txdai]

Integrates the PSF-based hashgrid parameter selection workflow from the ICLR poster PSF study into Nerfstudio so growth-factor tuning can be computed a priori instead of swept empirically.

Highlights:

• Added a new utility module: nerfstudio/utils/hashgrid_psf.py.
  • Implements revised axis-FWHM approximation for multi-resolution hash encodings.
  • Solves per-level growth factor (b) from a target empirical FWHM using binary search.
  • Converts solved growth to Nerfstudio-compatible max_res.
  • Estimates pixel-to-world scale from train cameras (perspective pinhole approximation) with robust scene-scale fallback.

Model integration:

• nerfstudio/models/nerfacto.py

  • Added opt-in config flags:
    • psf_guided_hashgrid
    • psf_target_fwhm_pixels
    • psf_broadening
    • psf_adjust_proposal_nets
  • Computes PSF-guided hashgrid parameters at startup when enabled.
  • Applies solved max_res to the main field.
  • Optionally propagates the same solved growth factor to proposal network max_res values.
  • Logs chosen b / max_res and whether camera-scale or fallback scale was used.

• nerfstudio/models/instant_ngp.py

  • Added opt-in config flags:
    • psf_guided_hashgrid
    • psf_target_fwhm_pixels
    • psf_broadening
    • num_levels
    • base_res
  • Computes and applies PSF-guided max_res at startup.
  • Logs selected parameters and scale source.

Pipeline plumbing:

• nerfstudio/pipelines/base_pipeline.py
  • Passes train cameras into model setup so PSF sizing can use real camera intrinsics/extrinsics.

Method preset:

• nerfstudio/configs/method_configs.py
  • Added new method entry: nerfacto-psf.
  • Provides a ready-to-use no-sweep preset with PSF-guided hashgrid enabled.

Tests:

• Added tests/utils/test_hashgrid_psf.py covering:
  • Growth-factor solve accuracy against target FWHM.
  • max_res conversion behavior.
  • Pixel-world estimation.
  • End-to-end parameter computation with both camera-based and fallback sizing.

Outcome:

• Enables PSF-guided, no-grid-sweep hashgrid initialization in Nerfstudio while remaining fully opt-in and backward-compatible by default.